If you have read the page about how an absorption spectrometer works, you will know that it passes a whole series of wavelengths of light through a solution of a substance (the sample cell) and also through an identical container (the reference cell) which only has solvent in it.For each wavelength of light passing through the spectrometer, the intensity of the light passing through the reference cell is measured. This is usually REFERRED to as Io - that's I for Intensity.

The intensity of the light passing through the sample cell is also measured for that wavelength - given the symbol, I.

If I is less than Io, then obviously the sample has absorbed some of the light. A simple bit of maths is then DONE in the computer to convert this into something called the ABSORBANCE of the sample - given the symbol, A.

For reasons to do with the form of the Beer-Lambert Law (below), the relationship between A (the absorbance) and the two intensities is given by:

On most of the diagrams you will come across, the absorbance ranges from 0 to 1, but it can go higher than that.

An absorbance of 0 at some wavelength means that no light of that particular wavelength has been absorbed. The intensities of the sample and reference beam are both the same, so the ratio Io/I is 1. LOG10 of 1 is zero.

An absorbance of 1 happens when 90% of the light at that wavelength has been absorbed - which means that the intensity is 10% of what it would otherwise be.