[Radiance-general] measuring color

[Alexa I. Ruppertsberg]

Hi Genevieve,

I definitely would recommend a more sophisticated method than using the 
macbethcolor checker card. Yes, it is true that the chromaticities are 
known for those colours, BUT this is under the assumption of a certain 
illumination source. Will you have that same illumination source? 
Probably not.

If you are after accuracy (rather than 'good looks')you might want to 
buy a spectroradiometer and a white reflectance standard, too. After 
all, what you measure with the spectro looking onto your wall is the 
colour signal, a combination of the surface reflectance and the 
illumination at hand (and some geometry). In order to model the material 
properties in RADIANCE, you will be interested in the surface 
reflectance and not in the overall colour signal. To retrieve the 
surface reflectance, you measure the spectrum of the white reflectance 
standard at the very same position where you measured the colour signal 
from the wall, then divide the colour signal (spectrum) by the spectrum 
from the white reflectance standard and you have your surface 
reflectance (this assumes a Lambertian surface). The spectrum from the 
white reflectance standard can be used to get the colour of the 
illumination source.
This approach will yield something with a clear tint, if the 
illumination source wasn't a natural light. If you want to have a 
good-looks picture then don't bother with the white reflectance standard
and model the light source as white (as described and recommended in the 
RADIANCE book).

Regards,
alexa

> Hi,
> In order to define colors for radiance models I'm looking for an 
> instrument to measure the colors of walls and of mural paintings of 
> archaeological sites. It should be portable, working with batteries and 
> non destructive to the material.  (max. price, $5000 US)
> Any recommendation?
> 
> genevieve lucet
> 

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Dr. Alexa I. Ruppertsberg
Department of Optometry
University of Bradford
Bradford
BD7 1DP
UK
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~