Luminous Intensity and the SI Unit: The Candela Explained

The SI Unit of Luminous Intensity: The Candela

The SI unit of luminous intensity is the candela (cd). It quantifies the power of light emitted by a source in a particular direction, weighted by the sensitivity of the human eye. Luminous intensity is distinct from radiant intensity, which measures raw electromagnetic power; the candela accounts for how bright a light appears to people, making it the cornerstone of photometry. Understanding the candela is essential for anyone working in lighting design, optoelectronics, or regulatory compliance.

Formal Definition of the Candela

The modern definition of the candela fixes the luminous efficacy of monochromatic radiation at a frequency of 540 × 10¹² hertz (roughly 555 nanometers, in the green part of the visible spectrum). At that frequency, one candela corresponds to a luminous intensity of 1/683 watt per steradian. This definition, adopted in 1979 and refined by the 2019 redefinition of the SI, ties the candela to the watt and the second, anchoring it to fundamental constants rather than a physical artifact.

Luminous Intensity vs. Related Photometric Quantities

Luminous intensity is often confused with luminous flux, illuminance, and luminance. Luminous flux, measured in lumens, describes the total visible light output of a source. Illuminance, measured in lux, is the flux arriving on a surface per unit area. Luminance, measured in candelas per square meter, describes the intensity of light leaving or reflecting from a surface in a given direction. The candela remains the base unit from which the lumen is derived: one lumen equals one candela steradian, representing the total flux within a solid angle.

A Brief History of the Candela

Before the candela, illumination was described using candlepower, based on the light from a standardized spermaceti candle. As measurement science advanced, the international candle and later the candela replaced this artifact-based approach. The 1979 definition tied the unit to a specific frequency of radiation, and the 2019 SI overhaul fixed the luminous efficacy constant Kcd at exactly 683 lumens per watt, making the candela a derived unit grounded in the Planck constant and the caesium-133 hyperfine transition frequency.

Why Luminous Intensity Matters in Practice

Luminous intensity governs how bright a directional light source appears to an observer. It is the key specification for LEDs, spotlights, automotive headlights, and signaling devices. Engineers use luminous intensity distributions, often plotted as polar diagrams, to predict how a luminaire performs across angles. In road lighting, theatrical design, and aviation signage, selecting a source with the right candela rating ensures visibility, safety, and energy efficiency.

Measuring Luminous Intensity

Accurate measurement requires a goniophotometer, which rotates a detector around the source in a dark room and records intensity at each angle. The detector is typically filtered to match the CIE photopic luminosity function V(λ), which models standard human brightness perception. Results are reported in candelas, and integrating the intensity over the full sphere yields the total luminous flux in lumens. Calibration traceable to national standards ensures that measurements are comparable across laboratories and industries.

Candela in Everyday Lighting and Displays

For consumers, the candela appears indirectly in product specifications for flashlights, bicycle lights, and projectors, where beam intensity determines reach and visibility. In display technology, luminance is expressed in cd/m², linking back to the candela. A typical smartphone display might emit around 600 cd/m² in bright outdoor mode, while a high-end television can exceed 1,000 cd/m². These values help users compare the perceived brightness of screens and lighting products with a common, physically grounded unit.