By BizLED Bureau
Nov 5, 2015: Good quality LED light that shows colors accurately and consistently is very important, particularly for commercial, medical and industrial sectors. Interestingly, consumers notice poor quality light but they fail to notice quality lighting and the difference in their color consistency.
What is color consistency?
Color consistency ensures incredibly uniform color performance between different luminaires. Color consistency is a parameter of light quality for color and white LED lights. In case of white LED light, correlated color temperature (CCT) indicates if the white light is warm, cool or neutral. CCT allows a range of variation in chromaticity, even when the CCT value is the same. Hence, ensuring color consistency is a major challenge for the LED manufacturers.
The more complicated the physics and chemistry of LED, the more difficult it is to manufacture it with consistent color properties. This is why manufacturers find it difficult to make LEDs with perfect color consistency, particularly because phosphor-converted LEDs shift in color over time. To reduce the variation in chromaticity, LED manufacturers use a color consistency system based on MacAdam ellipses (Wyszecki and Stiles 1982).
Two LED lights that are slightly different in chromatic properties may not be detected as different by human eye. D L MacAdam has shown that a noticeable difference in the colors of two lights was about three times different in the standard associated with making color match between a reference light and a test light (MacAdam 1942, Wyszecki and Stiles 1982).
MacAdam had also pointed out that the noticeable difference form an elliptical pattern of “constant discriminability” in chromaticity space, centered on the chromaticity of a reference light. Thus, a human eye cannot detect the color difference between a reference light having a chromaticity at the center of the ellipse and any other light within the elliptical pattern of constant discriminability.
To explain it technically, the temperature in correlated color temperature refers to black body radiation?it is the light emitted by a solid object with certain properties, heated to the point of incandescence?and is expressed in degrees K (Kelvin). As a black body gets hotter, the light it emits goes through a number of colors?from red to orange to yellow to white to blue. This sequence of colors indicated a curve within a color space.
The actual temperature of a 2700 K LED source is about 80° C, even though the LED source emits light of the same color as a filament heated to a temperature of 2700 K. (source: Philips Color Kinetics)
Business reason for color consistency
No doubt good quality LED lights with color consistency is essential, but beyond that there is business reason for good quality LED lighting, particularly in sectors like hospitality, retail, medical, manufacturing, etc. Besides energy efficiency, the best thing about LEDs is their quality of light.
Retail sector: Good quality lighting with color consistency plays an increasingly important role in creating the right ambience in, for instance, the retail market?in fact, it directly impacts sales. But the best thing about LED-based lighting is that it can be engineered for optimum impact and effect. As a result, retail stores are shifting from conventional lights to LED lights.
Medical sector: LED lighting definitely impacts the wellbeing of people?it also affects their behaviors. Visual examination is an important part of medical treatment and it is very important for doctors and nurses to see colors correctly, like skin tones?a particular color of the skin may indicate that something is right or not so right. Hence, color consistency plays a very important role in the medical sector.
Different technologies available
Today lighting designers have started giving importance to consistency of lumen output and color. Manufacturers are also developing technologies to take careof color consistency of LED lighting.
Xicato claims that its LED-based modules use remote- or cold-phosphor technology and provides a five year warranty on them. This means that LED luminaire manufacturers who use Xicato modular light engines can ensure customers consistency in light output and color for a minimum of five years. According to Xicato, the consistency is because phosphor gets deposited on the secondary optics and not directly on blue LEDs, hence it isolates the phosphor from heat which causes color shift.
On the other hand, Optibin developed by Philips Color Kinetics, uses an advanced bin selection formula that, the company claims, exceeds industry standards for chromaticity to guarantee uniformity and consistency of hue and color temperature for Philips lighting products.
The American National Standards Institute (ANSI) has developed standards for LED light sources–chromaticity standard C78.377A, published in 2008, defines 8 nominal CCTs ranging from 2700 K (warm) to 6500 K (daylight).
Philips Color Kinetics? white LED lighting fixtures offer industry leading color consistency and uniformity using Optibin. For instance, new line of white LED cove lights from Philips Color Kinetics exceed both ANSI and Energy Star standards for color uniformity in interior general illumination.
Achieving consistency of color temperature and hue in cove lighting applications is particularly challenging for lighting designers and installers. In cove lighting applications, light sources are positioned very close to the illuminated surfaces?usually white walls or ceilings?so there is very little room for mixing, and the appearance of the light is strongly angle-dependent. Viewed from a distance, even small variations in color temperature and hue are clearly visible.
However, designers argue that users give more value to lower maintenance costs than color consistency aspect when they choose an LED-based light. However, they believe that with the awareness level of the consumers increasing, they, too will realize the important role of color consistency in LED lighting.
NEMA / ANSI ANSLG C78.377-2008: Specifications for the Chromaticity of Solid State Lighting Products for Electric Lamps. American National Standards Institute, 2008.
DOE?s Energy Star LED Lighting Program
For more info: www.energystar.gov/index.cfm?c=ssl_res.pt_ssl_program
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