Talk:Luminance

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Contents 1 First comment 2 merge articles 3 Split article? 4 Luminosity is not a synonym for Luminance 5 Luminance (video) 6 Examples

[edit] First comment

Is the unit for L = luminance cd / m² ?

yes, indeed. But other common units are footlambert with abbrev. fl , 1fl = 3.426 cd/m² or the nit, 1nit = 1 cd/m²

- Jurgen Jacobs -

[edit] merge articles

If luminance and luminosity are supposed to be synonyms, shouldn't those two articles be merged? --Abdull 12:00, 23 Mar 2005 (UTC)

No. Not all meanings of each term are synonyms for the other. --Srleffler 07:42, 15 November 2005 (UTC)

The same is then true for Luminance and Radiance, which have the same equation (F replaced with phi or vice-versa, but the same), and almost equal wording? 5:18a 19 Apr 2007 (UTC)

Yes, same thing is true: they are distinct quantities. One based on radiant flux, the other on luminous flux. One an energy-based measure, the other based on ability to simulate the human eye to perceive brightness. Dicklyon 05:33, 19 April 2007 (UTC)

[edit] Split article?

I'm wondering if the TV signals materal should be split off into its own page, perhaps Luminance_(video). It seems to be a completely different concept from the physical quantity.--Srleffler 07:42, 15 November 2005 (UTC)

I split the article. The TV contents are now at Luminance (video), and links have been adjusted to point there where appropriate.

I'm OK with the split, but the concept is really very close conceptually. The video luminance or "luma" signal, or the luminance (L or Y) in a colorspace such as Luv or Lab or XYZ, represents the luminance of the scene, pretty nearly, with some exposure normalization and nonlinear encoding. It's still the same concept.

[edit] Luminosity is not a synonym for Luminance

In the original version of the page, it was stated that The word luminance, a synonym for luminosity, means "the quality of being luminous; emitting or reflecting light and also Luminance is also a technical term in photometry. OK, those are both sort of true. But now that the article of about the latter term, the former statement is no longer operative. I suggest we remove the false suggestion that photometric luminance is sometimes called luminosity. Dicklyon 06:52, 19 May 2006 (UTC)

[edit] Luminance (video)

After someone added a weighted RGB expression for luminance, I made a section to explain exactly what it meant. Then, srleffler removed it, saying it's the wrong sense, because of the separate article Luminance (video). I half agree, but it also seems we're missing the opportunity to tie these two senses together, which is what I had tried to do in that section by specifying the exact meaning of the proportional relationship between photometric luminance and a colorspace luminance. This might not have been the best way or place to go about, but it's a possible worthwhile bit of info. Suggestions? Dicklyon 04:59, 16 November 2006 (UTC)

NB. At least according to the articles, that formula applied to luma (video) rather than luminance (video). Those articles warn about the risk of confusing the two. The fact that the section that was added here appeared to confuse the two was a factor in my decision to delete.

And you were correct, as I later verified in trying to find the right numbers. Dicklyon 16:03, 16 November 2006 (UTC)

Is the video usage at all related to the photometric usage, or is it just a coincidence of terminology? (Yes they are both measures of amount of light, but there are a lot of measures of amount of light. They are not all the same, and should not be confused even when they happen to have the same name.--Srleffler 07:54, 16 November 2006 (UTC)

Yes, the video and photographic meanings are the same. It's basically a colorimetry term related to the photometry term, which is applied in all forms of color reproduction. It's the "Y" term in XYZ space, or proportional to it in systems where the gain is a free variable.

I think the thing to do is get rid of luminance (video), and to talk about that concept in both the luminance article and the luma article, so that the relations can be drawn where relevant. Dicklyon 16:03, 16 November 2006 (UTC)

I'm not sure what you mean by "the same". The photometric term denotes a quantity that has dimensions of luminous flux per unit area per unit solid angle. The dimensions of the video quantity are unclear to me. Is it a measure of the actual luminance per pixel? If so, what is the conversion to cd/m²? Or, is it actually a misnamed measure of luminous flux, luminous intensity, or illuminance? (I trust that it is at least proportional to all of these, for all choices of R, G, and B component.)

I definitely favour merging Luma (video) and Luminance (video), although it's not clear which title is best. They were one article until not long ago. Someone split them. I agree with discussing it in this article, but how it is presented depends critically on the answer to the question above.

