What Is Absorbance?

Absorbance is a term commonly used by chemists in relation to IR spectroscopy, including both mid- and near-infrared spectroscopic analyses.

Absorbance is closely related to transmittance and reflectance. Absorbance is the amount of light that remains in the sample, as it has been absorbed by the sample. The absorptions of infrared wavelengths are related to the vibrational energy modes of the molecules it contains, and to lattice vibrations of certain materials. Absorbance is calculated as:

A = - log 10   S sample - S dark S reference - S dark

Absorbance is typically used by chemists, who are not interested in the light itself, but rather on the sample that is being measured. Absorption is a property of the chemical compounds that are of interest. The absorption spectra are, effectively, infrared region fingerprints, which can be used to identify and quantify properties of the samples. One property of the infrared absorptions is that their relative intensities are different with changing concentrations. This nonlinearity is partially eliminated by using the log10 calculation, which makes the data more linear, but not fully linear over large concentration differences.


To save absorbance values in SensorControl, both dark signal and reference signal values must be saved to memory of the program.