Photometry (optics)

Photometry is a branch of optics that deals with measuring light in terms of its perceived brightness to the human eye.

In modern photometry, the radiant power at each wavelength is weighted by a luminosity function that models human brightness sensitivity.

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

Photometry is typically based on the eye's photopic response, and so photometric measurements may not accurately indicate the perceived brightness of sources in dim lighting conditions where colors are not discernible, such as under just moonlight or starlight.

[1] Photopic vision is characteristic of the eye's response at luminance levels over three candela per square metre.

Study of the chemical effects of ultraviolet radiation led to characterization by the total dose or actinometric units expressed in photons per second.

For example, offices are typically "brightly" illuminated by an array of many recessed fluorescent lights for a combined high luminous flux.

Radiant energy outside the visible spectrum does not contribute to photometric quantities at all, so for example a 1000 watt space heater may put out a great deal of radiant flux (1000 watts, in fact), but as a light source it puts out very few lumens (because most of the energy is in the infrared, leaving only a dim red glow in the visible).

In a radiometric sense, an incandescent light bulb is about 80% efficient: 20% of the energy is lost (e.g. by conduction through the lamp base).

The lumen is defined as amount of light given into one steradian by a point source of one candela strength; while the candela, a base SI unit, is defined as the luminous intensity of a source of monochromatic radiation, of frequency 540 terahertz, and a radiant intensity of 1/683 watts per steradian.

Within the visible spectrum, wavelengths of light are weighted according to a function called the "photopic spectral luminous efficiency."

Photometric measurement is based on photodetectors, devices (of several types) that produce an electric signal when exposed to light.

Lamps and lighting fixtures are tested using goniophotometers and rotating mirror photometers, which keep the photocell stationary at a sufficient distance that the luminaire can be considered a point source.