# Spectrophotometer Data to Radiance Surface Descriptions

If I have diffuse and specular XYZ surface data from a spectrophotometer—six numbers—, how do I convert that into Radiance material descriptions? Can I calculate the "specularity" for types plastic and metal from that data? Or do I need to use a function or data-based material type?

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Randolph Fritz • RFritz@lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs

Hi Randolph,

I believe I had a script or .cal file or something at one time to compute the specularity from separate "specular included" and "specular excluded" measurements, but I can't find it. The calculation is pretty simple, though. Just subtract the "excluded" from the "included" Y values and divide by the the "included" Y measurement to get the specular percentage for a metallic material with colored highlights. For a plastic material, the specular fraction is the "included" minus the "excluded" Y over 100 (assuming a 0-100% scale for Y).

The material color is the "included" Yxy converted to RGB for metal, and the "excluded" Yxy converted to RGB for plastic. The conversions are given in ray/src/cal/cal/xyz_rgb.cal. You can quickly spit out the matrix coefficients for this conversion using:

rcalc -n -f xyz_rgb.cal -o cmat.fmt

Best,
-Greg

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From: R Fritz <RFritz@lbl.gov>
Date: July 26, 2010 12:59:59 PM PDT

If I have diffuse and specular XYZ surface data from a spectrophotometer—six numbers—, how do I convert that into Radiance material descriptions? Can I calculate the "specularity" for types plastic and metal from that data? Or do I need to use a function or data-based material type?

Hi!

The material color is the "included" Yxy converted to RGB for metal, and
the "excluded" Yxy converted to RGB for plastic. The conversions are
given in ray/src/cal/cal/xyz_rgb.cal.

In cases where you have the spectrum as it comes from the spectrometer, you can also use mgfilt's ability to handle reflection spectra and convert to rgb...

Cheers, Lars.

Thanks, Greg & Lars.

BTW, there are apparently no "mgfilt" (and mgf2inv) man pages. Did these get lost in the shuffle a while back?

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Randolph Fritz • RFritz@lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs

On 2010-07-27 04:49:45 -0700, Lars O. Grobe said:

Hi!

The material color is the "included" Yxy converted to RGB for metal, and
the "excluded" Yxy converted to RGB for plastic. The conversions are
given in ray/src/cal/cal/xyz_rgb.cal.

In cases where you have the spectrum as it comes from the spectrometer, you can also use mgfilt's ability to handle reflection spectra and convert to rgb...

Cheers, Lars.

MGF isn't technically part of Radiance, so only mgf2rad and rad2mgf man pages are included. The others are at:

-Greg

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From: R Fritz <RFritz@lbl.gov>
Date: July 27, 2010 12:14:20 PM PDT

Thanks, Greg & Lars.

BTW, there are apparently no "mgfilt" (and mgf2inv) man pages. Did these get lost in the shuffle a while back?

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Randolph Fritz • RFritz@lbl.gov
Environmental Energy Technologies Division • Lawrence Berkeley Labs

On 2010-07-27 04:49:45 -0700, Lars O. Grobe said:

Hi!

The material color is the "included" Yxy converted to RGB for metal, and
the "excluded" Yxy converted to RGB for plastic. The conversions are
given in ray/src/cal/cal/xyz_rgb.cal.

In cases where you have the spectrum as it comes from the spectrometer, you can also use mgfilt's ability to handle reflection spectra and convert to rgb...
Cheers, Lars.

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