If it's not clear what I'm getting at, think about a CRT display and an LCD display (for sake of argument), each with a region of the same physical size displaying the same luma. Will the photometric luminance be the same between the two? The LCD spreads the emitted light into a narrower angular cone. --Srleffler 23:04, 16 November 2006 (UTC)

What I meant by "the same" is that the luminance in video is, or is intended to be, a weighted integration of the emitted or reflected spectral energy of the display, using the same spectral weighting as the photometric luminance. There is an absolute "Y" value that has the same units as luminance, but usually that's ignored and all values are taken relative to whatever arbitrary luminance is defined as white. In this sense, for any given display system, the video luminance is proportional to the actual display luminance, with an unknown constant of proportionality. So it's the same concept, but "uncalibrated" with respect to absolute units; or "normalized" to a reference white level, if you prefer. Dicklyon 23:59, 16 November 2006 (UTC)

In the encoding of video signals, luma is used and this is different than luminance/photometry. The term "luminance" is often misused in video to refer to luma, while people get it confused with luminance/photometry. In video however, luminance/photometry is used in defining standards such as monitor brightness. i.e. SMPTE RP 166 I believe defines 35footlamberts (I forget how many nits this is) for evaluation of television pictures. DCI (digital cinema) specifies 14fL, theatrical film projection 16fL (open gate; when you have film in the projector, the film blocks some light so white becomes ~14fL).

Sometimes the color science (i.e. CIE) definition of luminance comes into play with video (if you consider digital cinema to be interrelated to video). DCI uses a color space with CIE's XYZ tristimulus components. (from what I understand) These components represent relative luminances, not absolute luminances. i.e. if your scene is shot in daylight, it really makes sense to code the image in relative luminances and not absolute luminances (because the projector isn't as bright as the sun).

(Ok re-reading my message.) Basically I think that there should be three articles. 1- Luminance (photometry). 2- Luminance (colorimetry/color science). These numbers specify relative luminances, not absolute luminances. These numbers are most similar in concept to luminance and not illuminance, luminous flux, and luminous intensity. 3- Luma (video). These numbers are derived from #2 (luminance in color science), but they use a different 'order of operations' for pragmatic performance/cost reasons. Glennchan 21:50, 19 November 2006 (UTC)

This is exactly what we've done in recent days, right? Dicklyon 00:15, 20 November 2006 (UTC)

Yeah I think so. The luminance (colorimetry) article should be worked upon, so it is a clear explanation of luminance in the context of color science. It feels too video oriented, and doesn't have to be. Well I'm confused by it, that's basically what I'm getting at. Glennchan 01:19, 20 November 2006 (UTC)

[edit] Examples

I think this could do with some examples. As I understand it, luminance describes how bright things look, but I'd like some verification before I make edits. Do the following all sound right?: If my monitor displays all white, then its luminance is x. If I put a 1-stop (50%) ND filter over it, I see its luminance as x/2. If the monitor displays all black except for one pixel, the luminance of that pixel is still x. If I have a point source, its luminance is infinite; if I put a diffuser around it with finite size, the luminance would become finite, say y. If the area of the diffuser were doubled, the luminance would fall by half. —Ben FrantzDale (talk) 11:38, 11 June 2008 (UTC)

I believe that's correct, subject to some question about the nature of the diffuser; instead of saying a diffuser, you can take an "average luminance" over the area, even when there's a delta function in it. Dicklyon (talk) 15:50, 11 June 2008 (UTC)

I'm not an expert on this but I believe luminance is a good indicator of how things look under certain circumstances. Visual perception is pretty complex. See also brightness. I think luminance is a good indicator of how bright a relatively distant, isolated object will appear. Replies to your statements:

If my monitor displays all white, then its luminance is x.

I wouldn't use a monitor as an example, due to confusion of terminology. The video industry uses the term "luminance" in several ways, not all of which exactly correspond to the optical concept.

If I put a 1-stop (50%) ND filter over it, I see its luminance as x/2.

Yes

If the monitor displays all black except for one pixel, the luminance of that pixel is still x.

I think so, at least for an ideal monitor.

If I have a point source, its luminance is infinite;

There are no point sources. Luminance is never infinite. This is not a trivial objection; it is fundamental.

if I put a diffuser around it with finite size, the luminance would become finite, say y.

If you put a diffuser around a small source, you decrease the luminance, because the surface area of the new combined source is larger than that of the small source within it.

If the area of the diffuser were doubled, the luminance would fall by half.

Only if the initial source is also diffuse, with the same emission profile as the diffuser. For example, both could be Lambertian.

Luminance is directional, and sources need not emit the same luminance in all directions (and generally do not). A diffuser will change the angular distribution of radiation as well as the emitting area. Examples will need qualifications to deal with the directional aspect. A Lambertian surface has the property that it emits (or reflects) with equal luminance in all directions.--Srleffler (talk) 16:47, 11 June 2008 (UTC)

I agree with all that. I think the basic idea of trying to confirm his intuitive understanding was about right. It would be more precise and objective to speak of the light received by a pixel in a sensor, covering some area, rather than putting an eye and a diffuser. A finite pixel area is a good way to average a luminance that includes a point (delta function). Dicklyon (talk) 20:23, 11 June 2008 (UTC)