What is Luminous Intensity?
Luminous intensity is a photometric quantity that measures the power emitted by a light source in a particular direction, per unit solid angle. Unlike radiant intensity, which measures total electromagnetic power, luminous intensity is weighted by the human eye's sensitivity to different wavelengths of light, meaning it only accounts for visible light.
The Candela: Unit of Luminous Intensity
The SI unit for luminous intensity is the candela (cd). One candela is defined as 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 specific definition ensures a consistent, reproducible standard.
Practical Application: Guiding Light Direction
A practical example of luminous intensity is found in flashlights or car headlights. While two flashlights might emit the same total amount of light (luminous flux, measured in lumens), the one that focuses its light into a narrower beam will have a higher luminous intensity in that specific direction, appearing brighter to an observer within the beam.
Importance in Lighting and Display Design
Luminous intensity is crucial in designing and evaluating lighting systems, especially where directional light is important, such as spotlights, traffic signals, and display screens. It helps engineers and designers ensure that a light source produces the desired brightness and visibility for specific tasks or viewing conditions, contributing to safety and user experience.