Liquids or other objects appear colored because theyreflect or transmit certain colors and in turn swallow (absorb) others. With a so-called photometer, those colors (wavelengths) can be determined, which are absorbed by liquids. The basic principle is simple: with a LED of a certain color you first shine through a cuvette filled with water or another solvent. A photodiode measures the incoming light intensity and converts it into a proportional voltage U0. This value is noted. Thereafter, a cuvette with the liquid to be examined is placed in the beam path and again measures the light intensity or voltage U. The transmission factor in percent is then simply calculated by T = U / U0 * 100. For the absorption factor A, then A = 100 T.
This measurement is repeated with differently colored LEDs and determines in each case T or A as a function of the wavelength (color). If you do this with enough LEDs, you get an absorption curve.”

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