Photometry is the science of the measurement of light , in terms of its perceived brightness to the human eye. In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity. Typically, this weighting function is the photopic sensitivity function, although the scotopic function or other functions may also be applied in the same way. The human eye is not equally sensitive to all wavelengths of visible light.
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It thus deals with visible light only, not with infrared and ultraviolet light, and takes into account the spectral sensitivity of the human eye. Note that the term brightness should be used only as a qualitative indication for perceived brightness, not as a measurable quantity although it is often used instead of radiance or luminance. The wavelength -dependent sensitivity of the human eye plays an essential role in photometry.
Many photometric quantities are obtained by multiplying or integrating a radiometric quantity with a weighting factor luminosity function.
Here, one needs to distinguish between two different situations:. The luminous flux , which is generated in a light source, for example, can be calculated using the photopic luminosity function i. Both mentioned luminosity functions apply to the typical healthy human eye. In practice, there are certain variations, e. Some individuals exhibit substantial deviations from the norm, for example in the form of color blindness. For them, the standard luminosity functions are not applicable.
There is a wide range of photometric quantities, most of which are directly related to radiometric quantities:. Some of those terms are frequently confused. For example, luminous efficiency is often used instead of luminous efficacy ; this can often easily be recognized from the used units. There are also various spectral quantities in photometry, which describe a distribution of some quantity over optical frequencies or wavelengths.
For example, the spectral luminous flux is the luminous flux per unit optical frequency or wavelength. For the measurement of photometric quantities, there are various kinds of photometers. In particular, there are instruments for measuring the illuminance and for the total luminous flux. Further derived quantities can often be calculated from measured illuminance values, for example.
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As your inputs are first reviewed by the author, they may be published with some delay. See also: radiometry , photometers , luminous flux , luminance , luminous efficacy and efficiency , luminosity functions , illuminance and other articles in the categories general optics , light detection and characterization. If you like this page, please share the link with your friends and colleagues, e. These sharing buttons are implemented in a privacy-friendly way! Virtual Library. Sorry, we don't have an article for that keyword!
Wavelength-dependent Sensitivity of the Human Eye The wavelength -dependent sensitivity of the human eye plays an essential role in photometry. The eye has three different kinds of such cones S, M and L with different spectral sensitivities. Under faint illumination conditions, the human eye uses the substantially more sensitive rod cells scotopic vision.
There is only one kind of those rods, and as a result one cannot perceive colors with them. The resulting visual impression is gray.
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