Editing Photometer (section)

==UV and visible light transmission photometry==
{{Main|Absorption spectroscopy}}
These are optical instruments for measurement of the absorption of light of a given wavelength (or a given range of wavelengths) of coloured substances in solution. From the light absorption, [[Beer's law]] makes it possible to calculate the concentration of the coloured substance in the solution. Due to its wide range of application and its reliability and robustness, the photometer has become one of the principal instruments in [[biochemistry]] and [[analytical chemistry]]. Absorption photometers for work in aqueous solution work in the ultraviolet and visible ranges, from wavelength around 240&nbsp;nm up to 750&nbsp;nm.

The principle of [[spectrophotometer]]s and filter photometers is that (as far as possible) [[monochromatic]] light is allowed to pass through a container (cell) with optically flat windows containing the solution. It then reaches a light detector, that measures the intensity of the light compared to the intensity after passing through an identical cell with the same solvent but without the coloured substance. From the ratio between the light intensities, knowing the capacity of the coloured substance to absorb light (the absorbency of the coloured substance, or the photon cross section area of the molecules of the coloured substance at a given wavelength), it is possible to calculate the concentration of the substance using [[Beer's law]].

Two types of photometers are used: spectrophotometer and [[Filter (optics)|filter]] photometer. In  spectrophotometers a monochromator (with [[prism (optics)|prism]] or with [[grating]]) is used to obtain [[monochromatic]] light of one defined wavelength. In filter photometers, optical filters are used to give the monochromatic light. Spectrophotometers can thus easily be set to measure the absorbance at different wavelengths, and they can also be used to scan the spectrum of the absorbing substance. They are in this way more flexible than filter photometers, also give a higher optical purity of the analyzing light, and therefore they are preferably used for research purposes. Filter photometers are cheaper, robuster and easier to use and therefore they are used for routine analysis. Photometers for [[microtiter plate]]s are filter photometers.