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This article’s lead section does not adequately summarize key points of its contents. A photometer, generally, is an instrument that measures light intensity or the optical properties of solutions or surfaces. Before electronic light sensitive elements were developed, photometry was done by estimation by the eye. The relative luminous flux of a source was compared with a standard source. The photometer is placed such that the illuminance from the source being investigated is equal to the standard source, as the human eye can judge equal illuminance. By 1861, three types were in common use. These were Rumford’s photometer, Ritchie’s photometer, and photometers that used the extinction of shadows, which was considered to be the most precise.
Most photometers detect the light with photoresistors, photodiodes or photomultipliers. Some photometers measure light by counting individual photons rather than incoming flux. Due to their individual photon counting nature, these instruments are limited to observations where the irradiance is low. The irradiance is limited by the time resolution of its associated detector readout electronics. With current technology this is in the megahertz range. The maximum irradiance is also limited by the throughput and gain parameters of the detector itself.
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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. Photometers are used to determine the correct exposure in photography. In modern cameras, the photometer is usually built in. A reflectance photometer measures the reflectance of a surface as a function of wavelength. The surface is illuminated with white light, and the reflected light is measured after passing through a monochromator.
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. Two types of photometers are used: spectrophotometer and filter photometer. 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.