What is the Difference Between Absorbance and Transmittance?

The main difference between absorbance and transmittance lies in the way they describe the interaction of light with a substance. Here are the key differences:

• Transmittance:
• Represents the fraction of incident light that is transmitted through the substance.
• Defined as T = I/Io, where I is the transmitted light and Io is the incident light.
• Can be expressed as a percentage (%T) by multiplying T by 100.
• Indicates the amount of light that passes through the sample.
• Absorbance:
• Represents the amount of light absorbed by the sample.
• Calculated from the transmittance (T) using the formula: Absorbance = 2 – log(%T).
• Also referred to as "optical density".
• Indicates how much of the light the sample absorbed.

Bothabsorbance and transmittance are measurements used in spectrophotometry, which is useful for determining the identity of an unknown substance and, with the use of a set of standards, determining a substance's concentration in a sample. While transmittance is a directly measurable quantity, absorbance must be calculated using the transmittance measurement.

Comparative Table: Absorbance vs Transmittance

Here is a table that highlights the differences between absorbance and transmittance:

In summary, transmittance is a measure of how much light passes through a substance, while absorbance is a measure of how much light is absorbed by a substance. These two properties are related through the Beer-Lambert law, which states that the absorbance of a solution is directly proportional to the path length of light through the material and the concentration of the absorbing substance.