What is absorbance?

Absorbance (A), also known as optical density (OD), is the quantity of light absorbed by a solution. Transmittance is the quantity of light that passes through a solution. Absorbance and % transmittance are often used in spectrophotometry and can be expressed by the following:

Absorbance equation

A = Log10 (I0/I)

where I0 is the intensity of the incident light, and I is intensity of that light after it passed through the sample

T = I/I0    and    %T = 100 (T)

The equation that allows one to calculate absorbance from % transmittance is

A = 2 - log10 (%T)


Determine concentration using the Beer-Lambert Law

The concentration of a sample can be calculated from its absorbance using the Beer–Lambert law, which is expressed as follows:

A = ε * c * p

Where ε is the molar absorptivity, or molar extinction coefficient, in L mol-1 cm-1
c is the concentration of  the solute in solution, in mol/L
p is the path length of the sample,  in cm, for example 1 cm for a cuvette



Ultraviolet (UV) measurements in microplates became possible when Molecular Devices introduced the first UV-capable microplate reader. Since then, the microplate measurements of DNA, RNA, and proteins that this enabled have become very popular. Learn more about how absorbance is measured, and some key applications that utilize absorbance.


Resources of Absorbance

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