[Radiance-general] Re: Spectral Reflectance values to Radiance format

[Alexa I. Ruppertsberg]

Hi Ivan,

you might find this helpful:

For RGB values, you have to convert the surface
reflectance function S( lambda) to XYZ tristimulus values by using
x(lambda), y(lambda) and z(lambda) matching functions (e.g. Judd-Vos 
1978 adjusted CIE matching functions) and Equations 1.1 to
1.3 [Wyszecki and Stiles 2000]
X = k Integral( S(lambda) x(lambda) dlambda) (1.1)
Y = k Integral( S(lambda) y(lambda) dlambda) (1.2)
Z = k Integral( S(lambda) z(lambda) dlambda) (1.3)
The XYZ tristimulus values can then be converted to RGB
with a conversion matrix T, which is based on the primaries of
the monitor. T is a 3 by 3 matrix (for a description on how to
obtain this matrix see [Travis 1991; Foley et al. 1992] among
others). We used RADIANCE’s in-built monitor primaries to
derive the conversion matrix T. Multiplying the XYZ
tristimulus values with T yields an RGB –triplet, which is then
used as the three colour descriptor values in the material file.

WYSZECKI, G. and W. S. STILES 2000. Color Science. John Wiley &
Sons.
TRAVIS, D. 1991. Effective Color Displays. Academic Press.

FOLEY, J. D., A. VAN DAM, S. K. FEINER and J. F. HUGHES 1992.
Computer Graphics: Principles and Practice. Addison-
Wesley Publishing Company.


Good luck!
alexa

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

Phone: ++44 (0) 1274 - 23 53 78
email: a.i.ruppertsberg at bradford.ac.uk
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