What is the Difference Between Optical Density and Absorbance?

Optical density and absorbance are both related to the absorption of light when it passes through a medium. However, they differ in the aspects they consider and how they are measured:

Optical Density: Optical density measures the degree to which a medium slows down the speed of transmitted light. It takes into account both the absorption and scattering of light. The optical density of a medium is defined as the logarithmic ratio of the intensity of incident light and the intensity of light transmitted by that region.
Absorbance: Absorbance measures the capacity of a medium or object to absorb light of a specific wavelength. It does not take the refraction of light into account and only considers the amount of light absorbed. The absorbance value can be computed using the formula given by $$\frac{100}{T\%}$$, where $$T\%$$ is the percentage of transmission. An absorbance of 1 means the medium or material absorbs 90% of the light, and an absorbance of 2 means 5 times the light is absorbed.

In summary, the key difference between optical density and absorbance is that optical density measures the absorption and scattering of light, while absorbance measures only the absorption of light. Both concepts are important in understanding the behavior of light when it passes through various media and have applications in fields such as analytical chemistry and spectroscopy.