FAQ: "From my reading on the subject, it seems that 0°:d and 8°:d are each acceptable according to CIE, but are not equivalent (i.e. one should not expect to obtain the same results when measuring a sample using different geometries). This is of concern to me as I use my color instrument to ensure that the color of paper is within a set specification."
This following statement, although not explanatory, from CIE Publication 15.2: 2004 Colorimetry allows an offset of the viewing angle up to 10 degrees, allowing equivalency of a d/0°, d/8° and d/10° sphere instrument geometries.
"Diffuse/normal (d/0) The specimen is illuminated diffusely by an integrating sphere. The angle between the normal to the specimen and the axis of the viewing bean should not exceed 10 degrees...."
In theory, given a smooth opaque material (non-metal or metal) sample and a measurement in reflectance specular included mode, the CIE states that instruments with a d:0°, d:6°, d:8° and d:10° geometry should be equivalent in terms of measure values.
In practice, given samples that are not perfectly smooth and uniform, I would tend to agree with your statement that d/8° and d/0° are acceptable to the CIE, but may not give exactly the same values in the RSIN specular included mode due to sample surface issues and other facets of the sphere (diameter, coating type, baffle size and locations, method of specular exclusion etc.) that are not precisely defined in CIE Publication 15.2: 2004.
Here is an overall approach for dealing with equivalency of CIE sphere instruments with the 10 degree view spread:
- The marketplace has simplified some of these choices. There are basically only two sphere geometries available in sphere instruments that measure color - a d/8° (most common) and d/0°, or its inverse 0°/d (still out there but very rare).
- If you want the best agreement, it might be best to specify that the instrument have a "CIE-conforming d/8° sphere geometry".
- For optimal agreement, specify a single model of d/8° sphere instrument from the same manufacturer.