The key difference between optical density and absorbance is that the optical density measurement takes both, the absorption and scattering of light, into consideration whereas absorbance measurement takes only the absorption of light into consideration.
Both optical density and absorbance are related terms. Optical density (OD) is the degree to which a refractive medium retards transmitted rays of light while absorbance is a measure of the capacity of a substance to absorb light of a specified wavelength.
CONTENTS
1. Overview and Key Difference
2. What is Optical Density
3. What is Absorbance
4. Similarities Between Optical Density and Absorbance
5. Side by Side Comparison – Optical Density vs Absorbance in Tabular Form
6. Summary
What is Optical Density?
Optical density (OD) is the degree to which a refractive medium retards transmitted rays of light. In other words, optical density is a term that describes the propagation of a light wave through a substance. The measurement of optical density is taken as the logarithmic ratio between incident radiation on substance and the radiation transmitted by the substance. Therefore, optical density influences the speed of light through a substance. The main factor affecting the optical density is the wavelength of the light wave.
It is important to note that optical density has no relationship with the physical density of the substance. Optical density expresses the tendency of the atoms or molecules of a substance to retain the absorbed energy. This retention occurs via electronic vibrations. Therefore, if the optical density of a substance is high, the speed of light through this substance is low (since light waves move slowly). Furthermore, optical density can be measured using the spectrometer.
Figure 1: A Graph Showing Optical Density of a Ribosome Sample
The refractive index of a material indicates the optical density of that substance. To be more specific, the ratio between the speed of light in a vacuum and the speed of light through the substance gives the refractive index. In other words, this explains how slow the speed of light in a substance in comparison to that in a vacuum.
What is Absorbance?
Absorbance is a measure of the capacity of a substance to absorb light of a specified wavelength. Specifically, it is equal to the logarithm of the reciprocal of the transmittance. Unlike optical density, absorbance measures the quantity of light absorbed by a substance.
Furthermore, spectroscopy measures the absorbance (using colorimeters or spectrophotometer). The absorbance is a dimensionless property, unlike most other physical properties. There are two ways to explain absorbance: as the light absorbed by a sample or as the light transmitted through a sample. The equation for calculation of absorbance is as follows:
A = log10(I0/I)
Figure 2: Incident Radiation and Transmitted Radiation
While A is absorbance, I0 is the radiation transmitted from the sample, and I is the incident radiation. This following equation is also similar to the above equation, in terms of transmittance (T).
A = -log10T
What are the Similarities Between Optical Density and Absorbance?
- Both optical density and absorbance measures the ability of a sample to retain electromagnetic radiation pass through the sample.
What is the Difference Between Optical Density and Absorbance?
Optical Density vs Absorbance |
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| Optical density is the degree to which a refractive medium retards transmitted rays of light. | Absorbance is a measure of the capacity of a substance to absorb light of a specified wavelength. |
| Measurement | |
| The optical density measurement takes both, the absorption and scattering of light, into consideration. | The absorbance measurement takes only the absorption of light into consideration. |
Summary – Optical Density vs Absorbance
Both optical density and absorbance are related terms in analytical chemistry. The key difference between optical density and absorbance is that the optical density is measured considering the absorption and scattering of light whereas absorbance is measured considering only the absorption of light.
Reference:
1. Delta Optical Thin Film A/S. “What Is Optical Density?” LinkedIn SlideShare, 7 July 2014, Available here.
2. “Absorbance.” Wikipedia, Wikimedia Foundation, 10 Apr. 2018, Available here.
3. AZoOptics, Written by. “What Is Optical Density?” AZoOptics.com, 21 Aug. 2014, Available here.
4. Helmenstine, Anne Marie, Ph.D. “Absorbance Definition.” ThoughtCo, Aug. 10, 2017, Available here.
Image Courtesy:
1. “Optical density of ribosome sample” By Vossman – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Transmittance” By Marmot2019 – Own work (CC BY-SA 4.0) via Commons Wikimedia