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

The candela (cd) is the SI base unit of luminous intensity in a given direction. In the current SI, the candela is defined by fixing the numerical value of the luminous efficacy Kcd of monochromatic radiation of frequency 540 × 10¹² Hz to 683 lm·W⁻¹. Equivalently, a source that emits monochromatic radiation at 540 THz with radiant intensity of 1/683 W·sr⁻¹ has a luminous intensity of 1 cd. ISO 80000‑7 standardizes photometric quantities (luminous flux, illuminance, luminance) and symbols, ensuring alignment between radiometry (physical power) and photometry (visual response). Pair this definition with our lux to lumens calculator whenever you convert between intensity and total light output.

Need an academically styled deep dive? Continue with the candela ISO 80000-7 analysis, then pair it with our new lumen and lux articles to keep every photometric quantity in context.

Use this guide alongside the ISO 80000-7 explainer and the SI overview so photometric reports, lighting proposals, and calculator outputs stay consistent from radiant power to perceived brightness.

Overview

The candela (cd) is the SI base unit of luminous intensity in a given direction. In the current SI, the candela is defined by fixing the numerical value of the luminous efficacy Kcd of monochromatic radiation of frequency 540 × 10¹² Hz to 683 lm·W⁻¹. Equivalently, a source that emits monochromatic radiation at 540 THz with radiant intensity of 1/683 W·sr⁻¹ has a luminous intensity of 1 cd. ISO 80000‑7 standardizes photometric quantities (luminous flux, illuminance, luminance) and symbols, ensuring alignment between radiometry (physical power) and photometry (visual response).

Historical Evolution

From flame standards to photometric definitions

Early realizations used flame or incandescent lamp standards with approximate spectral output. The candela evolved to a radiometric basis weighted by the human visual response, culminating in the present definition via a fixed luminous efficacy at a reference frequency near the peak of photopic sensitivity. This anchors photometry to physical power while honoring the standard observer model of human vision. Check the ISO 80000-2 guide to ensure angular geometry and steradian notation stay correct while comparing legacy and modern standards.

Conceptual Foundations

Photometry vs radiometry

Radiometry measures electromagnetic power in watts; photometry weights that power by the eye’s sensitivity V(λ) under specified adaptation (photopic or scotopic). Key relations include luminous flux Φv = Kcd ∫ Φe(λ) V(λ) dλ (in lumens), luminous intensity Iv as luminous flux per solid angle (cd = lm·sr⁻¹), illuminance Ev as flux per area (lux = lm·m⁻²), and luminance Lv as intensity per projected area (cd·m⁻²). Candela, lumen, lux, and related units are derived coherently when radiometric quantities are correctly spectrally weighted.

Standard observer and colorimetry

The CIE photopic function V(λ) represents average daytime human visual sensitivity. Photometric measurements must account for spectral mismatch between real sources or detectors and V(λ). While candela is defined at a single frequency, real-world photometry integrates across spectra with instrument corrections. Use the lux to foot-candles converter to keep international lighting data synchronized while accounting for these perceptual weightings.

Realization and Traceability

Primary and practical routes

  • Detector-based scales: Cryogenic radiometers establish absolute radiant power; spectral responsivity standards and accurately characterized filters approximate V(λ). Luminous intensity is realized by spectral power measurement and weighted integration with Kcd.
  • Source-based scales: Standard lamps (tungsten-halogen, LED) with calibrated luminous intensity or flux disseminate photometric quantities to laboratories and industry.
  • Goniophotometry: Measures angular intensity distributions to determine luminous intensity, total luminous flux, and beam patterns.

Calibration reporting

Calibration reports include geometry, spectral mismatch corrections, temperature, and uncertainty components (type A/B), preserving metrological traceability to SI. Combine these documents with calculators like the room lighting lumens tool to translate calibrated data into lighting specifications.

Measurement Considerations

  • Spectral mismatch: The closer detector responsivity tracks V(λ), the smaller the correction. LED sources with narrow spectra require careful mismatch evaluation.
  • Angular and geometric factors: Aperture area, distance, and alignment drive uncertainty; inverse-square law applicability depends on source extent.
  • Stray light and flare: Optical baffling and spectral purity are essential, especially in high-dynamic-range measurements.
  • Temporal behavior: Modulation (PWM dimming), flicker, and warm-up transients influence photometric results; measurement bandwidth and sampling must be specified.

Use the lux to lumens and therms to kWh calculators together when balancing lighting levels against energy budgets and environmental constraints.

Applications

Lighting and energy efficiency

Lamp and luminaire ratings (lumens, candelas, efficacy) guide product design and regulation. Photobiological safety, glare metrics, and human-centric lighting depend on accurate photometry. Urban planning and roadway lighting use candela distributions for visibility and safety.

Displays and imaging

Luminance (cd·m⁻²) defines display brightness; colorimetric specifications ensure visual consistency across devices. HDR standards and cinema projection rely on calibrated photometry.

Safety, signaling, and standards

Aviation beacons, automotive headlamps, and maritime aids specify minimum luminous intensity in defined directions. Traceable candela ensures interoperability and compliance.

Vision science and health

Clinical photometry, light therapy dosimetry, and circadian lighting research require well-characterized luminous quantities and spectra to interpret biological effects.

Why the Candela Matters

The candela connects physical power to human perception through a rigorous standard, enabling consistent specification of how bright a source appears across industries and applications. By defining a fixed luminous efficacy at a reference frequency and integrating spectral weighting via V(λ), the SI provides a stable foundation for photometry. ISO 80000’s standardized symbols, names, and print rules ensure that luminous intensity, flux, illuminance, and luminance are communicated unambiguously in standards, regulations, and engineering documentation. As solid-state lighting, display technologies, and human-centric illumination evolve, a robust, SI-traceable candela remains essential for quality, safety, and energy performance.

Continue your base unit exploration with the ampere, kelvin, and mole explainers to keep electrical, thermal, and photometric projects aligned under ISO guidance.

Related resources on CalcSimpler

Explore these guides to expand your measurement toolkit and connect theory to hands-on calculations.

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

    Understand how photometric units, spectra, and ISO notation align with the candela.

    Read more

  • ISO 80000-2: Mathematical Signs and Symbols

    Review the geometry and angle symbols that underpin steradians and luminous flux calculations.

    Read more

  • International System of Units (SI)

    See how luminous intensity fits with the other SI base units and constant-based definitions.

    Read more

  • The Kelvin (K): The SI Base Unit of Thermodynamic Temperature

    Link radiometric standards to temperature control and spectral realization of light sources.

    Read more

Calculators that keep the candela practical

Keep these tools nearby when drafting lighting layouts, comparing regulations, or optimizing energy performance.

  • Lux to Lumens Calculator

    Convert illuminance and area into total luminous flux for lighting design.

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  • Room Lighting Lumens

    Estimate lamp output requirements based on room size and target illuminance.

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  • Lux to Foot-Candles

    Translate between SI illuminance and imperial lighting specifications.

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  • Therms to kWh Converter

    Compare lighting energy budgets against heating or electrical consumption using consistent units.

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