# Optical Density

## Definition

The process of transmission of light or any other electromagnetic radiation is known as optical density. The phenomenon of absorption takes place when a beam of light comes in contact with the absorbing atoms. The absorption depends upon the thickness of the sample and the concentration of absorbing atoms. The absorption and emission of these electromagnetic waves depend upon their wavelength that includes fundamental particles like atoms, electrons, ions, etc.

It is also called absorbance and is the ratio of incident radiant power and transmitted radiant power. Incident radiant power is the power of light hitting the material, and transmitted radiant power is the power of light exiting the material after hitting the surface.

In simpler words, the ability to block light is known as optical density. Optical density is a dimensionless quantity and does not have any scientific unit.

The expression for the optical density is given by-
Log10 (1/T), where T is the transmittance.

Note- 1) Since optical density is inversely proportional to transmittance, the higher the optical density, the lower will be the transmittance.
2) The ten times loss in the optical density is equivalent to the loss in transmission in decibels. For instance, the optical density of 0.5 signifies the loss of transmission of 5dB.

While making the eyewear, optical density is used in the laser industry that uses Class 3b and Class 4 lasers to protect the eyes from injury. Class 4 lasers are strong enough to cause permanent damage to the eyes. These lasers can also cause skin burns and may cause fire hazards. The optical density of eyewear depends upon the wavelength of the light. Therefore, the eyewear is rated based on the intensity of light it reflects and the protection of eyes from injury. It is also used to make personal protective equipment absorb a specific wavelength of light and save the person from getting damaged by the light.

## Relation between optical density and absorbance

1. They both are not the same. However, both measure the absorption of light when the light passes through an optical medium.
2. Optical density is the ability of a material to transmit light through it. It measures the speed of light passing through a substance and is affected by the wavelength of light. At the same time, absorption is the transfer of wave energy when the wave passes through a medium. Absorption of light occurs in a denser medium.

## Measurement of optical density

Optical density is measured at the maxima of the absorbance spectra. This is because, in that region, the chances of absorbance with the change in wavelength are the least. This measurement is essential to study parameters like biomass production, the concentration of cells, etc.

Absorbance A = - log10 (I / Io)
Here, I = intensity of light passing through the sample
Io = initial intensity of light

## Optical turbidity

Optical turbidity is used to measure the microbial biomass and cell processes in mammals. The scattered light by the culture that is tested depends upon the species, concentration of cells, and the strain of microorganisms. Growth calibration must be done before experimenting each time. During a fermentation process, the measurement of cell density is a significant parameter. It is helpful to monitor the fermentation process using a sterile probe inserted directly into the fermenter. These tests detect particulates of every kind present in the sample – debris and nonviable cells. However, these measurements can also be made using a spectrophotometer to ease the studies.

## Spectrophotometer

The optical density of a spectrophotometer is given by,
OD = A / L
Here, L = thickness of the sample
A = absorbance of light

Example
The intensity of light transmitted is 1/500 and the initial intensity of light of 2000 units. Find the optical density.
Solution

We know, optical density = log10 (I / Io)
= log10 (2000 / 500) = log10 4
= 2 log10 10= 2
Therefore, the optical density is 2.

## Frequently asked questions on absorption

Ques: What is the meaning of 1 absorbance?
Ans: Absorbance can range from 0 to infinity. 0 absorbance means the light does not pass through the medium. Absorbance 1 means the medium absorbs 90% of the light. Absorbance 2 means the medium absorbs 99% of the light. Absorbance 3 means 99.9% and so on.

Ques: How to calculate absorption?
Ans: Absorption is calculated by the intensity of light passing through a medium when a light ray is an incident. It is also calculated by the intensity of the ray of light reflected by any surface. One of the most suitable examples of absorption is the blackbody. It absorbs all the wavelengths of light. When heated, it radiates light of varying wavelengths in the form of energy.

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