Photometry (optics)

Photopic (daytime-adapted, black curve) and scotopic [1] (darkness-adapted, green curve) luminosity functions. The photopic includes the CIE 1931 standard [2] (solid), the Judd-Vos 1978 modified data [3] (dashed), and the Sharpe, Stockman, Jagla & Jägle 2005 data [4] (dotted). The horizontal axis is wavelength in nm.

Photometry is a branch of optics that deals with the measurement of light in terms of its perceived brightness to the human eye.^[1] It is concerned with quantifying the amount of light that is emitted, transmitted, or received by an object or a system.

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 weightings are standardized by the CIE and ISO.^[2]

Photometry is distinct from radiometry, which is the science of measurement of radiant energy (including light) in terms of absolute power.

^ Bass, Michael, ed. (1995). Handbook of Optics: Volume II – Devices, Measurements and Properties (2nd ed.). McGraw-Hill. pp. 24-40–24-47. ISBN 978-0-07-047974-6.
^ ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.

[Bass95-1] Bass, Michael, ed. (1995). Handbook of Optics: Volume II – Devices, Measurements and Properties (2nd ed.). McGraw-Hill. pp. 24-40–24-47. ISBN 978-0-07-047974-6.

[ISO_CIE_23539_2023-2] ISO/CIE 23539:2023 CIE TC 2-93 Photometry — The CIE system of physical photometry. ISO/CIE. 2023. doi:10.25039/IS0.CIE.23539.2023.

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