A photometer and a light meter are instruments for measuring luminance, the intensity of light. While a photometer is sometimes called a light meter, there are distinctions between these devices. Whereas a photometer measures the intensity of light that passes through a solution, a light meter measures the amount of ambient or reflected light in a given environment. Both are available in portable, battery-powered models, and a photometer is also available in benchtop form. Test results are generally displayed on a digital readout. A photometer is used for water quality testing in industrial and scientific applications in the laboratory and in the field. A light meter is used for environmental testing of light pollution, determining proper lighting for photography, and checking conditions for plant growth.
Photometers test a solution for specific substances or properties. They may be calibrated for a certain element, compound, or chemical such as ammonia, chlorine, phosphates, or surfactants; or for a certain property such as hardness, color, or pH. While some photometers test for a single parameter, others test for multiple parameters. Many photometers accommodate a cuvette for holding samples. Reagents are available for performing particular testing methods. A photometer is a simplified version of a spectrophotometer, which measures the amount of light of a specified wavelength that passes through a sample.
Light meters usually consist of a handheld display unit and a light sensor probe. They measure light in lumens, showing measurements in either lux (1 lumen per square meter) or foot-candles (1 lumen per square foot). Some meters toggle between both units of measure. Measurement range, accuracy, resolution, and sampling rate vary from meter to meter. For example, a typical single-range light meter measures up to 2,000 foot candles (fc)/20,000 lux whereas a high-intensity light meter with multiple measurement ranges, such as the Extech EA33, may have a top range of 99,990 fc/999,900 lux. Among the features a light meter may have are storage and recall of readings, data logging, comparator function, averaging function, and measurement hold. Light meter reviews can be a source of information to help consumers select the best meter for an application.