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How LEDs Produce White Light

 

There are two conventional methods of producing white light with LEDs. This provides underpinning information to understand how tuneable LED solutions are produced.


 

Conventional Method 1 : RGB Method

 

A mix of red, green and blue LEDs in one module according to the RGB colour model, white light is produced by the proper mixture of red, green and blue light. The RGB white method produces white light by combining the output from red, green and blue LEDs. This is an additive colour method which is often counterintuitive for people accustomed to the more everyday subtractive colour system of pigments, dyes, inks and other substances which present colour to the eye by reflection rather than emission. For example, in subtractive colour systems green is a combination of yellow and blue; in additive colour, red + green = yellow and no simple combination will yield green. Additive colour is a result of the way the eye  detects colour, and is not a property of light. There is a vast difference between yellow light, with a wavelength of approximately 580 nm, and a mixture of red and green light. However, both stimulate our eyes in a similar manner, so we do not detect that difference.

RGB white gives you control over the exact colour of the light, and it tends to make colour “pop”. But RGB white light is hardware-intensive, since it requires three LEDs, and it tends to render pastel colours unnaturally, a fact which is largely responsible for the poor colour rendering index of RGB white light. Colour rendering learning page

*Colour rendering is the ability of a light source to reproduce the colours of various objects faithfully in comparison with an ideal or natural light source.*


 

Conventional Method 2: The Phosphor Method

 

The Phosphor white method produces white light in a single LED by combining a short wavelength LED such as blue or UV, and a yellow phosphor coating.  The blue or UV photons generated in the LED either travels through the phosphor layer without alteration, or they are converted into yellow photons in the phosphor layer. The combinations of the blue and yellow photons combine to generate white light. In some modules, the yellow phosphor is remote. 

 

Phosphor white offers much better colour rendering that RGB white, often on a par with florescent sources. Phosphor white light is also much more efficient than RGB white. Because of its superior efficient and colour rendering (typically Ra70 to 85), phosphor white is the most commonly used method of producing white light with LEDs. Whilst colour rendering is good in the pastel shades, the spectral density is not close to daylight and there are problems rendering the more saturated colours such as red (R9). Read more about this in the section on colour rendering

In a typical phosphor white manufacturing process, a phosphor coating is deposited on the LED die. The exact shade or colour temperature of white light produced by the LED is determined by the dominant wavelength of the blue LED and the composition of the phosphor.

The thickness of the phosphor coating produces variations in the colour temperature of the LED. Manufacturers attempt to minimize the colour variations by controlling the thickness and composition of the phosphor layer during manufacturing.

Over time, the blue die and the yellow phosphor will degrade. This results in the delivered light shifting in colour. It will also produce unexpected colours if the device is operated at a different current or operating temperature.

 

LED Reflector Cup PhotonStar

 

ChromaWhite™ Advanced Colour Mix Technology

 

ChromaWhite™ is an advanced colour mix technology using multiple LEDs in one module. The colour mix approach results in a highly efficient white light delivered to a specific colour point. The spectral density is closer to daylight (but does not emit UV or IR). The colour rendering matches or exceeds popular conventional sources such as tungsten halogen, peaking at Ra97. Colour rendering is excellent in pastel and saturated shades as well as skin tones. The challenging saturated reds (R9) is typically greater than 90.

ChromaWhite™ does not rely on a precise mix, instead using ColourLoc to monitor the delivered light and adjust it to the required colour point. This eliminates the problems of colour shift caused by degradation over life, variations in operating temperature and variations in current (usually when dimmed).

Read how colour tuneable LED solutions work.

 

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