Photometry

Photometry is the science of measuring light, in terms of its perceived brightness to the human eye. It is distinct from radiometry, which involves measuring light in terms of absolute power. In practice, photometry involves the use of light meters and other devices to measure the intensity or brightness of light sources, the transmission of light through materials, and the illumination of surfaces.

Principles of Photometry

Photometry is based on the human eye's sensitivity to light, which varies with the wavelength of the light. The standard human eye responds most strongly to light in the green part of the spectrum, around 555 nanometers, and less so to light at the edges of the visible spectrum — violet and red. This response is described by the luminosity function, a standardised model of human visual brightness perception.

Photometric measurements are usually expressed in lumens, a measure of the total quantity of visible light emitted by a source. Other common units in photometry include:

• Lux (lumens per square meter) - used to measure illuminance
• Candela (lumens per steradian) - used to measure luminous intensity
• Foot-candle - an older unit of illuminance, still used in some contexts

Photometric Devices

Several devices are used in photometry to measure different aspects of light:

• Photometers - measure the intensity of light.
• Lux meters - specifically measure the illuminance of light.
• Integrating spheres - used to measure the total light output from a source.

Applications of Photometry

Photometry has a wide range of applications:

• In architecture and interior design, to ensure that spaces are adequately lit.
• In astronomy, to measure the brightness of stars and other celestial bodies.
• In the lighting industry, to design lighting fixtures and to ensure that they meet regulatory standards.
• In photography and cinematography, to determine the optimal exposure settings.

Challenges in Photometry

One of the main challenges in photometry is ensuring that measurements accurately reflect human brightness perception under various lighting conditions. Different light sources can emit light at different wavelengths, and the human eye's response can vary accordingly. This necessitates the use of correction factors in some cases to ensure accurate measurements.

See Also

• Colorimetry
• Spectrophotometry
• Luminosity function