Candela
The candela is the base unit of luminous intensity in the International System of Units (SISI). It is one of the seven SI base units and is used to quantify the perceived power of light. Here are some key points about the candela:
- Definition: The candela is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540×10¹² Hz, Kcd, 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 s³, where the kilogram, meter, and second are defined in terms of h, c, and ΔνCs. This definition relates the unit to the wavelength of light that is most efficiently perceived by the human eye, which is around 555 nanometers (green light).
- Historical Context: The term "candela" is derived from the Latin word for candle. Originally, the unit of luminous intensity was based on the light emitted by a standard candle. The modern definition was adopted to provide a more precise and reproducible standard.
- Use in Measurements: The candela is used to measure the luminous intensity of a light source in a specific direction. It is different from other measures of light, such as lumens, which measure the total amount of visible light emitted by a source, or lux, which measures illuminance.
- Applications: The candela is used in various fields, including lighting design, photography, and vision science. It helps in specifying the brightness of light sources, like LED and incandescent bulbs, and in assessing the intensity of light in specific directions.
- Photometry: The candela plays a crucial role in photometry, the science of measuring visible light, as perceived by human eyes. Photometric measurements take into account the varying sensitivity of the human eye to different wavelengths of light.
The candela is essential for standardizing the measurement of light intensity and is widely used in industries and scientific research involving lighting and visibility.