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To me, there's a mistake in the very beginning of the article:
"If a surface exhibits Lambertian reflectance, light falling on it is scattered such that the amount of light an observer sees, the surface luminance, is the same regardless of their angle of view."
If exhibiting a lambertian reflectance is the result of lambert's cosine law, then we can't really talk of "amount" of light. While I agree with the use of the term 'luminance', it implies a notion of brightness, and not really of quantity or amount. Lambert's cosine law says that the amount of light decreases with the cosine of the angle between the normal of the surface and the line of sight. But as the apparent surface the observer sees decrease with the same law, the brightness of the source remains constant. However I may be wrong... palleas - 04/03/2006
- Yes, you're right. I'll try to reword it. What the person who wrote that probably meant was that the amount of light an observer sees per unit area of the surface, as projected on the plane perpendicular to the observer's angle of view. That would be correct, but isn't exactly elegant. The description must not use the term "brightness", though. The use of "brightness" as a description of or measure of light is strongly discouraged, because it tends to lead to confusion between luminance, radiance, luminous and radiant intensity, irradiance, illuminance, etc. As discussed at brightness, the latter term should be used only to describe a person's subjective impression of light, not actual physically measurable "amounts" of light.--Srleffler 17:22, 3 April 2006 (UTC)
- I guess luminance (or radiance in energy units) is the term to use here, though it doesn't show clearly that there is the cosine factor in it. Maybe the article should have a first part that could be more 'instinctive', and say that a lambertian reflector is as bright whatever the line of view, and then a second part that explains it scientifically. I totally agree that the photometry terms are a hassle, and I'm french, so I have to deal with them in french and english, and as you can well guess, it's often not the same....--Palleas 9:10, 4 April 2006 (UTC)
I still somehow disagree with the definition. The term 'amount' annoys me. I know it's a very difficult definition to write properly, without using ambiguous terms... Or maybe keep amount of light, but make clearer the fact that it's not constant per unit area, but per perceived unit area? Palleas 07:25, 18 April 2006 (UTC)
quit it with the annoying overzealous merging. when i look up lambertian i want to know what people are talking about first, not what lambert's freaking cosine law is. i don't care about his cosine law, i want to know what lambertian means. if i don't make sense to you question yourself, not me.
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Correct me if I'm wrong, but I think the equation is wrong if N and L don't have unit length. Shouldn't it be: ID = cos(alpha) * C * IL = (L dot N) / (|L| * |N|) * C * IL? Update: Sorry, just noticed that the article mentions that L and N have to be normalized.
This statement (first two sentences) is at complete odds with the Lambert Cosine Law that this article is linked to. I would simply refer to the law which clearly explains the effect. This should be repaired now...the other comments are rather old. November, 2007
