The Color of White— Color Consistency with Leds Ron Steen, Xicato

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.

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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 lightingL 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 quantity. Both topics have a plethora of texts and standards, but when we throw the acronym 100

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- F lux per package (lumens) nology and many other items that are not Cost per lumens ( U S dollars) 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 significant 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

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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

4,000 N eutral 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. Not only is there color CCT although the white colors are very dif- variation between manufacturers, there is Picture 2. Example of CFL color ferent in appearance. significant color variation within a single inconsistency. Approximately one decade later, with manufacturing run. With LEDs, the playing the advent of film, Kodak Company became very interested in color. In the early 1940’s, Kodak tasked David MacAdam to map the recently created 1931 color space and Blue White light try and determine when the human can Chip 1 Phosphor see the difference in color. MacAdam car- + ried out a series of experiments where he Phosphor Yellow Light showed different color swatches to a group Blue Light of people and came to a determination of when the observer could see the difference in color. The experiments resulted in a series of ellipses (Figure 5). The size of the ellipse varies throughout the color space InGaN LED and shows how the human eye has different sensitivities at different color points. For example, a significant change in X and Figure 6. White LEDs are created using a blue LED and a phosphor coating. Y color point is required within the green region prior to 50% of the observing pop- ulation being able to see the difference in field is not limited to just color while very little movement in X, Y in a few major players but is the purple region will yield the ability to occupied by hundreds of recognize differences. While this particu- manufacturers making lar metric is not commonly used, it is quite LED products. handy as a reference. It should be noted that The factors affecting MacAdam’s data is under pressure for being LEDs color differences valid as only 200 subjects were tested, and arise from three main there was no real age, gender or ethnic con- areas: the wave length siderations. Be that as it may, the definition of the LED, the formu- serves as a point to discuss the ability to see lation of the phosphor the difference in color, which becomes the and the coating tech- real issue when we begin to discuss white niques. Of course white color differences with White LEDs. light can be created using We have all probably experienced RGB or RGBA arrays of seeing differences in color from source to LEDs, but the predomi- source within a built environment, espe- nant method for creat- cially within office buildings, when we ing white LEDs is with a can witness bluish or greenish or pinkish blue LED and a phosphor hues from fluorescent tubes. Picture 2 is coating (Figure 6). We of an installation using compact fluores- will discuss the phosphor cent lamps and obviously highlights the conversion approach rel- issues with color point consistency even ative to color issues. within incumbent technology. Most light- The blue LED, or the ing designers know to specify a particular “pump” color, has a varia- lamp manufacturer to avoid this problem. tion band of about +/- 5 Individual lamp manufacturers have con- nanometers. The phos- trolled color point within “tolerable” speci- phor has variation in the fications, but then again there are only a conversion characteris- Picture 3. The phosphor is a powder that needs to be mixed few big players in the conventional lamp tics. Some of these vari- with a binding material, such as a clear silicone, to be dis- market. Each large manufacturer would ables include particle size pensed over the top of the chip.

