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Title The color of white— Color consistency with LEDs Ron Steen, Xicato If the two basic attributes of color and quantity are not correct, the very essence of lighting is somehow missed. 12 – Global LEDs/OLEDs – Summer 2011 www.globalledoled.com The color of white—Color consistency with LEDs ight can be boiled down to two LEDs are not providing enough light and discrete color is expressed in terms of main attributes: amount and not being efficient enough to displace the nanometers (nm) and referred to as wave- color. While this is a very sim- incumbent technology. In both cases we lengths of light. Within the visible spec- plistic way to look at the com- know the LED is improving and it is simply trum of light, red, green and blue colors plex world of lighting, these a matter of time before a solid state solu- can be combined to create the color pallet. two very basic attributes define what most tion has the amount of required flux and For a relevant discussion, a basic Llighting professionals care about. Of course the correct LPW to carry the day. A third grounding in lighting metrics is important, it can be argued many other things come reason for not converting is due to the and the first lighting metric was provided into play, such as aesthetics of beam, con- color and quality of LED light. in the 1840s by Lord Kelvin (Picture 1). struction techniques of fixtures, trims, Color in and of itself is a highly com- Wikipedia says: glare, and the list can go on and on. But if plex topic, and again there are text books The Kelvin is a unit of measure- the two basic attributes of color and quan- and PhD courses assigned to the topic. But ment for temperature. It is one of the tity are not correct, the very essence of when it comes to LEDs, additional nui- seven base units in the International lighting is somehow missed. sances come into play. The remainder of System of Units (SI) and is assigned We have all heard the phrase “Quality this article shall attempt to discuss these the unit symbol K. The Kelvin scale of Light,” and we probably all have a defini- nuisances and inform the reader of things tion in our mind’s eye of what quality light- to beware of when evaluating the color ing is. I would suspect this definition is in quality of LEDs. the form of a picture and changes relative to Let us begin with the basics: Light our setting. Quality of light may be a cloud- is energy and is expressed as the visible less blue sky at noon while in the park, a spectrum represented by the colors of the well-lit kitchen where colors for food look rainbow (Figure 2). The measure of each good and you can see what you are doing, 2 a dimly lit restaurant which has created the 10 right mood but still provides enough light to read the menu. In any of these examples the definition of what constitutes “Quality” 101 can be derived by the color and the quan- tity. Both topics have a plethora of texts and S dollars) 0 standards, but when we throw the acronym U 10 LED into the conversation, it seems the or entire context of the discussion changes, and immediately we start talking about 10-1 semiconductors, energy efficiency, tech- lux per package (lumens) F nology and many other items that are not Cost per lumens ( necessarily core to the quantity and color. 10-2 First, let’s address the issue of quantity of light with LEDs. This topic has clearly been the focus of LEDs since their incep- 10-3 tion, and a great deal has been written on 1965 1970 1975 1980 1985 1990 1995 2000 2005 the topic, which is why we will not spend any more time than this paragraph on it. What is important to point out is the “Race Figure 1. Haitz’s Law: Every 10 years light output has increased by a factor of 20 while cost to Flux.” It seems the entire focus with per lumen has fallen by a factor of 10 LEDs has been, until recently, to get higher lumen packages and higher lumen per Watt (LPW), or efficacy. Clearly the vast majority of government spending for solid state lighting (SSL) has been going into the goal of LEDs being the energy saving and sustainable solution. This promise has been well documented and best shown in what is known as Haitz Law (Figure 1), which clearly shows the trend of LEDs increas- ing in the amount of light produced while price is decreasing, creating a very compel- ling story of Lumens per Dollar and sig- nificant energy savings. LEDs seem to be a panacea to solve all things lighting in every application and, as most lighting profes- Figure 2. Light is energy and is expressed as the visible spectrum represented by the colors sionals know, this is just not the case for of the rainbow. (Source: Max-Planck-Institut für extraterrestrische Physik) many reasons. Two reasons for not going to www.globalledoled.com Global LEDs/OLEDs – Summer 2011 – 13 The color of white—Color consistency with LEDs 10,000 9,000 8,000 Cool Tones Black body locus HID Light 7,000 6,000 Figure 4. The 1931 CIE color diagram. 5,000 eutral 4,000 N Tones 3,000 2,000 Warm Warm Tones Halogen 1,000 Figure 3. Color temperatures in K (degrees Kelvin). is an absolute, thermodynamic tem- While Kelvin was searching for a thermo- Figure 5. David MacAdam’s ellipses. perature scale using as its null point dynamic scale, a useful by-product was absolute zero, the temperature at the ability to measure the temperature of which all thermal motion ceases in fire. As most of us have learned from an X and Y grid is established to help define a the classical description of thermo- early age, the blue part of a flame is the particular color point. This diagram is usu- dynamics. The reference point that hottest part and the orange part is less ally shown with a line which runs the white defines the Kelvin scale is the triple hot. Ironically we refer to bluish light as portion of the color diagram. The line is point of water at 273.16K (0.01˚ “cool” and orange light as “warm” but suf- known as the “black body locus.” Wikipedia Celsius). The kelvin is defined as fice it to say the Kelvin scale is still used to defines this locus as: “In physics and color 1/273.16 of the difference between this day to discuss color temperature of a science, the Planckian locus or black body these two reference points. white light source (Figure 3). As a point of locus is the path or locus that the color of The Kelvin scale is named after reference our standard incandescent lamp an incandescent black body would take the Belfast-born engineer and physi- is ~2850 Kelvin and a halogen source is in a particular chromaticity space as the cist William Thomson, 1st Baron ~3000 Kelvin. blackbody temperature changes. It goes Kelvin (1824-1907), who wrote of We now fast forward to 1931 where from deep red at low temperatures through the need for an “absolute thermo- a group of scientists assembled in France orange, yellowish white, white, and finally metric scale”. Unlike the degree and formed an International Commission bluish white at very high temperatures.” Fahrenheit and degree Celsius, the on Illumination (CIE). Based on a body of Very much like the flame above, the black kelvin is not referred to or typeset research done in the 1920’s, the CIE formed body locus serves as a well defined refer- as a degree. The kelvin is the pri- what is known as the 1931 CIE color dia- ence point within the lighting community mary unit of measurement in the gram (Figure 4). This diagram is still com- Another representation on many CIE physical sciences, but is often used in monly used in the industry today and pro- diagrams is CCT lines. CCT is a correlated conjunction with the degree Celsius, vides a frame of reference to discuss color color temperature that is simply the corre- which has the same magnitude. and color point. Going back to the colors lation of the Kelvin temperature (K) super- Absolute zero at 0 kelvin is −273.15˚ of the rainbow, each wavelength is repre- imposed onto the 1931 color diagram, Celsius. sented going around the diagram while an using the black body locus as the zero point. What becomes important to know 14 – Global LEDs/OLEDs – Summer 2011 www.globalledoled.com The color of white—Color consistency with LEDs is that CCT does not define color point but claim a portion of the color space specifica- simply defines a color continuum, which tion, and as long as an individual built envi- is why the CCT is represented with a line ronment was populated with a single man- rather than a point. Using 3000 CCT as a ufacturer, everything would be fine. After reference, one can follow the line “north” or time, though, as alternative lamps become upward on the vector and the color moves stocked and installed, the color consistency to the yellow region of the chart just as you issue arises. can follow the line “south” or downward This problem is exacerbated with and the 3000 CCT becomes pink in shade. LEDs due to the inherent variation within In both cases the color is defined as 3000 the LED process.

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