[Radiance-general] Luminance contrast calculation

[Marija Velickovic]

Hi,

I'm trying to calculate luminance contrast between various surfaces or
Radiance image, and compare it to recommended values.
Idea is to calculate for each suface (eg. paper, desk, walls, floor etc)
average luminance, and obtain contrast as ratio between these luminances


Question: How to calculate average luminance for a surface on Radiance
image?
My first idea was to identify pixels for each surface, and calculate their
average luminance as simple average value:
*Average_lum = sum(pixel_luminance)/num_of_surface_pixels,*
where only pixels for specific surface are taken into account.

But after some thinking, maybe that wouldn't be correct approach.
Maybe solid angle of each pixel should be taken into account to have
physically correct value
*Average_lum =
sum(pixel_luminance*pixel_solid_angle)/sum(pixel_solid_angle), *
where only pixels for specific surface are taken into account.

So my real question would be: if we calculate average luminance for some
part of radiance image, should we take into account size of each pixel
(solid angle) or not?

###
Related to contrast calculation I'm still thinking if images I'm rendering
and analysing should be hemispherical (like for evalglare) or have some
human-like field of vision (-vtv -vh 110 -vv 70 for example)
Suggestions?

###
Another question is more theoretical - when we are talking about contrasts,
should we take into account average values (for work task, environment
surfaces etc), or extreme values like maximal luminance.

I know this question is somewhat close to glare calculation, so maybe Jan
has some experience with luminance calculation for various surfaces on the
scene.

Thanks in advance,
Marija
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