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Target CCT and Target Duv and Nominal CCT1 tolerance (K) tolerance 2700 K 2725 ± 145 0.000 ± 0.006 3000 K 3045 ± 175 0.000 ± 0.006 3500 K 3465 ± 245 0.000 ± 0.006 4000 K 3985 ± 275 0.001 ± 0.006 4500 K 4503 ± 243 0.001 ± 0.006 5000 K 5028 ± 283 0.002 ± 0.006 5700 K 5665 ± 355 0.002 ± 0.006 6500 K 6530 ± 510 0.003 ± 0.006 Flexible CCT T2 ± ΔT3 D 4 ± 0.006 (2700-6500 K) uv 1 Six of the nominal CCTs correspond to those in the fluorescent lamp specification [2]: 2700 K, 3000 K (Warm White), 3500 K (White), 4100 K (Cool White), 5000 K, and 6500 K (Daylight), respectively. 2 T is chosen to be at 100 K steps (2800, 2900, …., 6400 K), excluding those eight nominal CCTs listed. Figure 7. Graphical representation of the chromaticity specification 3 ΔT is given by ΔT = 0.0000108×T2 +0.0262×T +8. 4 2 of SSL products in Table 1, on the CIE (x,y) chromaticity diagram. Duv is given by Duv = 57700× (1/T) −44.6× (1/T) + 0.0085. (Source: American National Standard Lighting Group) Table 1. Nominal CCT categories. (Source: ANSLG.) and density. The phosphor is a powder that needs to be mixed with a binding material, such as a clear silicone, to be dispensed over the top of the chip (Picture 3). This is now the other variable, the mixture of the phosphor within the binding matrix, and the amount of phosphor material that is dispensed over the LED. Varying amounts of phosphor will change the conversion char- acteristics of the blue light into white. The variation issues mentioned above all compound and have created the need for the LED manufacturer to create “bins” of product. A bin is simply a sorting of the LEDs that come off the end of the assem- bly line. The bins are traditionally sorted on three variables: flux (amount of light), forward voltage (vF—amount of voltage it takes to turn on the LED) and color. At the end of the line, each LED package is measured for the aforementioned charac- Figure 8. Typical binning structures from LED manufacturers. (Source: Philips, Osram, Cree, teristics and put into a bin. When product Nichia web-based data sheets.) is purchased, bin codes can be specified to assure you are receiving what you want. The big problem with this approach is the body locus and draws a box around the ples, Manufacturer A has chosen a single inherent nature of variation. The LED man- intersection point. For 3000 CCT the box bin strategy of only using the ANSI stan- ufacturer cannot guarantee which parts spans approximately 7 MacAdam Ellipses. dard. Manufacturer D on the other hand are going to be coming off the end of the The reasons for the specification are two- has subdivided each ANSI box into 18 dif- line, and the buyer does not have a predict- fold: 1) The current CFL specification is a ferent boxes. It would seem manufacturer D able source of supply if extremely tight bin 7 step MacAdam Ellipse and 2) if the stan- would be a good selection by simply speci- codes are required by the user. dard were any tighter the yields for the LED fying a single box where the color point is Because of this variation relative to manufactures would fall and prices would desired, but as mentioned before, it is diffi- color, NEMA, ANSI and the US DoE potentially go up. Since the standard has cult if not impossible for the LED manufac- teamed up to create a standard for color been created, the major LED manufactur- turer to guarantee steady supply of a spe- binning LEDs. The standard is ANSI ers have mostly followed the basic boxes set cific bin selection throughout the lifecycle C78.377 (Figure 7 and Table 1). This stan- out by ANSI. Each of these boxes have then of a product. Another complexity arises if a dard basically uses the intersection point been subdivided into color bins. Figure 8 “bin per project” strategy is employed. This of a particular CCT vector with the black is a snap shot of typical binning structures strategy requires the ability to track exactly from LED manufacturers. In these exam- which color bin was used for the project

