[Radiance-general] Luminance to RGB

[Lars O. Grobe]

Hi Chantal,

it would be great of we could point you to the book "Rendering with 
Radiance", which has a chapter on material data aquisition. 
Unfortunately it has been unavailable for a while, and you need a lot of 
luck to find an old copy in some library. Maybe you still try.

You question was not 100% clear to me as what data you have:

> of materials (RGB plus roughness and specularity). If I want to simulate
> an existing material in Radiance I need to
> decompose it in the 3 primary colours.

You need that ONLY if you are interested in color in your simulation 
results, too. Photometric units are typically not aware of color 
(luminance, illuminance) as they are the weighted integral over the 
perceived wavelength range of light.

> As I understand Radiance uses Hemispherical Reflectance (ratio of flux
> leaving the surface to the incident flux). I have this
> data from the site...but how to get it to RGB in the case that I dont
> have an spectrophotometer to measure. One way I know
> could be using a grey scale chart, but then I also need to know under
> which light source were those reflectances calculated
> in order to compare them to my luminances.

You should compare your grey chart with the sample under daylight or 
incadescent lighting condition. As long as you have a continous source 
spectrum, the grey chart should(!) perform rather stable, and unless you 
use a green LED to lid your sample :-) you should get acceptable 
results. Keep in mind that the idea is not to get reflectance data in 
high accuracy ranges, but to find out whether your wall is bouncing back 
light more like 30 or more like 60% here...

You should take a grey chart with you. In many cases, there is measured 
data available by manufacturers, so it is always a good idea to take 
your notebook and write down any material/product information on site. 
One other way is to try using calibrated photographs (there is a tool 
called macbethcal coming with Radiance) if you have a color chart. This 
is useful if you need color and not just integrated reflectance. Still 
take the chart and write down a rough guess of the reflectance and 
compare to what you get from your calibration efforts.

The objective of all this is
> to obtain illuminance calculations.

Again - for this, you typically need no color information, just 
reflectance - all surfaces will be grey, all sources white.

Cheers, Lars.