Absorbance is defined as "the logarithm of the ratio of incident to transmitted radiant power through a sample (excluding the effects on cell walls)".
[2] The term is used in many technical areas to quantify the results of an experimental measurement.
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.
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.
is called an attenuation constant (a term used in various fields where a signal is transmitted though a medium) or coefficient.
The amount of light transmitted is falling off exponentially with distance.
If a size of a detector is very small compared to the distance traveled by the light, any light that is scattered by a particle, either in the forward or backward direction, will not strike the detector.
(Bouguer was studying astronomical phenomena, so this condition was met.)
as a function of wavelength will yield a superposition of the effects of absorption and scatter.
Consequently, this is often referred to as absorption spectroscopy, and the plotted quantity is called "absorbance", symbolized as
A common expression of the Beer's law relates the attenuation of light in a material as:
There is, however, a property called absorbing power which may be estimated for these samples.
The absorbing power of a single unit thickness of material making up a scattering sample is the same as the absorbance of the same thickness of the material in the absence of scatter.
[5] In optics, absorbance or decadic absorbance is the common logarithm of the ratio of incident to transmitted radiant power through a material, and spectral absorbance or spectral decadic absorbance is the common logarithm of the ratio of incident to transmitted spectral radiant power through a material.
Many people, including scientific researchers, wrongly state the results from absorbance measurement experiments in terms of these made-up units.
[7] Absorbance is a number that measures the attenuation of the transmitted radiant power in a material.
where According to the Beer–Lambert law, T = 10−A, so and finally Absorbance of a material is also related to its decadic attenuation coefficient by
An instrument must be calibrated and checked against known standards if the readings are to be trusted.
Many instruments will become non-linear (fail to follow the Beer–Lambert law) starting at approximately 2 AU (~1% transmission).
It is also difficult to accurately measure very small absorbance values (below 10−4) with commercially available instruments for chemical analysis.
In such cases, laser-based absorption techniques can be used, since they have demonstrated detection limits that supersede those obtained by conventional non-laser-based instruments by many orders of magnitude (detection has been demonstrated all the way down to 5×10−13).
The theoretical best accuracy for most commercially available non-laser-based instruments is attained in the range near 1 AU.
The path length or concentration should then, when possible, be adjusted to achieve readings near this range.
Typically, absorbance of a dissolved substance is measured using absorption spectroscopy.
The transmitted spectral radiant flux that makes it through the solution sample is measured and compared to the incident spectral radiant flux.
To use this machine, solutions are placed in a small cuvette and inserted into the holder.
The machine is controlled through a computer and, once it has been "blanked", automatically displays the absorbance plotted against wavelength.
Some filters, notably welding glass, are rated by shade number (SN), which is 7/3 times the absorbance plus one:[10]