Instruments used to measure the intensity of scattered light.

What Is a Photometer?

Photometers are instruments that measure electromagnetic radiation, from ultraviolet to infrared, including the visible spectrum. They convert light into electricity using a photoresistor, photodiode, or photomultiplier. For measuring at a defined wavelength or for analyzing the light’s spectral distribution, the light may also be passed through a filter or a monochromator.

What Is a Photometer Used for?

A light meter, a specific type of photometer, is used to measure the amount of light present. In photography, light meters help the photographer determine the correct shutter speed and f-number selection for the desired exposure. They are also used for cinematography, scene design, architectural lighting design, and for plant growth systems.

Science-related applications for light meters include the measurement of:

• UVA (315 to 400nm) and UVB (280 to 315nm) light for phototherapy or skin treatment
• UVC (100 to 280nm) output from disinfection and sterilization lamps (readout in watts per square centimeter)
• The brightness (luminance) of exit and other signs or displays
• Light source emissions that may help plants grow
• The UV light emission used to harden glues, plastics, or other protective coatings (readout in joules per square centimeter)
• Daylight or output from tungsten, fluorescent, or mercury lamps

Most light meters are portable and may measure light in only a defined section of the spectrum, such as UV light . Some light meters allow you to select the light source and report the results in one or more of the common units: lux, foot-candles, lumens, candelas, or candelas per square meter. Other units include watts or joules per square centimeter.