What is the basic principle from visible spectrophotometry?

Spectrophotometry is an important branch of spectroscopy that focuses on the technique of measurement. In this technique, the amount of light that a sample absorbs at a particular wavelength is measured and used to determine the concentration of the sample by comparison with appropriate standards or reference data. The most useful measure of light absorption is the absorbance (A), also commonly called the optical density (OD). The absorbance is defined as A = log I0 / I where I0 is the intensity of light that is incident on the sample and I is the intensity of light that is transmitted by the sample. The absorbance of a sample can be related to the concentration of the absorbing species through Beer's law: A = ε cl where c is concentration, usually measured in moles per liter; l is the length of the light path, usually 1 cm; and ε is a proportionality constant known as the molar extinction coefficient, with the units of liters per mole per centimeter. The value of ε is a function of both the particular compound being measured and the wavelength.