Absorbance from Transmittance

What is this formula?

This formula converts transmittance into absorbance, one of the most important quantities in spectrophotometry. Absorbance provides a logarithmic measure of how strongly a sample attenuates light passing through it.

When to use it

Use this formula when a spectrophotometer reports transmittance and absorbance is required for quantitative analysis, Beer–Lambert Law calculations, calibration curves, or concentration determination.

Example

A sample has a transmittance of:

T = 0.25

Formula:

A = -log10(T)

Substitution:

A = -log10(0.25)

Result:

A = 0.602

The absorbance of the sample is 0.602.

Applications

- UV-Visible spectrophotometry

- Beer-Lambert Law calculations

- Calibration curve generation

- Concentration determination

- Pharmaceutical analysis

- Environmental monitoring

- Biochemical assays

Note

Absorbance is a logarithmic quantity. Because of this relationship, equal changes in absorbance do not correspond to equal changes in transmitted light. Most spectrophotometric quantitative analyses are performed using absorbance rather than transmittance because absorbance is approximately proportional to concentration under Beer-Lambert conditions.