LED lighting technical support
1.2-1 ----Characterising a light source
A light source used for general lighting is characterised by:
its color temperature or its position on the chromaticity diagram (see point 1.4)
its color rendering
its luminous flux
1.2.1Color temperature, hot or cold light
The light source color temperature is determined by comparing its spectrum with that of a black body(1) heated between 2000K and 10000K.
For multi-chromatic fluorescent or electroluminescent (LED) sources, which have an irregular spectrum and thus resemble less than the spectrum of a black body, a Correlated Color Temperature (CCT) is calculated. In practice, we speak of color temperature and not of correlated temperature for all multi-chromatic light sources.
A light of low color temperature such as the standard incandescent lamp is called hot, its spectrum contains lots of yellow and red which gives the impression of a flame; it is perceived as sof and comfortable. From 5000K onwards, a light sources is called cold; it becomes blinding and is not used for general lighting.
Figure 4: Color temperature of different light sources
The technical sheets of lighting products give the color temperatures of sources. For example, for office lighting, we recommend using light having temperature ranging from 3000 to 4000K..
The white light of an LED generated from a blue source exciting a yellow phosphor gives a cold color ranging from 5000 to 10000K (see the different methods for producing white light from LEDs in point 6.1). When the color temperature is not specified by the manufacturer, it is generally more than 7000K.
Hot white LEDs have appeared recently with the development of phosphors that convert blue into a large spectrum or by adding a yellow phosphor and a red phosphor to the blue LED that reduces the color temperature between 2500 and 5000K. The reduction of the color temperature also results in the reduction of the LED yield associated with the use of thicker phosphors or with the addition of an additional phosphor to give the red color.
1.2-2----Characterising a light source
1.2.2 Color rendering
The color rendering index gives the capacity of a source to restore the colors of an illuminated object with regards to an ideal source. The color rendering depends on th spectrum of light emitted. The more this spectrum is conditions and thereby close to that of sunlight, the better shall be its rendering. A light source may be used for general lighting as soon as its color rendering exceeds 80%.
For an incandescent light source
An incandescent light source has a color rendering index close to 100%.
For a discharge luminescent source or LED
A TL tube or a compact fluorescent lamp may have color rendering indices from 80 to more than 90%
A white LED produced from a blue LED covered with yellow phosphor generally had bad color rendering (<80), as the emitted light lacked red and did not restore this color correctly. Today, we find white LEDs with a color rendering index greater than 80.
When the color rendering index exceeds 80 it is specified in the technical characteristics of the LED.
A low-pressure sodium lamp (used for lighting roads) has a rendering of 22, nearly monochromatic emission of yellow.
1.2.3The luminous flux (in lumen [lm])
This is the visible energy/time unit. A power of 1W radiated by a source at a wavelength of 555nm has a luminous fulx of 683 lumens (see point 1.3.2).