Optical density is the measure of the transmission of an optical medium for a given wavelength.Higher OD lower transmittence and vice versa e.g; optical density of 1 means 90% of incident light is absorbed.

Optical density is the absorbance of an optical element for a given wavelength λ per unit distance: Where: : l = the distance that light travels through the sample (i.e., the sample thickness), measured in cm Aλ = the absorbance at wavelength λ T = the per-unit transmittance I0 = the intensity of the incident light beam I = the intensity of the transmitted light beam
optical density is the measure of the transmition of an optical medium for a given wave length.