Defining the Candela (cd)
The candela (cd) is the International System of Units (SI) base unit for luminous intensity. It quantifies the power emitted by a light source in a particular direction, weighted by the luminosity function, a standardized model of the human eye's sensitivity to different wavelengths of light. Essentially, it measures how bright a light source appears to the human eye when observed from a specific angle.
How the Candela is Defined
Historically, the candela was defined based on the light emitted by a specific candle (hence its name). The modern definition, established in 1979 and refined in 2019, states that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10¹² hertz and that has a radiant intensity in that direction of 1/683 watt per steradian. This precise definition ensures a consistent and reproducible standard across the globe.
Practical Examples of Luminous Intensity
A common candle emits light with a luminous intensity of approximately one candela. A typical LED flashlight might produce light with an intensity of tens or hundreds of candelas, focusing its light into a narrow beam. For instance, a spotlight designed to illuminate a distant object needs a high candela value to project a bright beam effectively, even if its total light output (lumens) isn't exceptionally high.
Importance and Applications
The candela is crucial in fields such as lighting design, photography, and display technology. It allows engineers and designers to accurately specify how focused or intense a light source will appear from a certain viewpoint, enabling the creation of efficient streetlights, effective vehicle headlights, and high-contrast displays. Understanding candela helps ensure that lighting solutions meet visibility and safety standards for various applications.