Editing Spectral power distribution (section)

{{Short description|Measurement describing the power of an illumination}}
{{for|the [[statistics]] and [[signal processing]] concept|Power spectral density}}
[[File:NormSPD with eye.png|thumb|[[Standard illuminant|CIE standard illuminant]] spectral power distribution comparisons referenced to the human visual system [[photopic vision|photopic response]]]]

In [[radiometry]], [[photometry (optics)|photometry]], and [[color science]], a '''spectral power distribution''' ('''SPD''') measurement describes the [[Power (physics)|power]] per unit [[area]] per unit [[wavelength]] of an [[illumination (lighting)|illumination]] ([[radiant exitance]]). More generally, the term ''spectral power distribution'' can refer to the concentration, as a function of wavelength, of any radiometric or photometric quantity (e.g. [[radiant energy]], [[radiant flux]], [[radiant intensity]], [[radiance]], [[irradiance]], [[radiant exitance]], [[Radiosity (heat transfer)|radiosity]], [[luminance]], [[luminous flux]], [[luminous intensity]], [[illuminance]], [[luminous emittance]]).<ref>{{cite book | title = Color Appearance Models | author = Mark D. Fairchild | isbn = 0-470-01216-1 | date = 2005 | publisher = [[John Wiley and Sons]]| url = https://books.google.com/books?id=8_TxzK2B-5MC&dq=light+source+%22spectral+power+distribution%22&pg=PA57 }}</ref><ref name=focal>{{cite book | title = The Focal Encyclopedia of Photography | author = Michael R. Peres | publisher = [[Focal Press]] | date = 2007 | isbn = 978-0-240-80740-9 | url = https://books.google.com/books?id=VYyldcYfq3MC&dq=%22spectral+power+distribution%22+555+560+nm&pg=RA1-PA383 }}</ref><ref name=intro>{{cite book | title = Introduction to Radiometry and Photometry | author = William Ross McCluney | isbn = 0890066787 | date = 1994 | publisher = Boston: Artech House }}</ref><ref name=meas>{{cite book | title = Optical Radiation Measurements (v. 1) | author = Franc C. Grum | isbn = 0123049016 | date = 1979 | publisher = New York: Academic Press }}</ref>

Knowledge of the SPD is crucial for optical-sensor system applications. [[Optical properties]] such as [[transmittance]], [[reflectivity]], and [[absorbance]] as well as the sensor response are typically dependent on the incident wavelength.<ref name=intro/>