# Luminance

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I have read many papers in my efforts to understand Luminance

I understand the other photometric terms (Intensity, Flux, Illuminance, Reflectance, Emittance)

and I have no problem with 3D geometry (solid angles, projected areas) or cosine laws etc.

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Maybe wikipedia?

http://en.wikipedia.org/wiki/Luminance

In short for a 3d graphics application, luminance often refers to specialy weighted and combined illumination for any given point of your scene. The only term however I heard about it being used practically was for HDR and tonemapping, where you would calculate the average logarythmic luminance of your entire scene from the cameras viewport, for the HDR equation.

While this is probably not what you wanted to know in the first place, you might get the most information about this topic by reading articles about HDR like:

http://msdn.microsoft.com/en-us/library/windows/desktop/bb173486%28v=vs.85%29.aspx

Unless there is some other viable source that I don't know yet...

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The only term however I heard about it being used practically was for HDR and tonemapping, where you would calculate the average logarythmic luminance of your entire scene from the cameras viewport, for the HDR equation.

Don't forget about YUV format. Y is for luminance, and contains the luminance at the given pixel. YUV (and it's variants YUV422, YV42, etc) is the preferred choice when dealing with JPEG, MPEG-2, MPEG-4 & H264 compression, also when broadcasting TV.

Also lately the YUV422 format is comming up in games because it halves the bandwidth and memory requirements for slight losses in quality.

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converting rgb radiance to luminance I do understand (spectral weighting)

but the wiki article is incoherent

it says that luminance is dependent on the angle of view!

and the terms in the equation it gives are ambiguous and I can't determine any set of terms that makes sense

for example: which solid angle, which area, which angle, which flux, etc. etc.)

I believe that:

luminance = illuminance * reflectance / pi

pi describes the reduction in intensity due to diffuse scattering

and you can use rgb albedo as reflectance

and that the units cd/m^2 is almost a play on words

in that luminance is a form of intensity - but for surfaces rather than points

and the dot lighting we use in games ignores the pi term for simplicity

(editor keeps dropping the first letter of lines)

Edited by skytiger

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Luminance isn't a radiometric quantity. It's basically radiance weighted by an observer's luminosity function. If you compare the definition of radiance and luminance, you will see they are exactly the same with the exception that radiance uses watts whereas luminance uses candelas (which are just watts weighted by "perceived brightness", loosely stated).

I have to agree with Juliean, this is probably not the definition of luminance you were thinking of, and I think what you are looking for is pretty much just what you already know - it's just radiance, weighted to match our tristimulus color matching functions (this is what photometry is about - how humans and other species perceive light, not just a radiometric physical interpretation of it)

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Your reflectance/pi formula is the Lambertian BRDF, which is off on a completely different tangent.

As above, luminance is a much more general concept -- if you've got some quality of colored (vector/spectral) light, how can we approximate how bright (1d/scalar/monochrome) it will be percieved to be. It's basically just "to greyscale", but with some carefully tuned weights.
You can calculate the luminosity of radiance, irradiance, or even a pixel from an uncalibrated JPEG.

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This seems to describe one of the problems I am having:

it describes apparently contradictory descriptions of lambertian scattering

is this "a different tangent" though, luminance is associated specifically with diffuse surfaces, which lambert is describing

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This seems to describe one of the problems I am having:

it describes apparently contradictory descriptions of lambertian scattering

is this "a different tangent" though, luminance is associated specifically with diffuse surfaces, which lambert is describing

But "luminance" is mentioned nowhere in that document, do you mean the two reflectance diagrams? If you understand the cosine law then the two are equivalent, it simply depends on how you are interpreting the reflected light, energy per solid angle (that is radiant intensity) or energy per surface area per solid angle (that is radiance). One thing I've learned about radiometry is that units are all over the place, and you need to be sure you know the difference between each definition or you'll get muddled up really quickly.

And as for luminance being associated with diffuse surfaces, that's because diffuse surfaces are the easiest type of surface to analyze, and so it is only natural to describe it using Lambertian reflectors. To be clear, luminance is the photometric equivalent of radiance, so it isn't really associated with anything, it's a broad, general concept, but its use in computer graphics is typically narrow and sometimes pedantically wrong (granted, it's difficult to get it right sometimes when you've got largely similar terms like "intensity", "brightness", "luminosity", "luminance", which in ordinary english are pretty much interchangeable)

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ok things are getting clearer

NOW I understand the cosine law ...

can you help my thought experiment along

point light with intensity I

surface element subtends solid angle O

incident flux is F = I * O

surface element has surface area A

irradiance is E = F / A

reflectance is 100%

so emittance M = E

and radiance is ... now I am stuck

I could say the radiance is also M

as we have already divided by an area and by a solid angle ...

but we haven't taken into account the diffuse scattering

Or I could say that radiance is M / pi

which accounts for the scattering, but is now an average over the hemisphere

now I can see how to calculate the flux entering an eye's pupil

(integrate solid angle subtended by pupil and surface area of surface)

but then projected area pops up ...

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I think I understand radiance / luminance now

I think that for a lambert diffuse surface that L = M / pi

And that dot product lighting in video games bypasses the concept of radiance and defines lights in terms of irradiance

emittance = irradiance * albedo * dot(N, L)

as a simplification that allows us to model "impossible" light sources such as: point and directional and skip the divide by pi (and the whole solid angle * projected area thing cancels out anyway)

"luminance" as discussed earlier is really a conversion from radiant emittance to luminous emittance by spectral weighting

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