Difference Between Absorbance and Transmittance

Main Difference – Absorbance vs. Transmittance

Absorbance and transmittance are two related, but different quantities used in spectrometry. The main difference between absorbance and transmittance is that absorbance measures how much of an incident light is absorbed when it travels in a material while transmittance measures how much of the light is transmitted. Due to the way they are defined, the two are not complementary quantities: i.e., adding transmittance to absorbance directly does not give the total incident light.

As light passes through a material, it is absorbed by molecules in the material. Consequently, the intensity of light decreases exponentially with distance as the light passes through the material. Transmittance through a sample solution is is easily measured by measuring the intensities of incident and transmitted light. Using the value for transmittance, it is then possible to calculate the absorbance of the sample.

What is Transmittance?

Transmittance ( $T$ ) is a measurement of how much light passes through a substance. The higher the amount of light that passes through, the larger the transmittance. Transmittance is defined as the ratio of the intensity of incident light: intensity of transmitted light i.e. if the intensity of incident light is $I_0$ and the intensity of transmitted light is $I$ , then

$T=\frac{I}{I_0}$

At times, this fraction may be represented as a percentage, where it is called the percentage transmittance ( $\%T$ ).

What is Absorbance?

Absorbance ( $A$ ) is defined as:

$A=\mathrm{log_{10}\left( \frac{1}{T}\right)}$

Consequently, the absorbance can also be given in terms of the percentage transmittance:

$A=2-\mathrm{log_{10}\left( \%T\right)}$

According to Beer-Lambert law, the absorbance of light, as it passes through a solution, is directly proportional to the path length of light through the material ( $l$ ) and the concentration ( $c$ ). So, we can write,

$A=\epsilon lc$

where $\epsilon$ is a constant called the molar absorptivity. This constant has a specific value for a given substance, provided the temperature of the substance and the wavelength of light passed through it are kept unchanged.

This is an extremely useful relationship which allows concentrations of unknown solutions to be found by measuring the absorbance of light through a sample.

If we make a solution, allow light to pass through it and plot how the transmittance changes as we change the concentration of the solution (while keeping the path length travelled by light unchanged), we get an exponential relationship between transmittance and concentration:

Transmittance vs. Concentration

However, if we calculate the corresponding absorbance values and then plot a graph of absorbance vs. concentration, we will get a straight line through the origin, as predicted by the Beer-Lambert law:

Absorbance vs. Concentration

If the gradient of this graph is $m$ , then from Beer-Lambert law we have,

$m=\epsilon l$

Then we can calculate the value of $\epsilon=m/l$ using the length $l$ through which light has travelled.

Once we have calculated $\epsilon$ , we can use it to measure concentrations of unknown solutions of the substance using the same setup (i.e., maintaining temperature, the wavelength of light and the path length of light the same).

In labs, a spectrophotometer can be used to measure the absorbance of light by a sample.

A Spectrophotometer