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The Terms of Illumination

Illumination and Lighting kinds

Lighting Sources


Terms of Illumination  :

Light: Electromagnetic radiation with a wavelength that is visible to the eye, according to the Wave Theory. It has a particular speed of diffusion, frequency and wavelength. Humans could perceive visible light, which is the portion of electromagnetic spectrum between the frequencies of 380 nm and 780 nm and which is perceived as the color.

Color: The perception by a human’s brain of radiation with different wavelengths. The color of a beam of light is determined by its spectral properties.


100 – 380 nm


380 – 436 nm


436 – 495 nm


495 – 566 nm


566 – 589 nm


589 – 627 nm


627 – 780 nm


780 – 10.000 nm

Table 1 : Wavelengths of the colors in spectrum

Vision: Visual perception of details in external environment, through sensorial tracks caused by the incoming light to the eye. In other words, vision is the perception of light as a result of the visual sense subject to quantitative or qualitative changes when passing through or reflecting from surfaces of the objects.

Solid Angle: The surface area, which is conical or pyramid-shaped, through which a specific luminous flux passes; and denoted by (Omega). The value of a solid angle seeing a plane of 1sqm is defined to be 1 steradian.

Luminous Flux: The portion of the flux produced by a light source, which portion perceived by the eye and denoted by . The unit of luminous flux is the lumen (lm) and represents the ability of the radiation to produce sensation of gloss. Total luminous flux is defined to be the aggregate of those luminous fluxes produced by a source and diffused to various parts of the sphere.

Luminous Energy: Total luminous flux produced by a specific source for a particular period of effect; denoted by Q. The unit of luminous energy is the lumen second or lumen hour.

Luminous Intensity: The luminous intensity of a point light source in a particular direction of is defined to be the ratio of luminous flux produced by a solid angle encompassing such direction to unit solid angle. The unit of luminous intensity is the candela, which is denoted by ‘cd’. The luminous intensity is 1cd where a luminous flux of 1 lumen is produced by a solid angle of 1 steradian.

Level of Illuminance: The total luminous flux incident on a unit surface and denoted by E. The unit of illuminance is lux (lx).

Luminance: The luminance in a given direction of is the luminous intensity that passes through or is emitted from a unit surface perceived from such direction, and is denoted by L. The unit of luminance is nit for objects, and stilb for sources of light.

Photographical Stimulus: This concept, which is widely used in photography, is proportional to the level of illuminance and its period of effect and is denoted by U. The unit of photographical stimulus is lux second.

Photometric Radiation: It is defined to be the luminous flux intensity of a light emitting surface and denoted by R. It is measured in phot. Its definition is similar to the level of illuminance, but photometric radiation is active in quantity, while illuminance is passive.


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