Candela (cd): The SI Unit of Luminous Intensity

Introduction

The candela (cd) is the base unit of luminous intensity in the International System of Units (SI) and the International System of Quantities (ISQ). Defined formally as “the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and has a radiant intensity in that direction of 1/683 watt per steradian,” the candela links human vision with measurable radiative power. Its definition ties optical radiation at 540 terahertz—near the green region where the human eye is most sensitive—to a precisely fixed luminous efficacy of 683 lumens per watt. This article explores the history, conceptual basis, applications, and scientific significance of the candela.

Historical Development

The challenge of measuring light predates the SI. Early photometry relied on comparative methods using candles or oil lamps as reference sources. In 1881, the international candle (based on a Carcel lamp) was proposed as a unit. By the mid-20th century, electrical lighting required a more reproducible standard. In 1948, the General Conference on Weights and Measures (CGPM) adopted a definition tying luminous intensity to the emissive power of a blackbody at the freezing point of platinum (about 2042 kelvins). This blackbody-based definition remained until 1979, when advances in radiometry and photometry enabled a shift to a constant-luminous-efficiency approach. The 16th CGPM redefined the candela by fixing the luminous efficacy of monochromatic radiation at 540 × 10¹² Hz. This choice aligns with the peak of the photopic spectral luminous efficiency curve (V(λ)) and ensures stability independent of artifacts.

Conceptual Foundations

Luminous intensity measures the power emitted by a source in a given direction, weighted by the spectral sensitivity of the human eye. Unlike radiant intensity, which uses watts per steradian and treats all wavelengths equally, luminous intensity accounts for human vision by applying the V(λ) function and the luminous efficacy constant. The fundamental relationship is:

where I_v is luminous intensity (cd), I_e(λ) is spectral radiant intensity (W sr⁻¹ nm⁻¹), and K_cd is 683 lm W⁻¹ at 540 THz. The candela thus bridges objective radiometric quantities and subjective visual perception.

Realization and Measurement

Realizing the candela requires accurate radiometric apparatus and knowledge of the V(λ) function. National metrology institutes (NMIs) typically use photometers calibrated against cryogenic radiometers. A photometer measures the luminous flux from a lamp or LED, and by controlling the geometry (solid angle, distance, apertures), luminous intensity is calculated. Highly stable lamps, lasers, or LEDs near 540 nm provide working standards. Calibration chains disseminate the candela to industry via luminous intensity lamps, photometers, and photometric test benches.

Applications

Lighting design and industry. Luminaire manufacturers specify luminous intensity distributions in candela versus angle (cd/θ). These “polar plots” inform lighting designers how light is distributed. For example, automotive headlamps must meet legal candela thresholds at specific directions to ensure adequate roadway illumination without glare.

Safety and signaling. Aviation, maritime, and railway signals use candela ratings to guarantee visibility at designated distances and backgrounds. The International Civil Aviation Organization (ICAO) defines minimum and maximum luminous intensities for runway edge lights and beacons in candela.

Photometric measurements. In photometry labs, candela calibration is the foundation for luminous flux (lumens) and illuminance (candelas per square meter). High-precision research, such as calibrating photodiodes or characterizing novel light sources like white LEDs, depends on accurate luminous intensity scales.

Importance and Future Prospects

The candela’s importance stems from its role as one of the seven SI base units. Its human-centric definition ensures that lighting standards remain linked to visual perception while being grounded in fundamental constants. As lighting technology evolves—from incandescent to LED to laser-based systems—the candela provides a stable reference for comparing performance and ensuring safety. Emerging fields such as human-centric lighting, which tailors spectra for circadian rhythms, may drive refinements in photopic and scotopic weighting functions. Nevertheless, the candela’s reliance on 540 THz and the fixed luminous efficacy constant anchors luminous intensity in an immutable physical reference, ensuring consistency across decades.

Related resources on CalcSimpler

Expand your photometry knowledge with these cross-linked guides.

• ISO 80000-7: Quantities and Units of Light and Radiation

  Connect the candela to the photometric framework that also defines luminous flux, illuminance, and luminance.

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• Lumen (lm): The SI Unit of Luminous Flux

  Trace how luminous flux integrates luminous intensity and why lumens keep lighting specifications coherent.

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• Lux (lx): The SI Unit of Illuminance

  See how illuminance links luminous flux to area so workspace design and sensor calibration stay compliant with ISO guidance.

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• The Candela (cd): The SI Base Unit of Luminous Intensity

  Compare this academically styled deep dive with our practical summary focused on everyday photometry workflows.

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Calculators that apply the candela definition

Use these tools to convert luminous intensity insights into practical lighting deliverables.

• Lux to Lumens Calculator

  Convert illuminance readings and surface areas into total luminous flux for lighting plans.

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• Lux to Foot-Candles Converter

  Translate SI illuminance values into foot-candles when collaborating with imperial-spec teams.

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• Room Lighting Lumens

  Estimate luminaire output requirements from target lux levels and room geometry.

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