Photometry (optics)

Lore & Background

Photometry emerged as a field of study as early as the end of the 18th century, with measurement techniques varying depending on the effects under study. The use of the human eye as a detector led to photometric units, weighted by the eye's response characteristic, while the total heating effect of infrared radiation as measured by thermometers led to radiometric units in terms of total energy and power. The study of the chemical effects of ultraviolet radiation led to characterization by the total dose or actinometric units expressed in photons per second. The human eye is not equally sensitive to all wavelengths of visible light. Photometry accounts for this by weighting the measured power at each wavelength with a factor representing how sensitive the eye is at that wavelength. The standardized model of the eye's response is given by the luminosity function, with different responses for photopic vision (light-adapted) and scotopic vision (dark-adapted). Photopic vision is characteristic at luminance levels over three candela per square metre, while scotopic vision occurs below 2 × 10−5 cd/m2. Mesopic vision occurs between these limits and is not well characterized for spectral response. Photometric quantities are related to their radiometric analogs through standardized luminous efficiency functions, typically the photopic sensitivity function, though the scotopic function or other functions may also be applied. These weightings are standardized by the CIE and ISO. For example, luminous flux (photometric) corresponds to radiant flux (radiometric), but every wavelength is weighted according to how sensitive the human eye is to it, while radiometric quantities use unweighted absolute power.

Reader's Guide

Photometry is significant because it provides a standardized way to measure light as it is actually perceived by humans, which is essential for lighting design, display technology, and vision science. Unlike radiometry, which measures absolute power across all wavelengths, photometry weights each wavelength by the eye's sensitivity, making it directly relevant to how bright a light source appears. Photometric measurement is based on photodetectors that produce an electric signal when exposed to light, with applications ranging from simple light meters to complex lighting control systems. The legacy of photometry lies in its ability to bridge physical measurement and human perception, enabling consistent specification of lighting in offices, homes, and public spaces.

Did You Know?

• Photometry is a branch of radiometry, but radiometric quantities are not spectrally weighted and cover other kinds of optical radiation.
• The human eye has different spectral responses for photopic vision (above 3 cd/m²) and scotopic vision (below 2 × 10⁻⁵ cd/m²).
• A 60 watt incandescent bulb emits a total radiant flux of about 45 watts, but most of that is invisible infrared radiation.