Photometry is a branch of optics that deals with measuring light in terms of its perceived brightness to the human eye. It is concerned with quantifying the amount of light that is emitted, reflected, transmitted, or received by an object or a system. Photometric quantities (e.g., luminous flux) are related to their radiometric analogs (e.g., radiant flux) through standardized luminous efficiency functions that model the spectral sensitivity of the human visual system. Typically, this wavelength-dependent weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way. The weightings are standardized by the CIE and ISO. Photometry is a branch of radiometry. Radiometric quantities are not limited to light (i.e., cover other kinds of optical radiation and are sometimes extended to other kinds of electromagnetic radiation) and are not spectrally weighted.
Photometry and the eye The human eye is not equally sensitive to all wavelengths of visible light. Photometry attempts to account for this by weighting the measured power at each wavelength with a factor that represents how sensitive the eye is at that wavelength. The standardized model of the eye's response to light as a function of wavelength is given by the luminosity function. The eye has different responses as a function of wavelength when it is adapted to light conditions (photopic vision) and dark conditions (scotopic vision). Photometry is typically based on the eye's photopic response, and so photometric measurements may not accurately indicate the perceived brightness of sources in dim lighting conditions where colors are not discernible, such as under just moonlight or starlight. Photopic vision is characteristic of the eye's response at luminance levels over three candela per square metre. Scotopic vision occurs below 2 × 10−5 cd/m2. Mesopic vision occurs between these limits and is not well characterised for spectral response.
Photometric quantities
Measurement of the effects of electromagnetic radiation became a field of study as early as the end of the 18th century. Measurement techniques varied depending on the effects under study and gave rise to different nomenclature. The total heating effect of infrared radiation as measured by thermometers led to the development of radiometric units in terms of total energy and power. The use of the human eye as a detector led to photometric units, weighted by the eye's response characteristic. Study of the chemical effects of ultraviolet radiation led to characterization by the total dose or actinometric units expressed in photons per second. Many different units of measure are used for photometric measurements. The adjective "bright" can refer to a light source which delivers a high luminous flux (measured in lumens), or to a light source which concentrates the luminous flux it has into a very narrow beam (candelas), or to a light source that is seen against a dark background. Because of how light propagates through three-dimensional space — spreading out, becoming concentrated, reflecting off shiny or matte surfaces — and because light consists of many different wavelengths, the number of fundamentally different kinds of light measurements that can be made is large, and so are the numbers of quantities and units that represent them. For example, offices are typically "brightly" illuminated by an array of many recessed fluorescent lights for a combined high luminous flux. A laser pointer has very low luminous flux (it could not illuminate a room) but is blindingly bright in one direction (high luminous intensity in that direction).
Photometric versus radiometric quantities There are two parallel systems of quantities known as photometric and radiometric quantities. Every quantity in one system has an analogous quantity in the other system. Some examples of parallel quantities include:
Luminance (photometric) and radiance (radiometric) Luminous flux (photometric) and radiant flux (radiometric) Luminous intensity (photometric) and radiant intensity (radiometric) In photometric quantities every wavelength is weighted according to how sensitive the human eye is to it, while radiometric quantities use unweighted absolute power. For example, the eye responds much more strongly to green light than to red, so a green source will have greater luminous flux than a red source with the same radiant flux would. Radiant energy outside the visible spectrum does not contribute to photometric quantities at all, so for example a 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but as a light source it puts out very few lumens (because most of the energy is in the infrared, leaving only a dim red glow in the visible).
Watts versus lumens
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