# Talk:Lux

WikiProject Physics (Rated C-class, High-importance)
WikiProject Measurement (Defunct)

## Disambiguation Page

I think there needs to be a disambiguation page for lux in terms of light and also the lux gene/operon in bioluminescent bacteria. —Preceding unsigned comment added by 99.225.24.69 (talk) 16:20, 12 February 2008 (UTC)

There is a disambiguation page for lux. It is at Lux (disambiguation).--Srleffler (talk) 16:38, 12 February 2008 (UTC)

## Units and symbols

From the SI article:

• Symbols are written in singular, e.g. 25 kg (not "25 kgs").

Similarly it is lumen not lumens

213.51.209.230 11:09, 9 Oct 2004 (UTC)

The symbols lm and lx are written in the singular. Lumen and lux are not symbols, they are ordinary words and follow ordinary grammatical usage. Lux is a special irregular case, because its plural happens to be lux.

OK, point taken 213.51.209.230 16:34, 9 Oct 2004 (UTC)
Bobblewik  (talk), whom I invited to comment (see below), is very knowledgeable on SI. Oh, by the way, welcome to Wikipedia, Sonett72. [[User:Dpbsmith|Dpbsmith (talk)]] 21:51, 9 Oct 2004 (UTC)
Plural unit names are used when they are required by the rules of English grammar. They are normally formed regularly, for example, "henries" is the plural of henry. According to Ref. [8], the following plurals are irregular: Singular - lux, hertz, siemens; Plural - lux, hertz, siemens. (See also Sec. 9.7.)

[[User:Dpbsmith|Dpbsmith (talk)]] 14:32, 9 Oct 2004 (UTC)

P. S. Although klx and klm are properly formed and presumably valid combinations of prefix and symbol, I don't recall ever, ever, ever having seen either of them. Are you aware of this as being real everyday usage (as opposed to, say,

1.00 × 103 lx ?

I would write klx, in preference to "1.00 × 103 lx". --Srleffler 05:19, 13 January 2006 (UTC)

### Plurals

The official SI website says "Unit symbols are unaltered in the plural" (http://www1.bipm.org/en/si/si_brochure/chapter5/5-2.html). This applies to the symbols, not to the unit names. So unit names can follow the local language and spelling when used in full. English language forms are:

• 50 metres and 50 m, but not 50 ms
• 50 kilograms and 50 kg, but not 50 kgs
• 50 seconds and 50 s, but not 50 s's
• 50 volts and 50 V, but not 50 V's
• 50 lumens and 50 lm but not 50 lms

Bobblewik  (talk) 14:58, 9 Oct 2004 (UTC)

### Prefixes

According to the official SI website, SI prefix names and the prefix symbols can be added to any SI unit (http://www1.bipm.org/en/si/prefixes.html). Therefore klx and μlx are indeed valid and unambiguous forms. Some prefixes and unit combinations are less common, but SI does not address the issue of familiarity. That is a matter between authors and their readers.

Computing has made more people familiar with small (micro, nano) and large prefixes (mega, giga). I think it is interesting that potentially useful unit multiples Mg and Mm are not more common.

Bobblewik  (talk) 15:16, 9 Oct 2004 (UTC)

### Prefixes revisited

I think these SI prefixed forms should be removed from the table. Although they are technically correct, the purpose of this table is not to educate the reader about SI prefixes, but to give an idea of different common illumination levels. Moreover, especially 'hlx' is hardly ever used in real life. If I do a google search on 'hlx illuminance', I see 5 technical pages where hlx is used as a unit, the rest being Wikipedia (copies) and lamps that have 'HLX' in their part number. Wikipedia should represent common usage, an not falsely give the impression that "hlx" is a common term. Han-Kwang (t) 08:29, 12 September 2008 (UTC)
I'm fine with that.--Srleffler (talk) 04:31, 13 September 2008 (UTC)

## Less dictionary/science, more real-world?

I came to Wikipedia to look at the entry to lux because I was wanting some idea of how a 0.01 lux camera would compare in low light to a 0.1 or 1 lux camera. Like a comparison of what the human eye would see, versus what a camera at 0.01 lux would see. This may be one of those area too specific for Wikipedia to really cover, but it would be nice if the lux definition had some more real-world data than scientific data. --Jmccorm 04:18, 13 January 2006 (UTC)

You're not really looking for information on lux, you are looking for information on camera technologies and human vision. Maybe you'll find what you are looking for at visual perception, visual system or camera, or one of the related articles. I presume that a "0.01 lux camera" is a camera that the manufacturer claims will take a viewable picture at that level of light. This sounds like something that might be pretty poorly defined—more a matter of marketing hype than detailed specification. You might want to check the manufacturer's website and see if they provide an explanation of what it means. --Srleffler 05:19, 13 January 2006 (UTC)

## 60 watts bulb example & a square meter?

the article says 1 lux = 1 lumen per square meter.

shouldn't the square meter be cube meter??

and, how many lumens is a average 60 Watts light bulb?

Xah Lee 07:21, 2004 Oct 12 (UTC)

1. No.
2. About 870: 14.5 lm/W (according to our light bulb article) × 60 W.

--Heron 08:25, 12 Oct 2004 (UTC)

Just to avoid confusion for someone reading this who is not familiar with these units: note that the 14.5 lm/W ratio is particular to incandescent light bulbs, and even then isn't all that accurate and only applies for bulbs around 60 W. Other types of light source have different ratings, and in fact the exact ratio varies from one type of light bulb to another. The article on incandescent bulbs has a nice table comparing the luminous flux of various light sources.--Srleffler 05:19, 13 January 2006 (UTC)

## Human perception?

I think the remark below is wrong, for the following reasons:

• Human perception of light intensity is not logarithmic, but follows a power law. That is, equal ratios of light intensity correspond to equal ratios, not equal increments of sensation. See Stevens' power law.
• Although ratios of illuminance can be expressed in decibels, in fact I don't believe they ever are. That is, the comment verges on original research.
• Something's wrong here, anyway, as lux is a unit of illuminance. I don't remember offhand what the unit of light-intensity-reaching-the-retina is, apostilbs or some damn thing like that? but in any case lux is not the right quantity. If this remark belongs anywhere it would be in the article about some other photometric unit, not the lux.

Radiometric measurements involving actual power, i.e. measurements derived from watts, might well use decibels (e.g. to express signal-to-noise ratios).

If I'm wrong about this, apologies. But I'd like to see a source. Dpbsmith (talk) 16:06, 11 May 2006 (UTC)

--I will try to provide a source, but I am starting to doubt. I think I heard it a few years ago at the Uni. I agree, it should be left out untill I verify my information. —Preceding unsigned comment added by 85.144.129.243 (talkcontribs)

### Human perception

Human perception of light, like that of sound, is logarithmic[citation needed], so an increase by a factor of 10 in lux can be expressed as an increase of 10 dB in illuminance. With this notation, an increase from 50 klx to 100 klx is an increase in illuminance of about 3 dB.

