What is Optical Density?

Optical density is a term used in the field of optical spectroscopy to describe the propagation of a wave through a material. It is often confused with absorbance as they both measure the absorption of electromagnetic waves by the medium through which they pass, however, they are slightly different.

It is calculated as the logarithmic ratio of the intensity of radiation incident on the material to the intensity of radiation transmitted by the material. This value is the total loss of the material which may be occurring due to the absorption, reflection and scattering of the wave as opposed to the value of absorbance which only calculates the loss due to absorption. The optical density of a material is different for different wavelengths.

Optical density has no relationship with physical density. The former describes the ability of an optical component or material to retard the transmission of light or other electromagnetic waves. A higher optical density indicates how much slower the wave travels through that material.

The refractive index of the material is an indicator of its optical density. The dimensionless refractive index value indicates the number of times slower a light wave would travel in the medium compared to its speed in vacuum.

In spectroscopy, the optical density and absorbance measurements provide information about different properties of the sample. For example, while measuring a suspension of microorganisms with a spectrometer, the optical density measurement helps to calculate the total number of microorganisms in the suspension, however, their individual sizes can only be calculated by measuring the absorbance of the sample.

Sources and Further Reading

• Optical Density – Princeton University
• Measuring the Optical Density of Bacteria – Hofstra University
• Optical Density and Light Speed – Physics Classroom
• PR Photonics
• Light, Linda B., et al. “How Does Light Absorption Intensity Depend on Molecular Size?” Journal of Chemical Education, vol. 71, no. 2, 1994, p. 105. https://pubs.acs.org/doi/abs/10.1021/ed071p105.

This article was updated on 27^th July, 2018.