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1x2 Step represents 2 beams with a color point spatial differential of 1 Ellipse in the Y direction and 2 Ellipse’s in the X direction. In field research by Xicato, the 1x2 step spatial difference is the maximum allowed variation. Additionally there is a generally accepted rule of thumb which defines less than 2 MacAdam Ellipses are required when adjacent fixtures are ren- dered against a white wall and less than 4 MacAdam Ellipses are acceptable when lighting a multi-colored scene such as pro- duce in a grocery store. In most cases the 7 step as accepted in the ANSI standard is considered unacceptable by lighting professionals but indeed may be good enough in non-critical areas. Way finding, path- way and street lighting may be an example where a 7 MacAdam Ellipse tolerance could Figure 9. Each MacAdam ellipse around a particular center point can define the magnitude of be accepted. As it is often said in lighting “It noticeable color difference. is all about the application.” Color consistency is no different. beam in each picture. Using the 7-step pic- Everything discussed so far has been ture as an example, the pattern on the left is about color point consistency of a prod- running north on the 3000 CCT vector line uct coming “out of the box.” The other towards the yellow region while the pattern important attribute is how well the prod- on the right is running south towards the uct maintains color overtime. So far, the Picture 4. Each of these light patterns is pink region of the 1931 CIE Diagram. If the LED lighting world has been worried about within the definitional confines of what is color point were to run “Northwest” in the lumen maintenance, and a standard has considered 3000 Kelvin or CCT by the ANSI 3000 CCT ANSI box from the center point, been issued by IES known as LM-80. This standard. the light would appear green in tone rela- standard lays out a test protocol to measure tive to a point in the “Southeastern” corner how well an LED holds the lumens over and any replacement products would have appearing reddish. time but mentions nothing about holding to be matched to the color bin if a replace- As we would assume, the closer we color over time. ment product is required. approach to 1 MacAdam ellipse, the less If we buy into the original premise that So why is all this color binning stuff color difference there is. The picture labeled light is about quantity and color, then our required if a standard has been created to say “what is good enough.” If we take the work of David MacAdam as being valid, then this metric can be applied to make the determination of what is good enough. Figure 9 shows how the MacAdam ellipse’s can be viewed. Just like rings of the trees show their age, each MacAdam ellipse around a particular center point can define the magnitude of noticeable color difference. Thus, we refer to these as the number of steps of ellipse. Another term often used is Standard Deviation Color Match (SDCM). The ANSI requirement at 3000K is approximately a 7-step MacAdam Ellipse. If we take MacAdam at his word, then with anything beyond one ellipse we can begin to see the shift in chromaticity. The question is how far we have to go before it becomes objectionable. Every light pattern shown in Picture 4 is within the definitional confines of what is considered 3000 Kelvin or CCT by the ANSI standard. While the camera does not represent the magnitude as well as the human eye, clearly we can see the dif- Figure 10. The mapping of color movement over time where each color grouping of points represents a device under test (DUT). ference in color between the right and left

18 – Global LEDs/OLEDs – Summer 2011 www.globalledoled.com The color of white—Color consistency with LEDs current lifetime standards are only address- ing one attribute of the two required compo- nents, and after all isn’t the LED story about energy efficiency and long life to make the sustainability story believable? Many in the LED industry are coming to the realization that color maintenance may be the limit- ing factor relative to lifetime. To confound the issue, while there are accepted models to extrapolate lumen depreciation to 50k hours of operation, there are no known statistical models to predict color spatial movement over time. Figure 10 shows the mapping of color movement over time where each color grouping of points represents a device under test (DUT). Each LED in the data set is performing well within 1 MacAdam Ellipse relative to itself and stay- ing mostly within the 1x2 MacAdam step box previously referenced and well within the 4 step MacAdam ellipse reference. In closing, color consistency and quality WWW.IES-ENERGIA.COM of color with LEDs are two critical compo- nents to the acceptance of the light source across all applications. This ra ticle has only addressed the color consistency portion of this subject but has not addressed the reasons to select a particular CCT, broached the issues with spectral power distribu- tions, outlined the pros and cons of com- peting LED architectures relative to color and has not addressed the very meaty subject of CRI (color rendering index) which has been much maligned in recent years. If the lighting professional does not ask the tough questions about LED color consistency, many consumers and end customers may be disappointed in the results. Some in the LED field have said the American public is not willing to pay for quality of light, and this becomes a very interesting premise. Is the real question, “How long will the American public purchase poor quality of light?” Since lighting is in transformation from the vacuum tube to solid state, the market will tell us all what is good enough.

Ron Steen is a veteran of the LED world and started playing with LEDs in 1996 while working at General Motors. Ron success- fully launched the first full function LED tail lamp on the Cadillac DeVille in 1999 and did pioneering work with LED headlamps. Ron moved to the general lighting field with Philips in 2004 and was director of product management solid state lighting systems and drivers prior to his current position as VP of business development NA with Xicato, which provides LED modules to fixture manufactures with a focus on light quality without compromise.