![]() ![]() Many different units of measure are used for photometric measurements. Study of the chemical effects of ultraviolet radiation led to characterization by the total dose or actinometric units expressed in photons per second. Use of the human eye as a detector led to photometric units, weighted by the eye's response characteristic. The total heating effect of infrared radiation as measured by thermometers led to development of radiometric units in terms of total energy and power. Measurement techniques varied depending on the effects under study and gave rise to different nomenclature. Measurement of the effects of electromagnetic radiation became a field of study as early as the end of 18th century. Photometric quantities Comparison of photometric and radiometric quantities Mesopic vision occurs between these limits and is not well characterised for spectral response. Scotopic vision occurs below 2 × 10 −5 cd/m 2. Photopic vision is characteristic of the eye's response at luminance levels over three candela per square metre. 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. The eye has different responses as a function of wavelength when it is adapted to light conditions ( photopic vision) and dark conditions ( scotopic vision). The standardized model of the eye's response to light as a function of wavelength is given by the luminosity function. 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 human eye is not equally sensitive to all wavelengths of visible light. The weightings are standardized by the CIE and ISO. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way. In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity. It is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power. Photometry is the science of the measurement of light, in terms of its perceived brightness to the human eye. The photopic includes the CIE 1931 standard (solid), the Judd-Vos 1978 modified data (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data (dotted). Photopic (daytime-adapted, black curve) and scotopic (darkness-adapted, green curve) luminosity functions. JSTOR ( December 2010) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed.įind sources: "Photometry" optics – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification. ![]()
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