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Neutral density filters are categorized by their optical density (D) which is defined as the logarithm to the base 10 of the reciprocal of the transmitted radiant power (T):

D=log10 1/T or T=10-D

Absorptive type neutral density filters attain their density by absorbing light within the substrate, whereas the metallic type attain their density by both reflection and absorption. In applications where neutral density filters are used as calibration filters, it is very important that these filters reflect very little light. Secondary reflections, or stray light, can cause an additive effect to the calibration and henceforth make the readings invalid. The absorptive type neutral density filter is preferred over the metallic type since the reflections are very low. To further eliminate the possibility of any unwanted reflected light, Andover coats both surfaces of all absorptive neutral density filters with a visible broadband anti-reflection coating. This coating reduces the surface reflection to approximately 0.5% per surface. Since the glass absorbs the light within the substrate, the optical density is very dependent upon the thickness of the substrate. Please refer to the specifications listed in the table on Andover Corporation's website to obtain the approximate thickness of each density filter.