Wrong. You need to read the famous paper by S. S. Stevens, "To Honor Fechner and Repeal His Law". He updated the Weber-Fechner law (logarithmic psychophysics) with the more accurate Stevens' power law. It's OK to cite decibels, but they should not be regarded as perceptually equal units. Dicklyon 07:26, 16 May 2006 (UTC)

### decibels in optics and photometry

The use of dB to compare optical powers is always risky, since the dB definition, 10log(power_ratio), conflicts with the definition used in video and image sensors, 20log(voltage_ratio) where the voltage is proportional to the optical power. So when you see dB, you often can't be sure which it is. No such problem arises in audio and most other uses of dB, where power is proportional to the square of voltge. Dicklyon 05:47, 21 May 2006 (UTC) Ů

### A clarification

Appreciation to the writer above for citing an important issue. Care should be taken in the application and interpretation of dB units when used with electro-optic equipment parameters. Care is warranted in such cases because electro-optic equipment often incorporates power-to-voltage converter circuits (photodetector circuits in particular), power-to-current converter circuits (photodetector circuits in particular), and their analogs: voltage-to-power converter circuits (photoemitter circuits in particular) and current-to-power converter circuits (photoemitter circuits in particular). The definition of decibels (dB) is consistent and unambiguous (including in such electro-optic equipment): 10*log(P1/P2), 20*log(V1/V2), and 20*log(I1/I2) at a given point (at a lens surface, or at a circuit node). Given a dB value for a given point, one can calculate any or all of the following unambiguously in conventional electro-optic equipment: P1/P2, V1/V2 and/or I1/I2. Given any of the following: P1/P2, V1/V2 and/or I1/I2, one can unambiguously calculate the dB value at the given point. Care should be taken by writers and readers in assuring that references in dB units are associated with the proper side of any photodetector or photoemitter interface. Persons working regularly with image sensor design or image sensor integration, i.e. persons working regularly on both sides of the photodetector interface, may wish to specify something like "dB optical-side" or "dB electrical-side" in conversation or written narrative; or refer to a specific point ("dB at the objective lens"). This should help to prevent the above-referenced problematic variety of error associated with the use of dB units in electro-optical equipment. In image sensors, for example, a 10dB loss on the optical side (10:1 power loss through an obscured window, for example) adheres to the definition above at the window, and this hypothetical window-obscurant loss will likely manifest as the same dB-loss (due to window obscurant) value throughout the rest of the (conventional) optics train. But the (photoconductive-mode, for example) image-sensor photodetector converts optical power to electrical current, such that the 10:1 optical power change described above elicits a 10:1 change in current at the photodetector output, i.e. a 100:1 change in power on the electrical side. The loss on the electrical side due to the window obscurant is therefore 20*log(I1/I2) = 10*log(P1/P2) = 20dB. The definition of decibels (dB) is consistent and unambiguous at any given point in the image sensor described above; care should be taken to apply the 2:1 change in dB values that occurs across many conventional photodetector and photoemitter interfaces. The writer above notes that "no such (issue) arises in audio and most other uses of dB". In support of this characterization, it may be said that as a percentage of citations of equipment dB values, E/O converters and RF modules with square-law responses may host the bulk of such (nonlinear interface) issues with the proper application and interpretation of dB units. Jplesa (talk) 06:17, 27 April 2010 (UTC)

## Definition

I changed the definition to make it clear the Lux is NOT a measure of perceived intensity, but rather a physical measure. To make the definition definite and complete, we should really include a table and/or plot of the Vlambda curve of luminous efficiency, which I expect is in the public domain, or at least widely copied from the IEC standard. Dicklyon 07:50, 16 May 2006 (UTC)

• Yeesh. When did THAT creep in? Even if it were a psychophysical unit it would be the wrong one, as it measures illuminance on a surface, not on the retina.
• Actually, illuminance of the retina (not of the scene being viewed) is what drives the retinal sensors and hence is an OK measure of the stimulus that is being perceived at that level. Similarly, total exposure of a photograpic sensor or film (in any small uniform patch) can be quantified as lux seconds. Dicklyon 14:49, 16 May 2006 (UTC)
• No, that wouldn't be lux. I can never remember exactly what it would be--apostilbs or something like that. Lux measures illuminance of the scene. You can quantify the light falling on the retina in many ways... just as you can quantify the light output of a light bulb in "watts"... but lux is not the appropriate. unit. Sorry, apostilbs is a unit of luminance. The unit I'm thinking of takes into account the aperture of the iris and the differential effectiveness of light falling at angles that are not normal to the retina. That, by the way, is one reason (maybe the reason?) why lux is an inappropriate unit. The sensitivity of the retina to light does not obey Lambert's law; light coming in at an angle is "worth" less than when it falls on a Lambertian surface. The unit I'm thinking of is one of those obscure photometric units I use about once a decade if that... Dpbsmith (talk) 15:25, 16 May 2006 (UTC)
• Stiles-Crawford effect, that's what I'm trying to think of... the appropriate unit for measurement of light on the retina takes account of the Stiles-Crawford effect... but what the heck is the name of that unit? Dpbsmith (talk) 15:31, 16 May 2006 (UTC)
• Troland! Retinal illuminance is measured in trolands, not in lux. But I was out to lunch above: the Troland takes into account the pupil size, but does not take into account the Stiles-Crawford effect. (Unless, of course, the Stiles-Crawford effect is included in determining "effective pupil size.") Dpbsmith (talk) 15:40, 16 May 2006 (UTC)
• Disagree! -- As the Troland article says, that's a measure of conventional retinal illuminance, which is not the same as actual retinal illuminance, which would be measured in lux if that's what you wanted to know. In cameras, for example, which are not so different from eyes, the film or sensor illuminance is quantified in lux, and the total exposure in lux-seconds. The actual illuminance is the actual physical stimulus that the eye or film reponds to (although for film it's not such a great measure, since the spectral sensitivity of film is not very close to the luminosity function as it is in the eye; nevertheless, it is used). Dicklyon 21:51, 17 May 2006 (UTC)
• We shouldn't include a plot of luminous efficiency, because the curve is explained at luminosity function. The article links there. Sorry about the "perceived intensity". That was probably me. I was trying to distinguish it from direct physical measurement of power, not to imply that it was a measure of retinal illumination. Photometric terms are hard to explain.--Srleffler 04:45, 17 May 2006 (UTC)
• Oh, don't make it so complicated, I just want to know the answer to a simple question: which is brighter, one watt or 10,000 candlepower? :-) Dpbsmith (talk) 13:51, 17 May 2006 (UTC)
• Yes, let's get rid of all the silly Nit-picking. (If you think I'm going to sign that, you're crazy...)
• That's easy. 1 watt can't make more than 683 candlepower (modern unit, candela), best case. So 10,000 candlepower is also brighter than 1 watt. Usually MUCH brighter. Now if you had asked 1 watt or 100 candlepower, the answer would require a lot more waffling. Dicklyon 05:56, 21 May 2006 (UTC)
• ???? One watt can't make more than 683 lumens. Can't it make as many candlepower as it likes (I deliberately used "candlepower" because that is the unit that is always used, usually without the word "beam," commercially in the U.S., e.g. [1], [2], etc. If you put it in a parabolic reflector and scrunch it down to 0.0683 steradians, you get 10,000 candlepower, don't you? I'd have thought a one-watt laser could easily exceed 10,000 candlepower. Dpbsmith (talk) 10:34, 21 May 2006 (UTC)
• Oops, you're right. Your intentionally-silly question didn't admit to the answer I thought it did. Thanks for the clarification, or intensification. Dicklyon 02:38, 22 May 2006 (UTC)
• Oh, good. I'm never completely sure about anything when it comes to this stuff. For me, this is all half-remembered stuff that I one three-quarters-understood about half of. I always need to go to some reference source and do the mental equivalent of "sounding it out." But at least I understand that there are reasons why there need to be so many different units, that that when it comes to measuring light, the usual answer to "how can I convert X to Y?" is "you can't."
• Take a look at User:dpbsmith/photometry... Dpbsmith (talk) 09:55, 22 May 2006 (UTC)
• The article says "A flux of 1000 lumens, concentrated into an area of 1 square metre, lights up that square metre with an illuminance of 1000 lux. However, the same 1000 lumens, spread out over 10 square metres, produces a dimmer illuminance of only 100 lux." ----- which doesn't appear to convey details clearly. Whoever wrote this first needs to explain to readers how that flux is distributed. For example, a laser beam focusing on a point right in the middle of a 1 square metre surface region. Is that supposed to be different than light that comes from a light bulb illuminating a 1 square metre surface region? This just means it's necessary to show a diagram of the distribution of the flux.KorgBoy (talk) 12:21, 26 April 2020 (UTC)
• The flux is presumed to be distributed uniformly in the specified area. A laser beam focused on a point is not "concentrated into an area of 1 square metre", and certainly is different from a light bulb illuminating a one square metre surface.--Srleffler (talk) 18:26, 26 April 2020 (UTC)
• Srleffler - in that case, the article should state that condition regarding uniform distribution. Missing vital details such as that appears to be the life story of a lot of information sources. KorgBoy (talk) 07:57, 8 May 2020 (UTC)

