WANI Dictionary

Optical density (OD)

Optical density (OD) is a measure of transmittance calculated by the following equation:

OD_λ = –Log₁₀[T_λ/I_λ]

where

• OD_λ = Optical density at the wavelength λ
• T_λ = Intensity of transmittance at the wavelength λ
• I_λ = Intentisy of incident light at the wavelength λ

Because I_λ is always higher than T_λ, Log₁₀[T_λ/I_λ] is always negative. Thus, OD defined as –Log₁₀[T_λ/I_λ] is always positive.

Definitions in the literature

• Optical density (OD) is a method of quantifying microbial growth in relation to the turbidity of an inoculum, i.e., its optical density [1].
• A measure of the transmittance through an optical medium. ... In spectroscopy, optical density is the measure of absorbance, and is defined as the ratio of the intensity of light falling upon a material and the intensity transmitted [2].
• It is not clear whether this definition in [3] is valid. “It measures the speed of light through a substance, affected primarily by the wavelength of a given light wave. The slower that light is able to travel through a given medium, the higher the optical density of the medium.”