Absorbance is a measure of the amount of light with a specified wavelength that a given material prevents from passing through it. Absorbance does not necessarily measure the amount of light that the material absorbs. For example, absorbance would also include light that is dispersed by the sample material. Absorbance may be calculated from the transmittance, which is the fraction of light that passes through the test material.

### Figuring It Out

Measure the transmittance of light. This is the amount of light that passes through a test material and may be expressed as (I/Io) where I is the intensity of the light after it passes through the sample material and Io is the intensity before it passes through the sample.

Define absorbance mathematically. This can be given as Ay = -log10(I/Io) where Ay is the absorbance of light with wavelength y and I/Io is the transmittance of the test material.

Observe that absorbance is a pure number without units of measure. Absorbance is based on the ratio of two intensity measurements, so the resulting value has no units. Absorbance values are frequently reported in "Absorbance Units" but these are not true units.

Interpret the absorbance value. Absorbance can range from 0 to infinity such that an absorbance of 0 means the material does not absorb any light, an absorbance of 1 means the material absorbs 90 percent of the light, an absorbance of 2 means the material absorbs 99 percent of the light and so on.

Define absorbance as Ay = -ln(I/Io) outside of spectroscopy. Other fields of study may use the natural logarithm (ln) instead of the base 10 logarithm to express absorbance.