The Candela: Measuring Light the Way the Human Eye Sees It
Source: BIPM. Published: 2025-09-01.
Why Light Needs Its Own Unit
Light is electromagnetic radiation — the same physical phenomenon as radio waves, microwaves, and X-rays. Yet light occupies a special position among electromagnetic waves because the human visual system evolved to detect it, and the sensitivity of that system varies dramatically with wavelength. The human eye is roughly 100 times more sensitive to green light (around 555 nm) than to red or blue light of the same power. A radiometric watt of green light appears far brighter than a radiometric watt of red light, even though both carry the same amount of electromagnetic energy. Lighting science — photometry — therefore uses units that account for this biological sensitivity weighting, producing quantities that correspond to how bright a source actually appears to a human observer rather than how much power it emits.
The Historical Candela: A Standard Flame
Before the modern definition, luminous intensity was measured in candlepower — literally, the brightness of a standard candle. The original international standard was the Carcel lamp, a specific oil lamp burning at a defined rate, followed by various 'standard candles' made from spermaceti wax. These physical standards were notoriously difficult to reproduce consistently. In 1948, a new definition was adopted: the candela was defined as the luminous intensity of a blackbody radiator at the temperature of freezing platinum (2042 K), measured through a small aperture of defined area. This definition was more reproducible but still required a specific material at a specific temperature, making it challenging to realise in laboratories without the right equipment.
The Modern Definition: Linking Light to the Watt
In 1979, the General Conference on Weights and Measures redefined the candela in a way that directly linked it to radiometric power — the watt. One candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz (approximately green light at 555 nm) and has a radiant intensity in that direction of 1/683 watt per steradian. The 1979 definition fixed the luminous efficacy of this specific green light at exactly 683 lm/W, chosen to match the pre-existing value of the candela as closely as possible. The 2019 SI revision did not change this definition but integrated it into the framework of fixed constants, making the candela fully consistent with the other six base units.
Lumens, Lux, and the Candela in Everyday Life
The candela is the base unit of luminous intensity, but two derived units are more commonly encountered: the lumen (lm), which measures total luminous flux — the total amount of light emitted in all directions — and the lux (lx), which measures illuminance — the amount of light falling on a surface per unit area (one lux = one lumen per square metre). Light bulb packaging has largely shifted from watts (which measure power consumption) to lumens (which measure actual light output) because this gives consumers a better guide to how bright the bulb will appear. A 60 W incandescent bulb produces roughly 800 lumens; an equivalent LED might achieve the same output using only 8-10 W.
NIST and Photometric Standards
NIST maintains primary photometric standards for the United States and provides calibration services for luminous intensity, luminous flux, luminance, and illuminance. The Photometry and Lighting Group at NIST develops and maintains detector-based standards for luminous intensity and luminous flux that are traceable to the SI definition of the candela. These standards are used to calibrate instruments used by manufacturers of LEDs, luminaires, display panels, and medical devices. Incorrect photometric calibration can affect everything from automotive headlight approval to the irradiance levels used in medical phototherapy for conditions such as neonatal jaundice. NIST's work in this area directly supports safety standards and consumer protection in the lighting industry.
Candelas and the Digital World
Screen brightness — often expressed in nits (candelas per square metre) — is a direct application of the candela. A typical laptop display operates at 200-400 nits; a smartphone in outdoor mode may reach 800-1000 nits; high-end HDR televisions can produce localised peaks of 1000-2000 nits. Standardised brightness measurement enables the specification of display performance for gaming, film grading, medical imaging, and accessibility compliance. A display sold as meeting a certain luminance standard has been measured against photometric references that trace back, through national metrology institute calibrations, to the SI definition of the candela — the unit that bridges human visual perception and physics.
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