Candela

Luminosity

Linear visible spectrum

Candela (cd) is the SI base unit of luminous intensity in a given direction. It is one of the seven base units in the International System of Units (SI). The candela measures the amount of light emitted in the range of a three-dimensional angular span. Unlike other light-related units such as lumens, which measure the total amount of light emitted by a source, the candela quantifies the intensity of light in a specific direction. This unit is crucial in characterizing the brightness of light-emitting diodes (LEDs), light bulbs, and other light sources in terms of how much illumination they can provide in a particular direction.

Definition

The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10^12 Hz, K_cd, to be 683 when expressed in the unit lm⋅W^−1, which is equal to cd⋅sr⋅W^−1, or cd⋅sr⋅kg^−1⋅m^−2⋅s^3, where the kilogram, meter, and second are defined in terms of Planck's constant (h), the speed of light (c), and the cesium frequency (Δν_Cs). This definition connects the candela with the fundamental physical constants and ensures that the unit is both precise and universally applicable.

History

The term "candela" comes from the Latin word for candle, originally reflecting the unit's early definition as the light emitted by a "standard candle". The evolution of the candela as a unit of luminous intensity mirrors advances in lighting technology and the understanding of human vision. Prior to the establishment of the candela, various countries and organizations used their own units of light intensity, often based on the light emitted by specific candle types or lamps. The need for a standardized unit became apparent with the increasing complexity of scientific research and the globalization of technology.

In 1948, the candela was adopted as an international standard at the 9th General Conference on Weights and Measures (CGPM). Since then, its definition has been refined to reflect advancements in measurement precision and the understanding of light's physical properties.

Applications

The candela finds applications in a wide range of fields, including lighting design, photometry, and vision science. It is used to specify the brightness of displays, such as televisions and computer monitors, as well as in the design of lighting schemes for buildings and outdoor spaces. In industry, the candela is used to ensure compliance with safety and regulatory standards for automotive lighting, emergency exit signs, and other illuminated signage.

Measurement

Measuring luminous intensity in candelas involves comparing the light source in question to a standard light source of known intensity under controlled conditions. Photometers and integrating spheres are commonly used instruments for this purpose. The measurement process must account for the spectral sensitivity of human vision, which varies with wavelength, by applying a weighting function known as the luminosity function.

See Also

• Lumen (unit)
• Lux
• Photometry (optics)
• Color temperature
• Lighting

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