## Biological clock

This seems wrong to me. I'd like to see a citation. In Czeisler's work, 10,000 lux is used to reset the clock. 1,000 lux is typical of ordinary workplace illumination and it is not sufficient to synchronize shiftworkers to their schedules, as witness Bhopal, the Exxon Valdez, and numerous other accidents that happened in the wee hours... Dpbsmith (talk) 12:46, 16 May 2006 (UTC)

I just removed the biological clock material from the table. An anon editor correctly noted that the cited article claims small adjustments of the biological clock at illuminances as low as 180 lux. That's fine, but it renders this issue not interesting or important enough to include in the table. Rather than a sharp threshold for resetting the clock, the reference shows a gradual increase in clock-resetting from very low levels of light to very high levels. One would presume that this variation is continuous, so there is no particular value that is distinct enough to be worth listing here. The clock-resetting level was only really interesting when it was higher than normal room light anyway. Light makes you feel more alert. Film at 11. (yawn).--Srleffler 23:58, 19 September 2006 (UTC)

## Picking Nits... just call me an apostilb, I mean apostate...

I don't believe the section "Relationship between illuminance and power" belongs here at all. It should go in the article on lumen. Or, it should go in an article about luminous efficacy or the luminosity function and all of the articles on matching photometric/radiometric pairs should link to it.

The current text:

Illuminance is not a direct measure of the energy of light, but rather of the illumination it provides as perceived by the human eye. Thus, the conversion factor varies with the wavelength composition or color temperature of light. At a monochromatic light frequency of 540 THz (approx. 555 nm wavelength; the middle of the visible spectrum) the power needed to make one lux is minimum, at 1.464 mW/m²; that is, the peak of the luminosity function is 683 lumens per watt, falling to zero in the infrared and ultraviolet wavelengths. Typical light sources have a luminous efficacy much less than this theoretical maximum (luminous efficiency much less than 1). For example, a typical incandescent light bulb has a luminous efficiency of only about 2%.

mixes up illuminance, energy, and power. All of the very confusing photometric (and radiometric) units have different dimensions and different factors that affect them. We really shouldn't be trying to compare thingies that differ by more than one doohickey at a time.

This section is really about the relationship between photometric and radiometric units. It applies to power (watts) versus luminous flux (lumens). It applies to other corresponding pairs of photometric and radiometric units.

It doesn't apply to energy versus illuminance.

Illuminance is not a direct measure of the energy of light, for many reasons other than the luminosity function. For example, a strobe light may put out the same amount of energy in 1/1000th of a second that a floodlight puts out in ten seconds... but for that thousandth of a second its illuminance will be ten thousand times higher. Similarly, a bare bulb and a bulb with a parabolic reflector around it may put out the same amount of luminous flux, but the parabolic reflector results in a much higher illuminance over a much smaller area. Dpbsmith (talk) 19:02, 17 May 2006 (UTC)

I agree; make sure there's a good explanation in the appropriate article, and replace this section by a brief statement about photometry being different from radiometry, and a link. Dicklyon 21:43, 17 May 2006 (UTC)
I agree that section isn't very good, but I think it should be replaced rather than removed. Because of the confusion between photometric and radiometric quantities, every article on photometric quantities and units should contain a paragraph explaining the difference between the two (either a general paragraph, or one comparing that specific quantity/unit and the corresponding radiometric quantity/unit). The articles on luminous efficacy and the luminosity function are (and should be) more technical articles for readers interested in a greater level of detail. The articles on the quantities and units should give a naive reader a reasonably clear qualitative picture of the difference between the photometric and radiometric quantities, without recourse to the more technical articles. --Srleffler 22:10, 17 May 2006 (UTC)
I agree. I'm not sure I exactly know how to do it... Dpbsmith (talk) 22:24, 17 May 2006 (UTC)
I'm not too pleased with the new long-winded version. The long discussion of the relationship between lumens and watts might be OK in the lumen article, but not here. Then we could reference that with a short paragraph. Also, I believe you've inverted the meaning of luminous efficacy and luminous efficiency, if the article on those is correct. Dicklyon 05:57, 18 May 2006 (UTC)

## Another nit: 540 THz versus 555 nm

I admit I was confused and some of my edits may need to be corrected. There's a difference between the standard frequency at which the 683 lumen/watt is defined (540 THz), and the peak of the luminosity function as adopted by the CIE, which is 683.002 lumen/watt at 555 nm. Dicklyon 21:08, 18 May 2006 (UTC)

## Starlight example

Currently, the article states starlight causes an illunminance of 0.00005 lux. In my opinion this greatly depends on which star you look at and from where you do it. --Abdull 19:42, 19 May 2006 (UTC)

No, the illuminance provided by starlight on earth has no influence from where anybody looks; it's a physical quantity that you can get by putting a lux meter on the ground on a clear moonless night, away from a city. There may be some other dependencies, like some parts of the sky being brighter than others, but that's a pretty small effect. Dicklyon 21:03, 19 May 2006 (UTC)
"Which star you look at" suggests a misunderstanding of what "illuminance" is. I'm going to try to keep this simple which (means I'm going to introduce some inaccuracies of my own) but... here, in this context, "starlight" means how brightly the ground is lit up by all of the stars in the sky shining on it at the same time. It is about light falling on the ground; it is something you can assess very roughly by looking at the ground, not at the sky.
(Of course, now I'm wondering how much difference there is in how many bright stars are contained in various celestial hemispheres. I'm quite prepared to believe that starlight when viewed from a point on the earth on the galactic equator might be a little brighter than starlight viewed from a point under the galactic pole...)
To take another example: in "A Visit from Saint Nicholas" ("'Twas the Night Before Christmas"), Clement Moore writes:
The moon on the breast of the new-fallen snow
Gave the lustre of mid-day to objects below.
When we say that "moonlight" is 1 lux, we are not talking about how bright the moon appears when we look at the moon. We are talking about the moon as a light source for illuminating other things (hence "illuminance.") That is, how well it lights up "the objects below." Dpbsmith (talk) 19:02, 20 September 2006 (UTC)
I would disagree with your assertion that using lux to describe a particular star is a misunderstanding. We have been discussing this further down. I maintain that lux is exactly the right unit to describe a light source like a star. It's equivalent to the illumination that that star casts on a surface perpendicular to rays coming from it.
Of course, that's not what the sentence about starlight was referring to. I do think that the amount of starlight on a horizontal surface is probably a lot greater when the Milky Way arches over one's head than when it is spread around the horizon. But I don't know the numbers.
Eric Kvaalen (talk) 17:08, 27 December 2013 (UTC)

### Starlight versus overcast moonless sky

IS there a source to that table? How can starlight be less light than the overcast sky without a moon (assuming you are far from a city)? Where does that extra light come from? -- Cuardin (talk) 06:49, 10 September 2008 (UTC)

I agree the table appears inconsistant and lacks sources. I found a reference and modified the table accordingly. Han-Kwang (t) 08:30, 10 September 2008 (UTC)

The current definition doesn't make sense to me: 1 lx = 1 lm/m2 = 1 cd·m2·m–4. Isn't this the same as saying: 1 lm·m–2 = 1 cd·m2·m–4, and therefore 1 lm·m–2 = 1 cd·m–2, leading to 1 lm = 1 cd , which is wrong.

