Optical devices play a major role in a wide variety of modern technologies. They are in CD, DVD and Blu-Ray players, fiber-optic cable boxes and optical components. They even have uses in certain biological laboratories, as seen in certain microscopes and spectrometers. When studying the science behind these devices, it is easy to get optical density and absorbance confused since both measure the amount of light "absorbed" when light passes through an optical component, but the two terms have some subtle differences.
TL;DR (Too Long; Didn't Read)
Though optical density and absorbance both measure the absorption of light when that light passes through an optical component, these two terms are not the same. Optical density measures the amount of attenuation, or intensity lost, when light passes through an optical component. It also tracks attenuation based on the scattering of light, whereas absorbance considers only the absorption of light within the optical component. Both optical density and absorbance can be tracked through the use of a spectrometer.
Optical density, sometimes written as OD, is a measurement of a refractive medium or optical component's ability to slow or delay the transmission of light. 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.
In contrast to optical density, absorbance measures the ability of a refractive medium or optical component to absorb light. This sounds incredibly similar but is not quite the same. Where optical density measures the speed of light passing through a medium, absorbance measures how much light is lost over the course of light's passage through the given medium. Optical density also takes the scattering, or refraction, of light into consideration where absorbance does not.
One way both optical density and absorbance are used differently is when studying the concentration of bacteria in a given suspension. Through the use of a spectrometer it is possible to examine the optical density to determine how much bacteria is present within the suspension. But it is only through the measure of absorbance that you can determine how large each of the bacterial molecules within that suspension are. Together, you can use the two measurements to get an accurate idea of the nature of these bacteria, but the information gleaned through one measure cannot be replicated by the other.