Photometry (optics)

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. 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.