But since the definition has survived for so long in this article, I suspect that I misinterpret it. How can something that looks so simple also look so wrong, and still remain uncorrected? In a similar fashion, the definition for Lumen (unit) is fishy. --HelgeStenstrom 08:43, 24 April 2007 (UTC)

This stuff always confuses me, too. I think the issue is that in the definition of lumen, there's a steradian that got dropped becuase it's 'non-dimensional'. Something like that applies to the lux, too. I usually end up back in books or tutorials that try to explain it better. I see what I can figure out the help resolve this... Dicklyon 14:48, 24 April 2007 (UTC)
I feel confused too. I quite suggest that the definition should be changed to: 1 lx = 1 lm/m2 = 1 cd·sr/m2. Yufeng.Zhang 13:51, 8 August 2007 (UTC)
This just illustrates the pitfalls of trying to do "math" on units. The results don't always mean what you think they do. You have to think about the physics, not just the math. In your derivation, you dropped a factor of 1 steradian, since 1 sr = m2·m-2 = 1. You can drop the steradian for purposes of dimensional analysis, but you have to keep it if you want to deal with units rather than dimensions. 1 lm ≠ 1 cd, even though they have the same dimensions, because these are units of different physical quantities. You can't equate units of distinct physical quantities, even if they happen to have the same dimensions. The correct interpretation of the correspondence between 1 lm and 1 cd derived above is the one stated in the article: "If a light source emits one candela of luminous intensity into a solid angle of one steradian, the total luminous flux emitted into that solid angle is one lumen." --Srleffler 00:15, 10 August 2007 (UTC)

## convert footcandles and lux to candelas

"Candelas are equal to the square of the distance multiplied by the number of footcandles. For example, if your meter is ten feet away from the light source and your meter reading is 10 footcandles, the equivalent candelas equals 10 feet squared (e.g. 100) times 10 -- which equals 1,000 candelas.

The conversion of footcandles and lux to candelas is most accurately achieved in the context of a single point source of light measured in the dark. If the light source is diffused, you should take several readings at different angles in order to calculate an "average" candela measurement." [3] -69.87.200.181 16:22, 24 May 2007 (UTC)

"The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away." As natural as this sounds, this style of name is now frowned upon, because the dimensional formula for the unit is not foot · candela, but lumen/sq ft. Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one meter away). A source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one footcandle. Since illuminance follows the inverse-square law, and since one foot = 0.3048 m, one lux = 0.3048^2 footcandle ≈ 1/10.764 footcandle."

So, lux can be converted directly to candela, without computing footcandles. Just multiply the lux reading by the square of the distance, measured in metres. -69.87.200.181 16:38, 24 May 2007 (UTC)

## Lux to take into account observer distance?

Lux takes into account the area of the light source, and its intensity. The observer's distance or own size (that the light shines upon) is not a consideration. With this in mind, why is "Candle at a distance of 30 cm (1 ft)" stat specifying a "distance of 30 cm"? Surely the lux would remain constant no matter how far away it is? 50,000cm or 50cm, does that same candle remain at 10 lux? At the moment, it sounds more like a measure of nits. --Skytopia 15:38, 4 October 2007 (UTC)

Not quite. There are two distinct things that are measured in lux: illuminance (light falling on a surface) and luminous emittance (light emitted from a source). You're thinking of luminous emittance, which is the luminous flux per unit area on the source. Illuminance is the flux per unit area at the location of the observer. The luminous emittance of a source does not depend on the observer, but the illuminance received by an observer decreases with distance. Two different things, but they both use the same unit.
The nit is a measure of luminance—an entirely different physical quantity.--Srleffler 17:40, 4 October 2007 (UTC)
Fair enough. Yes, the fact that Lux is used in two slightly different ways was confusing at first. The "SI photometry units" section also shows two 'types' of lux to support what you say. However, given all of this, would it not be a good idea to further highlight this on the page about the two different types. In particular, it would be a good idea to mention that the benchmarks given in the table are representing illuminance rather than luminous emittance. For example "Moonless clear night sky" actually refers to the density of light (i.e. luminous emittance) received by Earth. It would seem the current definition on there now could be ambiguous to the reader. --Skytopia 06:30, 10 October 2007 (UTC)
The column in the table is clearly labelled "illuminance". You're right though that the article should call attention to the two distinct uses of this unit.--Srleffler 10:54, 10 October 2007 (UTC)

## When invented?/history

When was Lux unit used from? --maxrspct ping me 15:36, 5 November 2007 (UTC)

## Lux measurement in Logitech's Solar Keyboard

If needed, I can provide an image of LUX being measured in the Logitech Solar App. As seen here. 71.90.29.110 (talk) 17:32, 8 March 2011 (UTC)

## Horrible confusion from the table of examples

The table of examples primarily cites http://stjarnhimlen.se/comp/radfaq.html#10, a table of things like "how bright is the sun". This is all good, since those are all actual light sources. But the table in our article lists things like "Family living room", which is confusing, since my living room as a whole does not glow. The brightness of a room is probably best measured in luminance, and therefore cd/m2, not Lux.

Not only should the table be fixed, text indicating this distinction should be inserted to avoid confusing the issue for readers (or editors!) in the future. — Preceding unsigned comment added by Mattdm (talkcontribs) 17:40, 24 March 2011 (UTC)

You have it almost completely backward. Illuminance (lux) is the appropriate measure for how brightly a room is illuminated by its light sources. If you want to think about how bright a distant object appears, you want luminance (cd/m2), not illuminance. You use illuminance if, instead, you want to know how well that distant bright object illuminates the Earth. So, if you want an answer to "How bright is the Sun?" you use cd/m2. If you want to compare how well you can see in a room lit by sunlight vs one lit by a lightbulb, you use lux.--Srleffler (talk) 03:58, 25 March 2011 (UTC)
Well, I'm not so sure he has it backwards. Lux is a measure of how much light falls on a surface in the living room, and of course it varies from place to place in the room. Nits (candela per square metre) is what a surface looks like when you look at it – its brightness. (It doesn't have to be a distant object.) The number of nits that you see looking at a certain spot on the wall doesn't depend only on the number of lux falling there – it also depends on the paint, the glossiness, the angle you look from, and the angles from which light is falling on that spot. In other words, the nits are what you see. The lux is not. Eric Kvaalen (talk) 17:08, 27 December 2013 (UTC)
Ironically, your last sentence is exactly the point you have been arguing against, below. I pretty much agree with you here: lux is a measure of illuminance—that is, how well a source illuminates surfaces. Nits are a measure of luminance, which is how bright a source (or an illuminated surface) looks to an observer.--Srleffler (talk) 07:08, 30 December 2013 (UTC)

Yes, I realized that I was arguing a point here that may seem to be contrary to what I have been arguing lower down. But here I'm talking about when you look at a surface. Lower down I'm talking about looking at a source of light, where what matters (most) is the total amount of light it gives you (per square metre of course), not the amount per unit of solid angle. Eric Kvaalen (talk) 12:56, 8 January 2014 (UTC)

## Newly added paragraph on stellar magnitudes

The newly re-added statement "Lux can be used as a measure of the brightness of a light source, for instance of a star or a distant light bulb." is misleading. Lux is not a useful measure of brightness of a source. The illuminance of a star measured at Earth (in magnitude or lux) tells you nothing at all about how bright the star itself is, because obviously the illuminance at Earth depends on how far away the star is. Similarly, the illuminance received from a distant light bulb is not a good measure of the brightness of that bulb.

