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Optical density, or O.D. as is it is commonly known, refers to the attenuation factor of the filter to the base 10. Therefore, a filter with an OD of 2 attenuates the laser by a factor of 10 to the 2nd or 100, an OD of 3 attenuates the laser beam by a factor of 1,000, an OD of 4 by a factor of 10,000.
