candela

candela. The candela is the SI base unit of luminous intensity, a fundamental photometric quantity in the science of light and vision. Defined precisely by the International System of Units, it quantifies the perceived power of light emitted by a source in a particular direction, as weighted by the standardized sensitivity of the human eye. Its development and standardization are overseen by international bodies including the International Bureau of Weights and Measures and the International Commission on Illumination.

Definition and history

The modern definition, adopted by the General Conference on Weights and Measures in 1979 and refined in subsequent meetings, anchors the candela to the luminous efficacy of monochromatic radiation. Specifically, a source emitting monochromatic green light at a frequency of 540 terahertz, with a radiant intensity of 1/683 watt per steradian, has a luminous intensity of one candela. This definition effectively ties the unit to the watt via a fixed numerical constant. Historically, the unit evolved from earlier standards based on the light of candles, such as the candlepower derived from a spermaceti candle, and later the Hefner lamp. The transition to a physically reproducible standard began with the adoption of the new candle, based on the luminance of a platinum radiator at its freezing point, a decision ratified at the General Conference on Weights and Measures in 1948.

Measurement and standards

Accurate realization of the candela requires sophisticated laboratory setups and adherence to international protocols. National metrology institutes, such as the National Institute of Standards and Technology in the United States and the Physikalisch-Technische Bundesanstalt in Germany, maintain primary standards. These often involve absolute radiometers, like cryogenic radiometers, to measure radiant power precisely, combined with filters that simulate the CIE standard photometric observer function, known as the V(λ) curve. Calibration chains disseminate the unit to industry and testing laboratories using standard lamps, such as those from the Osram group, and photometers. The International Commission on Illumination provides essential data on spectral luminous efficiency, while the International Organization for Standardization publishes norms, like those in the ISO 23539 series, governing photometric measurements.

Relationship to other units

The candela is foundational within the SI derived unit system for photometric quantities. When combined with the steradian, it defines the lumen, the unit of luminous flux; one candela times one steradian equals one lumen. Illuminance, the luminous flux incident on a surface, is measured in lux, which is lumens per square meter. Similarly, luminance, which describes the brightness of a surface, is expressed in candelas per square meter, a unit also called the nit. These relationships are codified in standards from bodies like the International Electrotechnical Commission. The connection to radiometric units, such as the watt, is fixed by the definition's 683 lumen-per-watt constant for photopic vision, a value established by the General Conference on Weights and Measures.

Applications

The candela is critical across diverse fields requiring precise control and measurement of light. In the automotive industry, regulations from bodies like the European Union and the National Highway Traffic Safety Administration specify maximum candela values for headlamp high beams and minimums for taillights. In aerospace, the intensity of airport runway and obstruction lights is strictly defined in candelas by the International Civil Aviation Organization. For consumer electronics, the brightness of LED indicators, smartphone screens, and television displays, often manufactured by companies like Samsung or Sony, is characterized using this unit. Furthermore, the film industry uses it to specify the output of studio lighting equipment, while architectural lighting design for projects like the Burj Khalifa relies on candela data for planning luminous environments.

Practical examples

Common light sources exhibit a wide range of luminous intensities measured in candelas. A typical domestic incandescent light bulb might emit roughly 100 candelas in most directions. In contrast, a focused LED flashlight from a manufacturer like Maglite can produce a beam exceeding 10,000 candelas. The intense carbon arc lamps once used in film projectors could reach hundreds of thousands of candelas. For perspective, a standard international candle approximates one candela, while the Sun, as observed from Earth, has a luminance of about 1.6 billion candelas per square meter. Regulatory examples include the Federal Aviation Administration mandating that a standard medium intensity strobe light on a antenna must have an intensity of thousands of candelas, and the Society of Automotive Engineers providing detailed test procedures for measuring the candela output of vehicle lighting. Category:SI base units