I will most likely delete or move the new paragraph. This article is about lux, and the "Explanation" section should focus on explaining what lux is. The table of examples in that section is there to give the reader an intuitive idea of how much a lux is, by comparison to sources and conditions that may be familiar. The discussion of lux values corresponding to various magnitude stars is off-topic, and out of place in this section. It might be possible to rework the material and include it in the "Non-SI units of illuminance" section, relating magnitude as another unit of illuminance.--Srleffler (talk) 04:35, 19 May 2012 (UTC)

Moved and reworked, as proposed above.--Srleffler (talk) 03:25, 1 October 2012 (UTC)
Thank you for not deleting it! (I'm the one who wrote it, and I'm glad to see that it's still there now when I wanted to remember the figures.) I do think the original placement was appropriate. What I meant was that the lux is the proper SI unit for designating how intense (or whatever word you want to use) a light source appears to the person perceiving it! You don't like the word "brightness", but I think it's the right word. Eric Kvaalen (talk) 12:00, 8 November 2013 (UTC)
No, it is not the correct SI unit for that. If you want to know how bright a source appears to an observer, you should be using luminance, which is measured in candela per square metre.
The trouble with "brightness" is that it is hopelessly ambiguous. If you want to know how "bright" something is, you might want to know about luminous flux, or luminous intensity or illuminance or luminance. These are all completely different things. What you mean by "brightness" varies depending on the context.--Srleffler (talk) 06:48, 9 November 2013 (UTC)
Yes it is the correct SI unit for what I'm talking about. Luminance is how bright a patch looks – it's the number of candelas per square metre. For a star that would be a very large number (like 109 cd/m2), and would depend on what kind of star it is, but it would not depend on how far away the star is. What I want is that integrated over the (tiny) solid angle of the sky covered by the star. That comes out to lumens per square metre, or lux. I am not talking about luminous flux or luminous intensity, which are also huge numbers for a star and do not depend on the star's distance from us. As you say, when I want to know how "bright" something is, I might want to know about illuminance. Well, illuminance is measured in lux. The brightness of a star in the sky is exactly the same as the amount of illuminance that it provides on a surface here on the earth oriented perpendicular to the rays coming from that star.
As for whether "brightness" is the right word for this, if you ask someone to compare two stars, he will say "That one is brighter than that one". That is the term in English for what we are talking about. Eric Kvaalen (talk) 14:24, 12 November 2013 (UTC)
What you are saying works for stars, but not much else (I was mistaken on this point, above.). If you asked an observer to compare the brightness of the moon to that of a streetlight, she would be comparing the luminance of the two sources, not the illuminance. Stars are an odd special case, because they subtend such a small angle that they cannot be resolved. Even ignoring atmospheric effects, imperfections in the eye, and any resolution limit in the retina, the apparent angular size of a star would be limited by diffraction. The result is that all stars appear to subtend the same angle, regardless of their size or distance. In this special case, it doesn't matter whether you consider the eye to perceive luminance or illuminance. As perceived by the finite eye, they are proportional to one another for a star. --Srleffler (talk) 04:17, 13 November 2013 (UTC)

Whether the light lands on one rod or cone in the retina or on several, it's still the case that we distinguish distant lights by the total amount of light that they cast into our eye. That is what we call the brightness of the light. And that is measured in lux.
It's not just for stars. Any planet, satellite, moon, lamp, candle, fire (distant or not) as perceived at a certain distance can be described by a number of lux at the location of the observer. This is equal to the number of candelas that the light source is sending in the direction of the observer divided by the distance squared. Eric Kvaalen (talk) 07:31, 13 November 2013 (UTC)
You're just mistaken here. As long as it is large enough to be resolved, the brightness of an object as perceived by a human observer is proportional to its luminance, not the illuminance at the observer's eye. If you double the size of a distant object but keep the total luminous flux it emits the same, the illuminance at your eye does not change, but the apparent brightness of the object's surface drops by a factor of four.--Srleffler (talk) 03:56, 14 November 2013 (UTC)

But I'm not talking about the luminance. I'm saying that the perceived strength of a light is equivalent to the illuminance that it casts at our location. If the light appears as a point, then we certainly call this the brightness. Even if it is not a point, we can still use the word brightness to mean what I'm talking about. "Brigtness" is not a well-defined term like "luminance" or "illuminance".
If someone standing 100 yards from you shines two flashlights at you, wouldn't you say that it's brighter than if he shines one flashlight at you? I would.
But the point I want to make (in the article) is not about the meaning of the English word "brightness". It's that the "perceived strength" (or whatever term you want to use) of a light source, perceived at a certain location, is measured in lux. For instance, a magnitude 0 star is 2.08 microlux. The unobscured sun is 93 000 lux. Eric Kvaalen (talk) 21:22, 15 November 2013 (UTC)
This is just the problem: "brightness" and "perceived strength" are ambiguous terms, whose meaning depends on context. If someone standing 100 yards away points two flashlights at you, assuming you can resolve them at that distance each flashlight appears just as bright as a single one would; there are just two of them. The two flashlights do double the illumination at your location, but illumination is not the same thing as the perceived brightness of a distant object. That's why we have two different physical quantities and two different units for these two different things. It's confusing only because people use terms like "brightness" and "perceived strength" for both. --Srleffler (talk) 07:38, 16 November 2013 (UTC)

So let's come up with a wording that we can agree on. I say that if someone points two flashlights at me, whether I can resolve them or not, it's twice as bright as if he points one at me. The point is, to measure what he's pointing at me, the unit is lux. In SI. Eric Kvaalen (talk) 10:50, 16 November 2013 (UTC)
I like your flashlight example. I think it helps clarify the issues.
For sake of clarity, imagine this situation: two people about 100 yards away each point a flashlight at you. They are far enough apart that you can see that there are two flashlights. There are several different questions you can ask about the amount of light in this situation. Some of them are:
1. What is the total amount of perceivable light being produced?
2. What is the "density" of that light at my location?
3. How bright do the sources appear to me?
You clearly are interested in the answer to question #2. The problem is that you keep trying to describe it as if it were question #3. You want to know how much light those flashlights "cast" at your location—how well they illuminate you. This is illuminance. This is a fundamentally different thing from how bright those flashlights appear to be when you look at them. If there are ten people pointing flashlights at you they illuminate you ten times brighter, but if you look at them each flashlight of course appears just as bright as if there were only one.
As I said before, stars are an odd special case, where the apparent brightness of the source is proportional to the illuminance because the source is too small to be resolved.--Srleffler (talk) 16:27, 16 November 2013 (UTC)

Well, I think the word "bright" is ambiguous when you can see the light as a patch rather than a point. When it's a point though, I think "brightness" is the right word for the number of lux that it casts.
Anyway, here is a compromise, to be placed in the first section, or even in the intro:
"The lux unit can also be used to measure the strength of a light source as perceived at a particular location. This is equal to the number of lumens that the light source casts per square metre on a surface oriented perpendicular to the rays coming from the source. For instance, a standard candle at a distance of a kilometre would be 1 microlux. A star of apparent magnitude 0 is 2.54 microlux outside the earth's atmosphere, and 82% of that (2.08 microlux) under clear skies.[stellar_magnitudes 1] A magnitude 6 star (just barely visible) would be 8.3 nanolux. The unobscured sun is about 93 000 lux."
How's that? Eric Kvaalen (talk) 18:17, 18 November 2013 (UTC)
I agree with your first paragraph here: without context, the term "bright" is ambiguous for extended sources. For point sources like stars, illuminance is as good as any measure of brightness. The first sentence of your proposal, however, is misleading as written. Readers should not be left with the impression that illuminance is a good measure of the apparent brightness of an arbitrary light source—it is not.
Does anyone actually use lux to measure brightness of stars? Yes, stellar magnitude is equivalent to an illuminance in lux, but if lux are not actually commonly used there is little value in dwelling on it here, beyond what is already included in the article. If lux are actually used in this way, I think it might be better to have a separate section on this, rather than trying to fit it into the lede or the first section. Otherwise, the discussion of stellar brightness in lux and magnitude is just providing a unit conversion between an SI and a non-SI unit for the same quantity. The section on non-SI unit conversions would then be the best place for it.--Srleffler (talk) 05:21, 19 November 2013 (UTC)

