[Radiance-general] Specularity Definition of Radiance Material

Thu Oct 22 12:23:33 PDT 2015

Greg, according to different ASTM measurement standards, a hemispherical reflectance meter is deemed as accurate for highly specular and non-specular materials.  ASTM standards encourage the use of multi-angle measurements for materials that have medium gloss.  So it is quite possible, per a given test standard that usage of the hemispherical meter may not yield accurate data.

A multi angle meter is analogous to a BRDF measurement, just less angles.  With respect to Radiance terms for Total Reflectance, Total Specular Reflectance, and Total Diffuse reflectance, is there anything we could extract from test data from a multi-angle meter that would allow us to determine terms for Total Specular and Diffuse reflectance, aside from using a BRDF function?

For instance, per Radiance, what angle range around the specular angle constitutes the definition of a specular reflection?  For Radiance, what angle range from the specular angle defines the diffuse reflectance?  Please see attached image.  Here, I am thinking that the definition of Total Specular Reflection should constitute some range of angles around the specular angle.  All other angles should fall within the Total Diffuse Reflection category. For instance if I define the Total Specular reflection as the flux that reflects +/- 15 degrees from the specular angle would that be a good definition?  Alternatively can we use an analogy from the lamp world for the definition of beam angle (% drop off by angle from maximum relative intensity)?

Thank you for your thoughts as we attempt to reconcile differences in measurement techniques and definitions for parameters for Radiance material classification.

Toni Clark, P.E.

From: Greg Ward [mailto:gregoryjward at gmail.com]
Sent: Thursday, October 22, 2015 12:08 PM
To: Radiance general discussion
Cc: Maida, James C. (JSC-SF311); Clark, Toni A. (JSC-SF)[WYLE INTEG. SCI. & ENG.]
Subject: Re: [Radiance-general] Specularity Definition of Radiance Material

Hi Andrei,

Actually, your original formula came through for me.  Maybe it got mangled on the way back to your inbox.

The formula you had for the specularity, where you divided by the "SCI" component, was correct if you use the Radiance "metal" primitive and apply the overall reflectance as your RGB values.  This is because the specular component is multiplied by the RGB color for metals.  I don't recommend using the "plastic" type in your case, but if you did, you would want to leave off the denominator as you say.

Cheers,
-Greg

From: "Kolomenski, Andrei (JSC-SF311)[WYLE INTEG. SCI. & ENG.]" <andrei.kolomenski at nasa.gov<mailto:andrei.kolomenski at nasa.gov>>

Subject: Re: [Radiance-general] Specularity Definition of Radiance Material

Date: October 22, 2015 8:39:14 AM PDT

Hi Greg,

Thank you for your reply.

Unfortunately, as I pasted in the Specularity formula from MS Word into my original post, the division sign was lost in the process. The formula is intended to be a spectral power distribution (SPD) summation of the difference between a specular included (SCI) and a specular excluded (SCE) divided by the SCI.

This formula defines a relative specularity as a percentage of the total reflectance. As I understand, Radiance's material specularity variable is the total specular reflectance normalized percentage, and not the relative specular percentage I was previously thinking.

So that: total reflectance = total specular reflectance + total diffuse reflectance

Using this methodology for a black material I compute a 5% Total Reflectance, and a 20% Relative Specularity, meaning that my Total Specular Reflectance is (5%*0.2) = 1%, and this value (0.01) is used to define Radiance's specularity variable. Also, the Radiance RGB triplet is scaled by Total Reflectance (0.05).

Does this sound correct to you?

Thank you for the insight,
Andrei Kolomenski
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