Violet chips bring sunlight indoors with LED technology
When we look at objects, the spectrum of light that falls upon them determines our perception of their colours. We are used to viewing objects in sunlight and in various forms of artificial light but the lighting industry assumes that sunlight represents the ‘perfect’ light source. The sun radiates light over a wide spectrum, from violet through green and yellow to red, which gives us the impression of ‘white’ light. While it’s true that the sun seems to change colour during the day, depending on its position in the sky, this is due to scattering of light as it passes through the atmosphere, not due to changes in the spectrum of light that it emits.
In terms of how colours are perceived, a measure known as Colour Rendering Index (CRI) was developed some 40 years ago as a way of assessing the quality of light emitted by a source, such as a lamp. A CRI of 100 means that the light source reproduces colours so accurately that it’s comparable to viewing the same objects in sunlight. For example, tungsten halogen lamps have a CRI of 100, but at the expense of very poor efficiency because most of the energy that goes into them is used to create heat, rather than light.
The National Institute of Standards and Technology (NIST) has developed a relatively new Color Quality Scale (CQS) that is proposed as a more accurate alternative to CRI as a measure of light quality. CQS assesses how white light illuminates 15 standard pastel colours, rather than the 8 colours that define CRI. Using the CQS scale is said to overcome inaccuracies of the CRI system, which is particularly poor at predicting how saturated colours will appear, and it’s these colours to which the human eye is most sensitive. However, CRI is still by far the most commonly used reference for expressing quality of light.
Can good light be efficient light?
As the world strives to reduce energy consumption and CO2 emissions, the demand for low energy light sources is growing. Increasingly, regulations demand that energy-efficient lighting it installed in new buildings. Organisations and consumers, facing spiraling energy bills, are also looking for higher efficiency from commercial and domestic lighting.
Compact fluorescent lamps (CFLs) have been adopted in recent years and some have reached quite high CRI levels. However, they suffer a number of disadvantages, not least slow warm-up times (which tend to be further extended as the lamps age), and the use of mercury in their construction. Mercury is a poisonous heavy metal that creates safe disposal problems when lamps reach the end of their life.
These issues have led to the rapid development and deployment of solid-state lighting systems, including LED lamps. They switch on and off quickly, are up to 5 times more efficient than conventional incandescent bulbs and offer very long life operation – up to 50,000 hours under normal operating conditions. These advantages more than compensate for their relatively high initial cost.
LEDs produce light, sometimes very efficiently, when electricity is passed through them. The first popular LEDs were red ones, typically used as status indicators on electronic products. Today’s LED lamps for general lighting simulate white light. White light contains the complete spectrum of colours visible to the human eye. LED lamps create a close approximation to white light in one of two ways. The first is to combine the output of red, green and blue LED chips into a fixture in order to produce white light. However, the manufacturing process is complex and relatively expensive. The second, and most common, is to create an LED chip that produces blue light, then coat it with a yellow chemical, known as a phosphor (Figure 1), which shifts the spectrum of the emitted light to create nominally white light. (For the technically-minded, blue light has a wavelength of between 450 and 500nm.)
Figure 1: White light is simulated using a blue LED chip coated with a yellow phosphor
LED lamps manufactured in this way achieve CRI of around 80, giving acceptable performance in many applications but lacking output in the green and red parts of the spectrum (Figure 2). This can cause something like a red apple to appear dull, almost brown, when illuminated with such lighting. Skin tones are similarly affected, and the result can be less than flattering.
Figure 2: The low green and red output of lamps based on blue LED chips distorts our perception of colours illuminated by such lamps
A small step in wavelength, a giant step in CRI
LED lamps with a CRI up to 98 are now about to appear on the market. These are based on violet chips, which produce light with around 10% shorter wavelength than blue chips. (Violet light is in the 400 – 450nm range). By producing light in this region, with chips made on gallium nitride substrate, then coating these chips with red, green and blue phosphors (Figure 3), Mitsubishi Chemical Corporation (MCC) has found a unique way to create light that’s very similar to sunlight in the way it renders colours. The technology is called VxRGB.
Figure 3: Coating a ‘VxRGB’ violet LED chip with red, green and blue phosphors creates LED lamps with CRI of over 90
In fact, from the perspective of the human eye, it’s virtually indistinguishable from sunlight. This is because the spectrum of light produced by the technique is very broad (Figure 4) and quite consistent in level across this spectrum.
Figure 4: MCC’s violet chip process produces a broad, even light spectrum
Making white light tunable
Another characteristic of white light is its colour temperature, defined in degrees Kelvin, (K). This relates to the apparent colour of the light source itself, rather than the ability of the source to render colours accurately. LED lamps, and other low energy lamps, are usually described as being ‘warm white’ or ‘cool white’. Warm white is a yellowish through to red light with a colour temperature between 2,700 K and 3,000 K. Over 5000 K, cool light appears as bluish in tone. This is the opposite of cultural associations attributed to colours, in which red is ‘hot’ and blue is ‘cold’.
In some situations it’s desirable to be able to ‘tune’ the temperature of white light and to change the colour appearance and saturation level of objects illuminated by the light source. Retail outlets, museums, offices, hospitality venues, and healthcare facilities, including operating theatres, can all benefit from the flexibility that tunable lighting offers in this respect. Even in homes, you may want a cool white light in the daytime for illuminating a workstation but a warmer light in the evening to create a more relaxing environment.
Verbatim’s future lighting systems based on MCC’s VxRGB technology will offer this flexbility using complementary electronic control circuits. Most importantly, the high CRI will be maintained over the entire colour temperature range, as will the light output level (measured in lumens). In addition, the lamps will be dimmable, without degradation of CRI. In professional applications, these lamps will give lighting system designers a new level of flexibility, enabling them to create ‘indoor sunlight’ and even mimicking this in a way that reflects the sun’s path across the sky during the day.
About the author: Jeanine Chrobak-Kando is Business Development Manager, LED EUMEA, Verbatim (a Mitsubishi Chemical Corporation Company)
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