You say "Readers should not be left with the impression that illuminance is a good measure of the apparent brightness of an arbitrary light source—it is not." But I didn't use the word "brightness"! I specifically avoided it because you object to it.
What I object to in the present article is that the idea that lux is a measure of the strength of a light source is relegated to a section called "Non-SI units of illuminance".
I think it's interesting and worthy of mention that the smallest light we can see is 8 nanolux. That's a fact. I happened to have found that out by looking at a reference that had to do with astronomy, but it's not an "astronomical" fact and it is not specific to stars. And as I say, any light source is measured in lux. Not just stars.
Eric Kvaalen (talk) 07:28, 22 November 2013 (UTC)
Lux is a measure of how strongly a surface is illuminated, not a measure of the strength of the source as perceived from a given location. These are in general two different things, and they are measured using different units. It doesn't matter whether you call it "brightness" or "strength...as percieved at..." The problem is the concept, not the words.
I think we're at the point where a reference would be needed to resolve this. If you want to include a general assertion like what you have proposed, you're going to need to be able to cite a source that makes such a general statement. Finding sources on the topic may help us figure out how to present it, too.
You picked an interesting "fact". If 8 nlx were, in general, the smallest light we can see that would certainly be worth mentioning in the article. It's not a fact, though. You read that a magnitude 6 star, which is just barely visible, provides 8 nlx of illumination, and you presumed that therefore 8 nlx is the limit of human vision. A magnitude 6 star is just barely visible because even under the best conditions the sky is not completely dark and your night vision is affected by the light from the rest of the sky (and the other stars).[4] The actual limit for being able to detect illumination from a faint distant point source is around 1.3–1.4 nlx. My point here is not that your number is wrong, but rather that context matters. You interpreted the number as an absolute limit, when in fact it is specific to one special case (and yes, it was an astronomical fact and very specific only to stars, and even then only under typical outdoor viewing conditions.)--Srleffler (talk) 06:15, 23 November 2013 (UTC)
But I disagree with your statement that the number of lux is "not a measure of the strength of the source as perceived from a given location" and that "these are in general two different things, and they are measured using different units". What units do you think are used to measure the strength of a source as perceived from a given location? Eric Kvaalen (talk) 16:34, 11 December 2013 (UTC)
We've been over this. The "strength" of a source as perceived from a given location is generally measured in candela per square metre. Part of the problem here is that "strength" is just as ambiguous as "brightness" is. I'll concede that by "strength" one might, in general, mean "how well does the source illuminate a surface" rather than "how bright does the source look to me". The qualifier "as perceived from" pushes me to assume the latter meaning of "strength", however.--Srleffler (talk) 03:01, 12 December 2013 (UTC)

I still say that "how bright it looks" may be taken to mean the number of lux, rather than the number of nits (candela per square metre).
The number of candela per square metre does not depend on the location of the observer, at least not on his distance from the source. (It may depend on his angular location since the source may not radiate the same in all directions.) So the number of nits does NOT depend on distance, whereas the number of lux does.
Eric Kvaalen (talk) 17:08, 27 December 2013 (UTC)
That's just the right dependence on distance, though. Look at a light bulb. Stand 1 metre from it and compare how bright it looks compared with standing 4 metres from it. The bulb appears to be the same brightness. It certainly does not appear to be 16 times less bright when you are four times further away. Your visual perception of brightness is not, in general, proportional to the illuminance at your eye.--Srleffler (talk) 07:32, 30 December 2013 (UTC)

I disagree. By the way, I ask'd my wife (a sample size of one) whether two (or ten) flashlights a hundred yards away are brighter than one, and she said yes. And especially this is true if the light source just looks like a point. Eric Kvaalen (talk) 12:56, 8 January 2014 (UTC)
I still would like an answer to the question I asked above: Does anybody actually use lux in the way you have proposed? Do we have a source that supports that? If not, then there is little point in dwelling on it in the article. Astronomers use magnitude for this. The article explains how to convert between lux and magnitude in the appropriate section.
Motivated by this discussion, I did add a section on the astronomical usage to Illuminance. I even used the word "brightness", for want of a better term.--Srleffler (talk) 04:51, 12 December 2013 (UTC)

I would still like an answer to my question! If it's not lux that should be used to measure the quantity or quality that I've been talking about, then what should be used? Certainly not candela per square metre. I'll admit that I haven't seen any place that says what I am saying. It's a case of a fact that can be arrived at by clear thinking but which cannot (according to you and many others) be put into an article because we don't have a reference! Do you concede that lux is the only proper unit for what I've been talking about? (Besides foot-candles!) Eric Kvaalen (talk) 17:08, 27 December 2013 (UTC)

I think I have already answered your question: candela per square metre are the correct SI units for describing how bright a surface appears to an observer. (This applies regardless whether the surface is a "source" or is illuminated by something else.) Surfaces that are too small or too far away to be resolved are an exception to the general rule. In the latter special case, illuminance is fine.--Srleffler (talk) 07:32, 30 December 2013 (UTC)

So how's this, to be added after the first paragraph of the section "Explanation":

"The illuminance provided by a light source (on a surface perpendicular to the direction to the source) at a given location is a measure of the strength of that source as perceived from that location. For instance, a star of apparent magnitude 0 provides 2.08 microlux at the earth's surface.[stellar_magnitudes 1] The dimmest point-source that can be perceived is around 8 nanolux (corresponding to a magnitude 6 star), but this depends greatly on the amount of background light and on how well the eye has adapted to the dark.[5] The unobscured sun perceived from the earth's surface is (or provides on a perpendicular surface) about 93 000 lux.

The lux is also a measure of luminous emittance, the number of lumens given off by a surface per square metre, regardless of how that light is distributed in terms of the directions in which it is emitted. It differs from the luminance (measured in candela per square metre or "nits"), which does depend on the angular distribution of the emission. A perfectly white surface with one lux falling on it will emit one lux.

