Lux meters or light meters measure illumination in terms of luxes (lx) or foot candles (fc). A lux is equal to the total intensity of light that falls on a one square meter surface that is one foot away from the point source of light. A foot candle is equal to the total intensity of light that falls on a one square foot surface that is one foot away from the point source of light. Most lux meters or light meters consist of a body, photo cell or light sensor, and display. The light that falls onto the photo cell or sensor contains energy that is converted to electric current. In turn, the amount of current depends on the amount of light that strikes the photo cell or light sensor. Lux meters read the electrical current, calculate the appropriate value, and output the results to an analog, digital, or video display. Since light usually contains different colors at different wavelengths, the reading represents the combined effects of all the wavelengths. Typically, standard colors or color temperatures are expressed in degrees Kelvin (K). The standard color temperature for the calibration of most lux meters is 2856° K, an amount that is more yellow than pure white.
Selecting lux meters or light meters requires an analysis of performance specifications, display types, and special features. Performance specifications include photo cell or sensor diameter, illumination range, accuracy, lux resolution, foot candle resolution, humidity range, and operating temperature range. Typically, lux resolution and foot candle resolution are minimum amounts. Several display types are available. Analog devices display values on a dial, usually with a needle or pointer. Digital devices display values as numbers and/or letters. Video outputs use cathode ray tubes (CRT), liquid crystal displays (LCD) or other multi-line forms. Bargraph displays are also available. Some lux meters or light meters are portable, hand held devices. Others are designed to sit atop a desk or benchtop. Special features include backlit displays, low battery indicators, low voltage alarms, remote light sensors, built-in memory, and comparator functions.
Lux meters or light meters are used to measure levels of light in schools, hospitals, production areas, laboratories, and passageways. They are also used to monitor light-sensitive displays in museums, art galleries and archives. In the United States, lux meters are used to ensure that workplace, clean room and industrial lighting complies with requirements from the Occupational Safety and Health Administration (OSHA). Specifications from the American National Standards Institute (ANSI) and the National Fire Protection Association (NFPA) also describe proper lighting levels. Other uses for lux meters or light meters include video, photographic, and architectural applications.