Difference Between Absorbance and Transmittance

Absorbance vs Transmittance

Absorbance and transmittance are two very important concepts discussed in spectrometry and analytical chemistry. Absorbance can be identified as the amount of light which is absorbed by a given sample. Transmittance can be recognized as the amount of light passed through that sample. Both of these concepts are very important in fields such as analytical chemistry, spectrometry, quantitative and qualitative analysis, physics and various other fields. It is vital to have a proper understanding in the concepts of absorbance and transmittance in order to excel in such fields. In this article, we are going to discuss what are absorbance and transmittance, their definitions, the applications of absorbance and transmittance, the similarities between these two, the connection between absorbance and transmittance, and finally the difference between absorbance and transmittance.

What is Absorbance?

In order to understand the concept of absorbance, one must first understand the absorption spectrum. An atom consists of a nucleus, which is made of protons and neutrons, and electrons that are orbiting around the nucleus. The orbit of the electron depends on the energy of the electron. Higher the energy of the electron, farther away from the nucleus it would orbit. Using quantum theory it can be shown that electrons cannot just get any energy level. The energies the electron can have are discrete. When a sample of atoms is provided with a continuous spectrum over some region, the electrons in the atoms absorb specific amounts of energies. Since the energy of an electromagnetic wave is also quantized, it can be said that the electrons absorb photons with specific energies. At the spectrum taken after the light is passed through the material, certain energies appear to be missing. These energies are the photons that have been absorbed by the atoms.

Absorbance is defined as Log­10 (I0/I), where I0 is the intensity of the incident light ray, and I is the intensity of the light ray which has been passed through the sample. The light ray is monochromatic and set to a specified wavelength. This method is used on spectrophotometers. The absorbance depends on the concentration of the sample and the length of the sample.

The absorbance of a solution is linearly proportional to the concentration according to the Beer – Lambert law, if the I0/I value lies between 0.2 and 0.7. This is a very useful law in spectroscopic methods used in quantitative analysis.

When absorbance is defined in fields other than chemistry, it is defined as Log­e (I0/I).

What is Transmittance?

Transmittance is the opposite quantity of absorbance. Transmittance gives a measurement of the light that passed through the sample. The value measured in most of the practical spectroscopic methods is the transmittance intensity.

The transmittance intensity divided by the source intensity gives the transmittance of the sample.

What is the difference between Transmittance and Absorbance?

  • Transmittance is a directly measurable quantity whereas absorbance must be calculated using the transmittance measurement.
  • Transmittance is a measurement of the amount of light passing through the sample, but absorbance is a measurement of the amount of light absorbed by the sample.