Eric Kvaalen (talk) 12:56, 8 January 2014 (UTC)
I still don't like that first sentence, but I don't seem to have a strong enough argument to dissuade you, so let's use it. I think this material would be better placed after the table in the Explanation section. The article should give the definition of the lux early, and the illumination examples in the table follow on from the first paragraph of the explanation section. Your paragraph is introducing a different concept of the "meaning" of the unit.
The statement that the dimmest point-source that can be perceived is around 8 nanolux is incorrect, and is outright contradicted by the source you cite. Qualifying it with "this depends greatly on the amount of background light..." is not sufficient. The statement is flat-out wrong, arising from your misinterpretation of the source(s) you are looking at. The dimmest star one can see is brighter than the dimmest point source one can see overall because the light from the other stars in the sky affects your night vision, even if there are no other sources of light present. The source you cite is explicit that under the best conditions most observers can see stars dimmer than 7th magnitude, but this would still not be the real limit for the dimmest source one can see in general, again because of the light from the other stars in the sky. If you want to say something general about the dimmest source people can see, you probably need a reference that addresses that question directly. --Srleffler (talk) 06:40, 9 January 2014 (UTC)

## Calculating lux on a work surface

Hi! I'm thinking about adding a new section about how to estimate (for household purposes, back-of-the-envelope way, ignoring trigonometry) the correct illumination of work and living places. For example, if I want around 200 lux for a bathroom of 10m2, and I'm going to light it with a single light bulb on the ceiling, how powerful must it be? (around 2000 lumen, which suggest using more than one light source). For this I was going to (very briefly) explain such as this:

 The recommendations for adequate lighting in a given space are given in lux (for example, 500lux for an office). This is a measure of how much light (illumination) there is on a given surface. Light sources such as lightbulbs report their power either as watts or lumens. When lumens are not indicated, you can estimate 1W ~ 15 lm (for incandescent bulbs) or 1W ~ 60 lm for compact fluorescent lamps, and 1W ~ 80lm for LED lamps. Keep in mind those are just approximations, and lights should clearly mark their light output in lumens. Lumen, which lightbulbs are measured in, is candela (luminous intensity) per steradian, a measure of solid angle. So, in layman terms, it measures how much light the bulb emits over a certain portion of space. The steradian has a useful quality: at a distance "A" from the center, the surface it covers is A2. Thus when you are at a certain distance (say, 2m) from a light source, the given light (measured in lumens) is spread over (2m)2, that is, 4m2. Thus, to estimate the lux on your work surface, you can add, for each light source in the room, ${\displaystyle Lumens/Distance^{2}}$. Example: Your work table is below a 1200lm light in the ceiling, 2m above your head. There is another light of the same power in the ceiling, 2m to your left (thus, ${\displaystyle {\sqrt {8}}m}$ from you). The first light is giving you ${\displaystyle 1200/(2m)^{2}}$, 300 lux and second one gives ${\displaystyle 1200/{\sqrt {8}}^{2}}$, 150lux, for a total of 450lux. Keep in mind that all of these are simple, back-of-the-envelope estimations for household use, and that for proper calculations on workplace settings you should contact a professional.

I think this article is the proper place to put this info in (I arrived here through lumen, and made the math myself after I failed to find other sources). If you disagree, could you please suggest an appropriate article? Scarbrow (talk) 07:31, 19 May 2014 (UTC)

That might not be appropriate material for Wikipedia at all. By policy, Wikipedia is not a how-to guide. A worked example of how illumination is calculated is fine, but the article already has that. Providing step by step advice on how to determine lighting for a work area is out of our scope.--Srleffler (talk) 16:55, 19 May 2014 (UTC)
I agree with you. --Matthiaspaul (talk) 21:46, 19 May 2014 (UTC)
Thank you for your comments, I won't add to the article then. Anyway, at least I got that written down. Some how-tos I found on talk pages on the past were most useful Scarbrow (talk) 11:10, 20 May 2014 (UTC)

## New images

Luminous energy and power
Luminous exitance
Illuminance
Luminous intensity
Luminance

The new images that were added today could use some discussion. First, the text in these images has horrible spelling errors, which must be fixed. Second, the solid angles should have their vertices at the source, not out in space behind it.

Before fixing them, though, it would be worth having a discussion of whether the concept works. Do these images help a reader understand the concepts? The images may need to be used together, for the shower analogy to be clear. I'm not sure that presenting only one or two of the images by themselves works. Perhaps these would work better in the generic photometry article than they do in specific unit and quantity of measure articles.

What do other editors think?--Srleffler (talk) 18:07, 29 November 2015 (UTC)

The distinction between all these notions is tricky, and using this kind of analogy to explain it seems like a good idea to me. However, as they are, these pictures leave much to be desired. Just looking at the first one:
• lumens have nothing to do with psychology, but with the spectral sensitivity of the eye
• the lumen is not the photometric equivalent of radiant flux, the luminous power is
• the line “unit: lumen, lm” looks like lumen and lm are two different units, and one needs to choose either
• only in programming languages the asterisk (*) is used for multiplication
• it's not “Lumen*sec” but either “lumen-second” (unit name) or “lm⋅s” (unit symbol)
Looking at the other pictures I see very much the same issues. The last one (luminance) is especially bad, to the point I doubt it provides any help in understanding the concept. On the other hand, explaining the concept of luminance with a simple picture is quite a challenge.
In my opinion, these pictures should be removed from the articles. If someone manages to rework them into something both correct and useful, then Photometry (optics)#Photometric quantities would be a good place to put them.
— Edgar.bonet (talk) 10:18, 30 November 2015 (UTC)
I suggest removing these images entirely. The comparison to water flow is unsuitable for an encyclopaedia; why not explain the process for light itself? Sure, it may be harder for an untrained reader to understand, but I'd say that's too bad; articles about physics will always be inherently difficult for the average person to read. Even if one does accept the comparison to water, these messy hand sketches look very unprofessional. Aren't we trying to create a world-class encyclopaedia here? Not a pop science blog. (143.119.162.27 (talk) 00:11, 18 October 2019 (UTC))

### Suggest new images need (major) rework

While I can appreciate what the user Yonoodle tried to achieve, I have to suggest that these images be completely redone. In their current form they are confusing, and ugly to look at. Also, I agree with the technical problems highlighted by user Edgar Bonet above. FMalan (talk) 15:06, 30 November 2015 (UTC)

### reply to suggestions by yonoodle

• Thanks for the responses, I'm very willing to remake the images, but to be efficient ,i might need to make sure what exactly i should remake, hope it will be more correct and useful to people. and i must apologize for my english for it's not my native.
• I'm not quite sure how should i move the vertices of solid angle to the source point , it will make the measure of luminous intensity and luminance varies with different measuring distance(different) ,due to the shower nozzle(light source) is not a point source ,should the luminous intensity be considered as the integral of all differential plane element? if it is , the vertex of solid angle in this illustration will automatically matches the source if a small area (a point source) is considered.
• the term "psychological" might be a misunderstanding, i thought the eye responding curve is measured by an experimental psychology experiment, (flashing the different spectral color and decide when participants unable to distinguish the blinking) , is there more correct way of term to simply deliver the idea? or should i just remove the term?
• other issue will be fixed as your suggustions, thanks!
• about where to put these images or if it helps, i thought it will help since a person major in physics as i faced a little difficulty understanding these concepts already(without illustration), it might be harder for people in other fields, but it really need more opinions where to put it, my opinion is some place to let more people can know this aid ,maybe a link to an other page as Srleffler mentioned?

Yonoodle (talk) 21:44, 2 December 2015 (UTC)

Hi Yonoodle. You are wise to gather responses and get a clear picture before beginning to revise the images. Technical drawing is hard: it's always difficult to capture everything that needs to be captured to present a technical concept in a way that is both completely correct and able to aid the reader's understanding.
All of the photometric quantities are integrated over differential elements of the source. Obviously, one often considers simple cases such as a point source, or a source where every element contributes equally. Since your shower head has five little nozzles, you could show five identical cones, with vertices on each of the nozzles. That would suggest that it is the angular spread that matters, not the position. (But perhaps someone else has a better suggestion.)
You are correct about how the luminosity function was obtained, but I wouldn't describe the phenomenon as "psychological". I'm sure there is a better way to describe it, but I'm not sure what that is right now. You could just say something like "Water here represents the amount of light energy, adjusted for the eye's response".
There are a lot of issues with the text on the images; we should proofread the text here so that you don't have to keep revising the images as people raise different concerns. I don't have time to go into that tonight, but will come back to this soon.--Srleffler (talk) 06:39, 4 December 2015 (UTC)

Yonoodle (talk) 00:23, 13 December 2015 (UTC)

Thank you for the kind reply, i'm giving the first remake version now to make the images more correct and readible, about the issue of drawing five identical cones seems not clear to deliver "total amount" and "per unit area" concepts , so I will keep it a single cone as origin version in this remake version, if i have any physic concept error please let me know to prevent misleading the readers , best regards.

