ImageBased Electric Lighting Illuminance Render Lacks Materiality

I apologize if this question has either been asked before or is uninformed; however, I am puzzled as to why an Image Based Recipe (set to Illuminance) in HB Legacy is returning renders lacking materiality.

When I change the recipe engine to the default of ‘Luminance’, I get the expected output. I am assuming that this might be occurring b/c I am dealing with ‘usable surface light’ rather than light reflected off of objects. If this (or some variation) is true, would a false color illuminance render take into account materials ?

Thank you.

Thank you.

luminance lum_illuminance.3dm (27.6 KB) (524.7 KB)

Yes, I believe that is why. Illuminance is the measure of luminous flux incident on a surface. So, materials have no role to play as we are only measuring the quantum of light that is falling on the surface. You will see numerically appropriate results in falsecolor though. Illuminance renderings, in general, only make sense as falsecolor renderings. The material properties do influence reflected light, so you will see a variation in results if you were to change materials in your scene. In the below example, luminance (left) and illuminance-falsecolor renderings are compared side-by-side. You can see the light reflecting off of the marble surface impacting the illuminance values (circled).

I think in electric lighting simulations falsecolor illuminance renderings are pretty useful to check on the distribution patterns of the luminaires on the walls (as seen with the light patterns on the paintings in the images below).


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Hi @sarith

I have question, left is luminance and right is illuminance, as you said:

But what about the illuminance on the water? There are reflections (2) from water seen from the left luminance rendering, meaning the surface should also receive the light which can be measured as illuminance (1). But I don’t see any illuminance value in the area (1) of the right rendering. Am I missing anything here?

@MingboPeng is always asking deep questions (wont expect any less from you :grinning:)!
I believe we are just seeing the reflection of the entire wall on the water. The paintings standout because they are lit-up and also a brighter surface. That is not translating to a lot of reflected light however… few more images.

You can see the reflection of the wall in the daytime image too…

This was from a homework assignment in 2014 when I was a grad student at Penn State. I still have the model lying around. So, I just decided to test a couple of views to see if the reflection profile on the water changes…and it does.

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Thank you for clearing this up. Your post validates what I was thinking regarding the apparent lack of materiality in an Illuminance render. Furthermore, it is useful to know that falsecolor Illuminance renders represent materiality; however, I have one more question now…

If falsecolor renders take into account materiality, it seems (if I am understanding correctly) that this post by Greg Ward could say contrary.

This post states that materials that don’t emit light are ignored by rpict -i.

How would it be possible that falsecolor renders can take materiality into account if they are ignored by rpict -i according to the above post?

(hopefully my Radiance terminology is correct and it might not be)

Thank you again for the help.

Hi Eric

The primary material in my scene is plastic, which is not present in the Greg’s list of materials that are ignored. So the material properties of most surfaces get taken into account. The pool/moat is dielectric that is modified by a noise function. And true to Greg’s explanation, the material properties of the pool are not taken into account in the illuminance falsecolor rendering.