Optical density (OD) of a medium is the measure of its transmittance for a radiation of particular wavelength. Higher is the value of OD, lower is transmittance. It is expressed as log10 (1/T), where T is the transmittance in percentage.OD is related to refractive index of a material. A higher value of refractive index means higher OD. This means that light travels slower, in a medium with high optical density.