The light colour describes the colour appearance of the light. LEDs do not directly produce white light. There are two ways in which white light is produced from LEDs.
Using a blue LED with a phosphor coating to convert blue light to white light by a process called fluorescence. Combining red, blue and green LEDs to produce white light. White light is produced by varying the intensities of the individual red, blue and green chips.
Correlated Colour Temperature (CCT)
The description used to describe the effect of heating an object until it glows incandescently, the emitted radiation, and apparent colour, changes proportionally to the temperature. This is easily demonstrated when considering metal in an oven that begins to glow red, then orange, and then white as the temperature increases. CCT is defined in degrees Kelvin (K).
Therefore, in addition to the colours of surfaces, it is also the light colour that determines a room’s atmosphere.What defines the colour temperature of a white LED is the thickness of the phosphor layer and the wavelength of the blue chip.
A measure of the degree of colour shift objects undergo when illuminated by the light source as compared with those same objects when illuminated by a reference source of comparable colour temperature. The reference source has a CRI of 100. Natural light (daylight) would also be 100.
Color Rendering Index – CRI indicates the accuracy with which a light source such as an LED can reveal the various colors of an object.
Tm-30-15 is a new method to evaluate colour rendition; it includes several measures and graphics t evaluate the Fidelity (Rf) and Gamut (Rg) of a source when compared to a reference illuminant. TM-30 uses the average value of 99 colour samples; CRI however only utilizes an average score of 8 colour samples. This metric is still yet to become widely adopted in the LED market.
Spectral Power Distribution (SPD)
A light source is the visible white light that we see which is made up of a spectrum of various colors of light, ranging from wavelengths of 380nm (violet) to 760nm (red). The SPD is a graph that shows the power (strength) of each wavelength of light produced by a particular light source.
An elliptical region on the CIE chromaticity diagram that contains all the colors that are indistinguishable to the average human eye, from the color at the center of the ellipse. Adjacent ellipses are ‘just distinguishable’ in terms of color. This system is used to refine the binning process of LED colors. Slight color differences in the appearance of LED light are measured in MacAdam ellipses or steps.