Photometer

A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum.

The relative luminous fluxes can then be calculated as the illuminance decreases proportionally to the inverse square of distance.

Due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low.

The light sensing element in photon counting devices in NIR, visible and ultraviolet wavelengths is a photomultiplier to achieve sufficient sensitivity.

In airborne and space-based remote sensing such photon counters are used at the upper reaches of the electromagnetic spectrum such as the X-ray to far ultraviolet.

Conversely, radiometers are typically used for remote sensing from the visible, infrared though radio frequency range.

This type of measurement has mainly practical applications, for instance in the paint industry to characterize the colour of a surface objectively.

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.

In spectrophotometers a monochromator (with prism or with grating) is used to obtain monochromatic light of one defined wavelength.

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.

Spectrophotometry in infrared light is mainly used to study structure of substances, as given groups give absorption at defined wavelengths.

Measurement in aqueous solution is generally not possible, as water absorbs infrared light strongly in some wavelength ranges.

The substance being tested is thoroughly mixed with specially purified KBr and pressed into a transparent tablet, that is placed in the beam of light.

In this way, the whole wavelength range can be analyzed simultaneously, saving time, and an interferometer is also less expensive than a monochromator.

The infrared spectrum typically has very narrow absorption lines, which makes them unsuited for quantitative analysis but gives very detailed information about the molecules.