Absorbance vs. Transmittance

The main difference between absorbance and transmittance in the case of spectrometry can be perfectly defined by the Beer’s Law which states that if all the light passes through a solution without any absorption then transmittance is 100% while absorption is 0% while if all the light is absorbed then transmittance is 0% and absorption is 100%.

Comparison Chart

Basis Absorbance
Definition The amount of light that is absorbed when it travels through a material. It is the amount of energy which is being transmitted during the process.
Co-relation When transmittance is 0% then absorption is 100%. When transmittance is 100%, then absorption is 0%.
Value Always less than one Value is greater than Absorbance
Measurement Can only be measured by laser instruments. Can be measured by the normal instrument.
Dependence depends on Transmittance for calculation. depends on Absorption for occurring.
Types Spectral Absorbance Hemispherical Transmittance, Spectral directional transmittance, Spectral hemispherical transmittance and Directional transmittance.

What is Absorbance?

To get a clear understanding of this term, one has to look at the absorption spectrum. Let us take a normal structural element which has a nucleus and is made up of protons and neutrons while electrons are orbiting around them. The main focus is that the faster an electron will move the further away it will be from the nucleus. It is a known fact that they cannot reach any level on their own but require a specific amount of rotation to reach any level. All the electrons require to absorb some energy for that purpose and since the wave is also quantized it can be written that electrons absorb the photons present with the energies. So it can be said that photons have been absorbed by the electrons. Therefore, it can be defined as the amount of light that is absorbed when it travels through a material. This term is closely related to attenuance and can be redefined as the total attenuation of the transmitted light power in a material. Absorbance can be caused due to several processes such as reflection, scattering, and others. The value of absorbance is always less than one. It has some equations which will require proper analysis and explanation to be understood.

There is no manual instrument that can calculate the amount of absorbance since it is very low therefore laser-based techniques are used which can be considered accurate. There is a method through which it can be measured, and that is known as absorption spectroscopy.


What is Transmittance?

It is perhaps the most underrated yet important term used in spectroscopy as explained in the first paragraph that electrons always require some sort of energy to move and that movement results in absorption, that is not the only phenomenon that takes place during this process. When the electrons are moving, they also emit some sort of energy which is necessary for them since they are in motion. As we know that every action has an equal reaction, similarly the energy being released by the election and the nucleus is known as transmittance. Explaining the term in simple words, it is the amount of energy which is being transmitted during the process. It is the total light that had passed through the material when it was under observation. The more the amount of light that passes through the substance the higher will be the value of transmittance.

There are lengthy equations to prove this point which will be out of the scope of this article. The known fact is that whenever there will be some sort of transmission in a system, there will always be absorption. The amount can vary from 0-100% depending on the situation. It is the quantity which can easily be measured and has instruments and equations for that purpose. It is to be noted that the phenomenon is dependent on other quantities such as absorption, scattering, reflection and others. Radiant flux transmitted by the surface and the radiant flux received by the surface can give the value of hemispherical transmittance.


Key Differences between Absorbance and Transmittance

  1. Both terms form the basis of the subject of spectrometry and are dependent on each other for various actions.
  2. According to Beer’s Law when the transmittance is 100% then absorption will be 0%, and when the transmittance is 0%, the absorption will be 100%.
  3. Absorption cannot be easily measured and requires laser-based technologies to perform the task while transmittance can easily be measured with the help of instruments.
  4. The value of absorption is always low and mostly less than 1 while the value of transmittance is relatively high.
  5. Transmittance depends on Absorption for occurring while absorption depends on transmittance for calculation.
  6. There is one main type of absorption which is known as spectral absorption while there are four main types of transmittance which are known as hemispherical transmittance, Spectral directional transmittance, Spectral hemispherical transmittance and Directional transmittance.

Video Explanation

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