Absorbance, What is Absorbance, Definition of Absorbance, Define Absorbance

In spectroscopy, the absorbance A is defined as
$A_\lambda = -\log_{10}(I/I_0)\,$ ,
where I is the intensity of light at a specified wavelength λ that has passed through a sample (transmitted light intensity) and I0 is the intensity of the light before it enters the sample or incident light intensity. Absorbance measurements are often carried out in analytical chemistry, since the absorbance of a sample is proportional to the thickness of the sample and the concentration of the absorbing species in the sample, in contrast to the transmittance I / I0 of a sample, which varies logarithmically with thickness and concentration.
Outside the field of analytical chemistry, e.g. when used with the Tunable Diode Laser Absorption Spectroscopy (TDLAS) technique, the absorbance is sometimes defined as the natural logarithm instead of the base-10 logarithm, i.e. as
$A_\lambda = -\ln(I/I_0)\,$ ,