[Radiance-general] Diffuse reflectance measurements and GretagMacbeth ColorChecker reflectance values

[Lars O. Grobe]

Hi Ilya!

> 1. Is there way to estimate diffuse reflectance without using of
> integrating sphere? Also, to some extent, can the reflectance measured
> under overcast sky conditions be referred to as diffuse reflectance?

Yes, the integrating sphere is only one possible instrument. "Diffuse 
reflectance" is usually referring to direct-hemispherical reflectance 
without whatever is defined as specular. The problem with turning it the 
way that you measure it as hemispherical-direct (incident light over 
hemisphere, measured at perpendicular angle) is that you must know the 
BRDF, and as far as I understand the idea, would need a uniform 
hemispherical source (overcast is non-uniform). A small integrating 
sphere is not all that expensive, by the way...

> 2. How do you calibrate your CCD cameras for taking luminance measurements?

Again - using an integrating sphere, a stabilized light source, and a 
calibrated detector as a reference. One example where we did that was 
presented at the Radiance Workshop in 2008:

http://www.radiance-online.org/radiance-workshop7/Content/Jacobs/jacobs-fribourg2008d.pdf

> 3. With uncalibrated camera, I guess the reflectance values can be
> estimated using HDR shots of GretagMacbeth ColorChecker chart as
> reference. The ColorChecker chart's reflectance values are known, right?
> So, it is possible to compare the values from the shot of a sample
> against the values from the shot of the chart to estimate the
> reflectance of the sample?! But where can I get the GretagMacbeth
> ColorChecker Rendition chart's RGB reflectance values? I wonder if
> anyone have measured them?!

They are printed on its back, and you can assume the chart to be a nice 
diffuse surface. However, the pixel values in you picture represent 
luminance (or radiance), not reflectance!

> 4. What are in the "macbeth_spec.hdr" and "macbeth_pub.hdr" files? What
> data are in these files?

I think the spec-file is the reference giving the chart, while the 
pub-chart may be used for calculations, but I am not sure now. The image 
files are used for macbethcal-"calibrations".

> 5. Having estimated BRDF for a particular material sample can we
> estimate the material's diffuse reflectance?

If you have measured the BRDF (which is done using other instruments 
then an integrating sphere, and a bit more complicated), you can derive 
the diffuse reflectance by integrating the transmission hemisphere's 
data and substracting the integral of the specular range. If you start 
with estimating a BRDF, I have my doubts that you will end up with 
correct values. Using a grey-chart and comparing using you eyes may be 
more promising. But maybe you do both and compare your results.

Cheers, Lars.

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