Editing Light (section)

==Measurement==
{{Main|Photometry (optics)|Radiometry}}

Light is measured with two main alternative sets of units: [[radiometry]] consists of measurements of light power at all wavelengths, while [[photometry (optics)|photometry]] measures light with wavelength weighted with respect to a standardized model of human brightness perception. Photometry is useful, for example, to quantify [[Illumination (lighting)]] intended for human use.

The photometry units are different from most systems of physical units in that they take into account how the human eye responds to light. The [[cone cell]]s in the human eye are of three types which respond differently across the visible spectrum and the cumulative response peaks at a wavelength of around 555&nbsp;nm. Therefore, two sources of light which produce the same intensity (W/m<sup>2</sup>) of visible light do not necessarily appear equally bright. The photometry units are designed to take this into account and therefore are a better representation of how "bright" a light appears to be than raw intensity. They relate to raw [[power (physics)|power]] by a quantity called [[luminous efficacy]] and are used for purposes like determining how to best achieve sufficient illumination for various tasks in indoor and outdoor settings. The illumination measured by a [[photocell]] sensor does not necessarily correspond to what is perceived by the human eye and without filters which may be costly, photocells and [[charge-coupled device]]s (CCD) tend to respond to some [[infrared]], [[ultraviolet]] or both.