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Concept
Absorbance
Summary
Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)". Alternatively, for samples which scatter light, absorbance may be defined as "the negative logarithm of one minus absorptance, as measured on a uniform sample". The term is used in many technical areas to quantify the results of an experimental measurement. While the term has its origin in quantifying the absorption of light, it is often entangled with quantification of light which is “lost” to a detector system through other mechanisms. What these uses of the term tend to have in common is that they refer to a logarithm of the ratio of a quantity of light incident on a sample or material to that which is detected after the light has interacted with the sample.
The term absorption refers to the physical process of absorbing light, while absorbance does not always measure only absorption; it may measure attenuation (of transmitted radiant power) caused by absorption, as well as reflection, scattering, and other physical processes. Sometimes the term "attenuance" or "experimental absorbance" is used to emphasize that radiation is lost from the beam by processes other than absorption, with the term "internal absorbance" used to emphasize that the necessary corrections have been made to eliminate the effects of phenomena other than absorption.
The roots of the term absorbance are in the Beer–Lambert law. As light moves through a medium, it will become dimmer as it is being "extinguished". Bouguer recognized that this extinction (now often called attenuation) was not linear with distance traveled through the medium, but related by what we now refer to as an exponential function.
If is the intensity of the light at the beginning of the travel and is the intensity of the light detected after travel of a distance , the fraction transmitted, , is given by: , where is called an attenuation constant (a term used in various fields where a signal is transmitted though a medium) or coefficient.
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