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Basic Knowledge of Lighting

What is light?

Light is the relatively small part of the spectrum of electromagnetic radiation that can be perceived by the human eye. The visible light spectrum ranges from 380 nanometres (nm) to around 780 nm. This electromagnetic radiation consists of vibrating packets of energy (quanta) emitted in waves by a light source. The time that light needs to travel from the point of origin to the observer's eye is determined by the speed of light.

In 1850, the French physicist Léon Foucault developed a method of measuring the speed of light using a rotating mirror. It proved highly accurate, establishing that light travels at a speed of 2.98 x 108 m/s. For light travelling in vacuum and air, this is generally rounded to 3 x 108 m/s – i.e. 300,000 kilometres a second.

What are spectral colours?

All colours perceived by the human eye are determined by the wavelength and frequency of visible light. Each wavelength evokes a particular colour. The spectrum of sunlight is continuous, ranging from short-wave violet (starting at approx. 380 nanometres) through blue, blue-green, green, green-yellow, yellow and orange to long-wave red (up to approx. 780 nm). Spectral colours are distinguished by the impression of colour evoked by radiation of a particular wavelength (monochromatic light). For each hue, they are the most intensive colour with the highest saturation.

1) Lumen (lm)

Lumen is the international(SL)unit of luminous flux, a measure of overall light emitted by a light source.


2) Candela (cd)

Candela is the international(sl) unit of luminuous intensity, a measure of light emitted by a light source in a particular direction.
Any given light source will have different luminous intensities in different direction considered.
[candela = lumen/solid angle]

 3) Beam Angle [Unit : degree]

Beam angle is the angle betweent two directions for which the luminous intensity is 50% of the maximum intensity as measured in a plan through the nominal beam centerline.Beam angle of a lamp give an idea of how the light beam spread away form the beam center.


4) LUX (lx)

Lux is the international(SL)unit of illuminance, a measure of light arriving at a surface, 1 lumen per square foot equals 1 footcandle, while 1 lumen per square meter equals 1 lux .
[lux = lumen/m2]


5) Color Temperature / Correlated Color Temperature (CCT)

A black-Body radiator will have different color when it is heated up to different temperature(in kevin). The Temperature at which the heated black-body radiator matches the color of the light source is the color temperature of the light source.


6) Color Rendering Index (CR)

Color rendering index ranges from 0-100 ,measures a light source's ability to reproduce the color of varies objects being lit by reference light source, black body radiator (CRI=100)

7) Efficacy (lm/W)

Eifficacy is the effcience of a light source to concert electricity power to light which is expressed in lumen output per unit power input (watt)
[Efficacy = lumen / wattage]


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