[[File:Luminosity.png|thumb|right|upright=1.5|Photopic (daytime-adapted, black curve) and scotopic [http://www.cvrl.org/database/text/lum/scvl.htm] (darkness-adapted, green curve) luminosity functions. The photopic includes the CIE 1931 standard [http://www.cvrl.org/database/text/cmfs/ciexyz31.htm] (solid), the Judd-Vos 1978 modified data [http://www.cvrl.org/database/text/lum/vljv.htm] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [http://www.cvrl.org/people/Stockman/pubs/2005%20Vstar%20SSJJ.pdf] (dotted). The horizontal axis is wavelength in nm.]]
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.