The optical density of a substance is the logarithmic ratio of the intensity of transmitted light to the intensity of the incident light passing through the substance. It is otherwise measured as the absorbed radiation of the corresponding wavelength. Optical density refers to the absorbance of a substance.
The optical density of a substance is also referred to as the index of refraction of the substance. This indicates the number of times the speed of light is reduced in that substance compared to its speed in vacuum. The speed of light in vacuum is 3 x 10^8 meters per second.