The optical density of any material means how dense an object is to pass light through it. It is measured in terms of the speed of light through that medium. Physical density is different and is measured as a ratio of mass per unit volume. Optical density is a property shown by transparent materials, as opaque materials don’t allow light to pass through them. The extent to which light is refracted away from the normal is called optical density. In this article, we discuss the meaning of optical density and how to measure it and also discuss some common applications of optical density.
Meaning of Optical density
The optical density of a material can be defined as the ratio of incident radiant power to transmitted radiant power. It is the measure of how opaque material is to transmit light through it. Like, for example, we can compare two materials for transparency based on the ratio of light transmitted through it. This concept is used in the estimation of concentration biomass in a solution. The more the concentration of biomass, the more optical dense will be the solution.
Formula of optical density
Measurement of optical density
Optical density is measured as a logarithmic measurement of the total light to transmit light through the material.
For absorbance measurements, the optical density (O.D.) is a logarithmic measurement of the percent transmission (%T) and it can be represented by the equation, A = log10 100 / %T.
Optical density meters or analyzers are used to measure optical density.
How to calculate concentration as a function of optical density?
In the case of homogenous colloidal solutions, the optical density is a function of the solution’s concentration. Therefore, optical density can also be expressed as the molar absorbance coefficient (the absorbance of the solution per unit length per mole of solute) x the molar concentration of the solution x the path length of the light.
Relationship between light transmittance and optical density
The percentage of light transmitted is inversely proportional to the optical density. Optical density has values between 0 and 4.
Applications of optical density
- Optical density (OD) is used as a rapid proxy measurement of suspended biomass concentration.Â
- Optical density measurements are most commonly used in microbiology laboratories to assess the growth of microbes in terms of OD.
- Optical density is used both qualitatively as the turbidity of culture and quantitatively as a measure of the intensity of light transmitted along a path through the culture of known path length.
- OD can also be used to estimate levels of pigments within cells when the chosen wavelength corresponds to chromophore absorption.
Research
You can read this article titled “Quantitative Monitoring of Mycelial Growth of Aspergillus fumigatus in Liquid Culture by Optical Density” to understand the role of optical density in microbiology. In this article, the authors have used optical density to quantify the dose-response of hyphal growth(a type of filamentous fungi) to antifungal agents. [Link: https://journals.asm.org/doi/10.1128/spectrum.00063-21]