### Text suggestions

I'm going to repost the images here, with suggested new text for each. I encourage other editors to edit the text in this section just like you would text in an article--just change what I wrote if you see a way to improve on it.--Srleffler (talk) 18:20, 6 December 2015 (UTC)

#### 1: Lumen

Treat light rays as water jets. Water here represents the amount of light energy, adjusted for the eye's response.

lumen
lm
lumen-second
lm·s
Luminous power
=water supply rate (amount per second)
Luminous energy
=accumulated water quantity over a period of time
=supply rate × time

#### 2: Exitance

Water here represents the amount of light energy, adjusted for the eye's response.

lux
lx
Radiometry equivalent: watt per square meter
Luminous exitance
=water supplied per second per unit source area
lx = lm/m2

#### 3: Illuminance

Water here represents the amount of light energy, adjusted for the eye's response.

lux
lx
Radiometry equivalent: watt per square meter
Illuminance
=water supplied per second per unit target area
lx = lm/m2

#### 4: Intensity

Water here represents the amount of light energy, adjusted for the eye's response.

candela
cd
Luminous intensity
=water supply rate per unit solid angle
cd = lm/sr

#### 5: Luminance

Water here represents the amount of light energy, adjusted for the eye's response.

candela per square meter
cd/m2
Luminance
= water supply rate per unit of source area, projected on plane perpendicular to propagation direction, per unit solid angle
cd/m2 = lm/(sr⋅m2)

#### Note

Either “per square meter” or “per meter squared” is fine, but use consistently. — Edgar.bonet (talk) 10:01, 7 December 2015 (UTC)

### Still need some work

I glanced at these in one of the articles tonight, saw the misspelled word "lumimous", and deleted them from all the articles since I thought they hadn't been edited. I put them back after I realized these were updated versions. The misspelled word needs to be fixed: it is luminous not lumimous. I don't have time to proofread these tonight; there are probably other issues.--Srleffler (talk) 06:01, 2 February 2016 (UTC)

The misspelling "Lumimous" still isn't fixed...removing. I don't see other major problems; if this spelling error were fixed in images 1 and 4 I would not object to the images being reinstated.--Srleffler (talk) 04:28, 26 April 2016 (UTC)

### Spelling corrected and source file uploaded

Dear Srleffler and all , Sorry for noticing the issue so late after 2 years , i've corrected the lumi"m"ous spelling in the images, and uploaded the pptx source file to google drive https://drive.google.com/open?id=1M0pXnHxnhkG_vgb8wHB--4qPwt3k5XSD it is welcome to everyone to correct the spelling issue or some phrases correction and re-upload from other wikimedia url. still i'm willing to do the job, if there's any suggestions , please also give me a short message at Facebook https://www.facebook.com/profile.php?id=100000740641519 which i will check every day. Yonoodle (talk) 07:26, 12 December 2017 (UTC)

### The images are so terrible, the article would probably improve if they were removed

• They're so ugly they actually discourage the reader to look at them. They even discouraged me from reading the text close by.
• The shower head water spray analogy was probably well-intended, but it doesn't work for me at all. Water behaves very different from light and I've built up some strong intuitions about how water behaves and the images just don't seem to make any sense in that frame of mind and I cannot really unthink this and focus on light while there's a naked person with a shower head in the picture.
• The pictures need to be about actual light sources at different light levels, showing the practical effects of different quantities expressed in the various units. Make it about light, make it real, make it visual, no analogies.
• I wonder if the images were put in the article to troll us?
• Maybe some editor is a photographer and can take some actual pictures and create new schematics based on those?
• I think whoever created the images in the article shouldn't have another go at it; let someone fresh have a stab at this.

What do others think? I'm no fan of the images and would be fine with seeing them removed.--Srleffler (talk) 23:12, 25 April 2021 (UTC)

I have never before used the discussion section of Wikipedia. But now I just wanted to say that those images should be removed. They are ugly and not work at all. 2001:14BB:A2:76A3:AF0C:DE59:8D40:AF3E (talk) 05:09, 10 August 2021 (UTC)

OK, I removed them. Removed wikicode is below, for reference. The images themselves are visible above.--Srleffler (talk) 02:35, 16 August 2021 (UTC)

{{multiple image|caption_align=center <!-- Essential parameters --> | align = right | width = 250 | direction = vertical | image1 = 1 luminous.PNG | caption1 = Luminous energy and power | image2 = 2 exitance.PNG | caption2 = Luminous exitance | image3 = 3 illuminance.PNG | caption3 = Illuminance | image4 = 4 intensity.PNG | caption4 = Luminous intensity | image5 = 5 luminance.PNG | caption5 = Luminance }}

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## Illuminance example values

Hi, the values listed in the table of lux values for different weather conditions don't really agree with my knowledge from the daylight simulation courses I have taken at my university. Most of the values I know nothing about, but something we often use is the overcast sky since this is used for daylight factor calculations. Here the standard value is 10,000 lux not 1,000 as suggested in the table here! As part of those courses I have also done some measurements to back up calculations, and from those I can confirm that 10.000 lux is pretty much what you get with a normal overcast sky (here in Denmark at least). Obviously the value can be lower if it's a really thick sky cover or something, but I think it should be noted that the 1,000 is a lower bound then. I also find it a bit confusing that there is a "full daylight (no direct sun)" category too. Should that be interpreted as clear sky conditions but only the ambient light (if so I think the value would be lower than with a thin sky cover?), or is that essentially an overcast sky? For more info see https://en.wikipedia.org/wiki/Daylight_factor Here overcast sky conditions are listed with a value of 11921 lux. — Preceding unsigned comment added by 94.189.3.178 (talk) 19:57, 24 January 2019 (UTC)

## Unit for luminous emittance?

For both illuminance and luminous emittance, the SI unit is lm/m2. But to my knowledge, lx=lm/m2 is used for illuminance only. See e.g. [6] and [7] — Preceding unsigned comment added by Wassermaus (talkcontribs) 15:26, 17 November 2019 (UTC)

Yes, after your edit today I did some quick searching myself and found other sources that suggested the same thing. I removed the section on luminous emittance altogether, since it is no longer relevant to the topic of the article.--Srleffler (talk) 22:01, 30 November 2019 (UTC)

## starlight lux rating errors?

the magnitude-0 star gives 2.08 microlux while magnitude-6 give 8 nanolux, according to the page. probably should be the other way around. i won't switch them around myself because i don't know whether the resulting values would be correct either Gridzbispudvetch (talk) —Preceding undated comment added 04:01, 29 November 2019 (UTC)

See Apparent magnitude. Larger magnitude means a dimmer star.--Srleffler (talk) 05:32, 29 November 2019 (UTC)

## Kudos to the editors

This article is awesome. Congrats to the editors! Now I finally understand what light bulb I need for my office. Don Ema Valecirro (talk) 07:08, 26 April 2020 (UTC)

## Relationship of lux and nit

The article should contain a few worked examples of the relationship between lux and nit in a few common situations, such as maybe a glowing sphere, surround lighting, the sun, a piece of paper, and so on. We got light source A, this emits so-and-so-much light, of which this much reaches object B which in turn itself becomes a light source of such-and-such brightness. I think it would help a great deal for conceptual understanding. — Preceding unsigned comment added by 77.61.180.106 (talk) 13:00, 1 October 2021 (UTC)

That should probably be in Luminance or Illuminance rather than here. The Luminance article has a section on the relationship between the two already.--Srleffler (talk) 02:04, 4 October 2021 (UTC)