Named after the two scientists Johann Heinrich Lambert (1728-1777) and August Beer (1825- 1863), states the correlation between the absorbance A, the light path traversed, and the concentration of the absorbent substance. “The absorption is proportional to the number of absorbent molecules and to the light path traversed”. The proportionality constant is called relative spectral absorption coefficient K(&lambda) and is a substancespecific function of the wavelength. In the appropriate concentration range and if using monochromatic light, Lambert-Beer's law is reliable with great accuracy.

In physics, light is regarded as an electromagnetic wave propagating (in a vacuum) with the velocity of light. If the wave passes through a medium, its propagation is determined by a material constant (the complex refractive index). Two variables characterise the the wave per unit area). The wavelength of visible light ranges from 400 nm (violet) to 700 nm (red).electromagnetic wave: the wavelength (determining the colour of light) and the radiant flux (determining the energy transported by the wave.

Cross-section of the concentrated light in the sample compartment (where the cell is) of a photometer.

Lambert-Beer's law states that the concentration of a sample can be calculated from the absorption value if the light path is known. This is why cells must be produced with a very high accuracy of light path or the individual light path of a cell must be determined later and must be confirmed by a test certificate.

Light consisting of waves of one wavelength only is called monochromatic. One then speaks of a pure spectral colour. The light from the sun and from artificial light sources (exception: laser) is polychromatic. It contains waves of different wavelengths.

General term for certain cases of light emission. If the excitation is caused by light, it is called photoluminescence. This is the case if molecules are excited by the absorption of optical radiant energy and emit radiation when they return to lower energy levels. Fluorescence: Immediately extinguishing luminescence. Phosphorescence: Luminescence with noticeable persistence.