The illumination value is a measure of the light, originating from the LED source, that reaches the light detector. The illumination value is reported in relative units because all the measurements made with the Polarimeter are made in reference to a blank sample.
The illumination value should not be used as a quantitative value for this device. If you choose to use it for this purpose, note that the illumination value is very sensitive to change. These slight variations can be brought on by several different variables in the sample, including: heterogeneity (such as particulates and bubbles), height of the sample in the cell, concentration of the sample, and aberrations in the glass cell or pathlength.