[Radiance-general] Determining average reflectance from RGB values

[Carsten Bauer]

Hi Steve !

Comparing measured values with one from simulation is always tricky, as
a lot of things have to be considered. 

To begin with the average reflectance question: From a radiation point
of view, you may average the three reflectances for red, green, blue, as
the rgb-channels simply represent three ranges of the spectrum with
equal weight. Turning over to illumination, things become different, as
the human visual response of the eye has to be taken into account. For
this purpose, within Radiance a so called brightness is used, which is a
weighted sum of the three rgb-channels

B = 0.265*r + 0.670*g + 0.065*b 

(only first three digits of the coefficients are given, their sum is of
course 1, multiplying this brightness with K = 179 lm/W makes it finally
a luminance value).

Example: a uniform grey r=g=b=0.5, illuminated by white light (r=g=b=1)
has of course a brightness of 0.5, (remember that the sum of coeficients
is one, so the above formula gives
 (0.265+0.670+0.056)*0.5 = 0.5)

Something more or less red, e.g. r=0.8, g=0.5,  b=0.2 with the same
average reflectance of 0.5 will however have a brighness (again, for the
light r=g=b=1) of 0.265*0.8 + 0.670*0.5 + 0.065*0.2 = 0.56, which is
different.

So, for comparing different materials, the brightness should be used.
Additional surface normal undulations will certainly have an effect on
the amount of received and reflected light. To determine this effect,
you have to do test runs with and wihout texture modification and
compare the results.

Last but not least, the color of the light source has to be taken into
account, too. It has to be the same as the one of your test equipment. 
(Pictures are often rendered with uniform white light , r=g=b=1, to
avoid color biases, although this is not exact from a scientific point
of view.) Color values for different lamp types can be calculated with
the Radiance "lampcolor" utility.


So, again, such a simple comparison is more tricky than one might think
at a first glance.

Carsten