What is the optical geometry of a spectrophotometer?

The optical geometry of the instrument is important. In some instruments an integrating sphere is used that enables the sample to be illuminated diffusely (from all angles equally) and the reflected light to be collected at at angle roughly perpendicular to the surface of the sample. Alternatively, other instruments illuminate the sample at a certain angle and collect light at another angle. For example, typically the sample may be illuminated at 45 degrees to the surface and light reflected measured at 0 degrees - this is known as 45/0 geometry. The converse to this is 0/45. The sphere-based geometries are known as D/0 and 0/D. It is extremely difficult to correlate measurements taken between instruments if the optical geometry is not identical.

The four CIE standard geometries are:

• diffuse illumination and light collection at the normal, D/0;
• normal illumination and diffuse light collection, 0/D;
• illumination at 45 degrees and light collection at the normal, 45/0;
• normal illumination and light collection at 45 degrees, 0/45.