The 191 Gonioreflectometer Heads measure the spectral reflectance of surfaces over the range of 380 to 1100 nanometers. Different model cover both specular reflectance, and diffuse reflectance, or both.  The illumination and collection microscopes are mounted in a calibrated semi-circular sector assembly with independent angular adjustment capable of measuring angles down to 20 degrees included angle, 10 degrees incident angle. Small area illumination and collection viewing allow the unit to separate front and back surface reflections of thin samples.

A quarts halogen lamp provides illumination through the illumination microscope. Reflected light from the test piece is collected in the collection microscope integrating cavity. The 191 is coupled via a fiber optic probe to the Gamma Scientific RadOMA spectroradiometer.

The 191C comes with a viewing eyepiece that is inserted in place of the integrating cavity for checking system alignment and a low reflectance glass sample traceable to N.I.S.T. Standards. Specular reflectance calibration data is provided in increments of 10nm from 300 to 2000 manometers at angles of an incidence of 10 to 45 degrees in 5 degree increments. They are provided in the form of ASCII text files directly compatible with Light Touch Software. Tabular listings are also made available.