The Beer-Lambert law relates absorbance to an absorbing species' concentration and path length by in a homogeneous medium using sufficiently narrow-band light. The coefficient depends on the species, wavelength and chemical solution conditions. This makes Ellman's reagent useful for quantitative thiol assays once its released colored species has been calibrated. [ref]
https://goldbook.iupac.org/terms/view/B00626. IUPAC Gold Book: Beer-Lambert law and validity conditions
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The Beer–Lambert law, also known as Beer's law, is a fundamental relationship in chemistry that describes how the absorption of light by a substance varies with the concentration of the substance and the path length of the light through the material. It is widely used in various applications, such as spectroscopy and analytical chemistry.