Lighting equipment basic concepts at a glance |
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Light and radiation Light or visible light is the portion of electromagnetic radiation that is visible to the human eye, responsible for the sense of sight. Visible light has a wavelength in a range from about 380 or 400 nanometres to about 760 or 780 nm, with a frequency range of about 405 THz to 790 THz. Units and measurements of lighting There are 4 important variables:
Luminous flux Luminous flux or luminous power is the measure of the perceived power of light that is adjusted to reflect the varying sensitivity of the human eye to different wavelengths of light. The SI unit of luminous flux is the lumen (lm). Luminous intensity luminous intensity is a measure of the wavelength-weighted power emitted by a light source in a particular direction per unit solid angle, based on the luminosity function, a standardized model of the sensitivity of the human eye. The SI unit of luminous intensity is the candela (cd). Illuminance illuminance is the total luminous flux incident on a surface, per unit area. It is a measure of the intensity of the incident light, wavelength-weighted by the luminosity function to correlate with human brightness perception. In SI derived units these are measured in lux (lx) or lumens per square meter (cd·sr·m−2). Luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through or is emitted from a particular area, and falls within a given solid angle. The SI unit for luminance is candela per square meter (cd/m2). Luminous efficacy Luminous efficacy is a figure of merit for light sources. It is the ratio of luminous flux to power. The color of light The color of light from the source is expressed with color temperature. Color temperature is conventionally stated in the unit of absolute temperature, the kelvin, having the unit symbol K. Kelvin scale starts from the absolute zero (0K=-273.15° C ). The color temperature of a light source is the temperature of an ideal black-body radiator that radiates light of comparable hue to that of the light source. The higher the temperature, the brighter the light. For example, warm-white incandescent is around 2600K, daylight fluorescent light temperature is 5000K. Despite the same light temperature, the color of light can differ due to the spectral structure of light. Color Rendering Artificial lighting should enable the human eye to sense colors as in natural daylight. Of course, this depends of the placement and intent of the light. The criterion is color rendering index. The color rendering index (CRI) is a quantitative measure of the ability of a light source to reproduce the colors of various objects faithfully in comparison with an ideal or natural light source. The smaller the CRI, the worse the color rendering. |
