Thanks for the clarification Blake! Let me add some more info on the type of reflectance standard Blake was talking about (thanks to Google's fast and relevant searches! :D ):
- PTFE is the abreviated name for the plastic resin that we all know and love as Telflon.
- I don't have access to CIE or ASTM standards, but this website: http://www.aviantechnologies.com/reflect.html
states that packed PTFE powder is one of the specified standard for measuring diffuse reflectance. (check under reflectance standards kit)
- I believe the relevant ASTM standard is E259-98 Standard Practice for Preparation of Pressed Powder White Reflectance Factor Transfer Standards for Hemispherical Geometry and Bi-Directional Geometries, available for purchase at www.astm.org.
I don't know what the relevant CIE standard is.
- also check out http://www.oceanoptics.com/products/ws1.asp
- As Blake mentioned, Stewart's literature specifically refers to their use of this standard.
I can see how there would be some confusion as Magnesium Carbonate is the original reflectance standard, although I believe that was from the days before Teflon even existed. Anyways, if you read the specs on those sites the PTFE standard is highly reflective at a wide range of wavelengths and a good enough lambertian/diffusive surface to be used as a reference standard. (i.e. it's very good :D )