Diffuse reflectance measurements and GretagMacbeth ColorChecker reflectance values

radiance-general
1

Very quickly, you can look at the "Tools for Materials" section of the SIGGRAPH '96 course notes here:

  http://radsite.lbl.gov/radiance/refer/sg96crs.pdf

The Radiance tool you want to check out is macbethcal, and you can look in src/px/macbethcal.c to find the MacBeth color checker measurements. There are some tips as well in Chapter 5 of "Rendering with Radiance" if you have that.

On a Mac, you can use Photosphere from <www.anyhere.com> to calibrate a camera, as described in Mehlika Inanici's technical reort:

  http://gaia.lbl.gov/btech/papers/57545.pdf

Best,
-Greg

···

From: "Ilya A. Zimnovich" <[email protected]>
Date: November 3, 2010 12:14:30 PM PDT

Dear mailing list members,

I have been experimenting with our new scientific grade CCD camera which I suppose can be used as an imaging photometer. Yes, after some extensive calibration!

I would like to take some measurements of diffuse reflectance. The classical setup will require an integrating sphere. Well... the University I work at will not afford such an expensive device. I'm sure there would be another way to estimate it. I guess it is possible to acquire BRDF and to make some diffuse reflectance estimation?

Below are my questions:

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?

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

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?!

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

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

Thanks,
Ilya

2

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.