The color of white. Color consistency with LEDs Ron Steen, Xicato

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1 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

2 Light can be boiled down to two main attributes: amount and color. While this is a very simplistic way to look at the complex world of lighting, these two very basic attributes define what most lighting professionals care about. Of course it can be argued many other things come into play, such as aesthetics of beam, construction techniques of fixtures, trims, glare, and the list can go on and on. But if the two basic attributes of color and quantity are not correct, the very essence of lighting is somehow missed. We have all heard the phrase Quality of Light, and we probably all have a definition in our mind s eye of what quality lighting is. I would suspect this definition is in the form of a picture and changes relative to our setting. Quality of light may be a cloudless blue sky at noon while in the park, a well-lit kitchen where colors for food look good and you can see what you are doing, a dimly lit restaurant which has created the right mood but still provides enough light to read the menu. In any of these examples the definition of what constitutes Quality can be derived by the color and the quantity. Both topics have a plethora of texts and standards, but when we throw the acronym LED into the conversation, it seems the entire context of the discussion changes, and immediately we start talking about semiconductors, energy efficiency, technology and many other items that are not necessarily core to the quantity and color. First, let s address the issue of quantity of light with LEDs. This topic has clearly been the focus of LEDs since their inception, and a great deal has been written on 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 to Flux. It seems the entire focus with 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 increasing in the amount of light produced while price is decreasing, creating a very compelling 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 professionals know, this is just not the case for many reasons. Two reasons for not going to LEDs are not providing enough light and not being efficient enough to displace the incumbent technology. In both cases we know the LED is improving and it is simply a matter of time before a solid state solution has the amount of required flux and the correct LPW to carry the day. A third reason for not converting is due to the color and quality of LED light. Color in and of itself is a highly complex topic, and again there are text books and PhD courses assigned to the topic. But when it comes to LEDs, additional nuisances come into play. The remainder of this article shall attempt to discuss these nuisances and inform the reader of things to beware of when evaluating the color quality of LEDs. Let us begin with the basics: Light is energy and is expressed as the visible spectrum represented by the colors of the rainbow (Figure 2). The measure of each Cost per lumens (US dollars) or Flux per package (lumens) discrete color is expressed in terms of nanometers (nm) and referred to as wavelengths of light. Within the visible spectrum of light, red, green and blue colors can be combined to create the color pallet. For a relevant discussion, a basic grounding in lighting metrics is important, and the first lighting metric was provided in the 1840s by Lord Kelvin (Picture 1). Wikipedia says: The Kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units (SI) and is assigned the unit symbol K. The Kelvin scale Figure 1. Haitz s Law: Every 10 years light output has increased by a factor of 20 while cost per lumen has fallen by a factor of 10 Figure 2. Light is energy and is expressed as the visible spectrum represented by the colors of the rainbow. (Source: Max-Planck-Institut für extraterrestrische Physik) Global LEDs/OLEDs Summer

3 10,000 9,000 Cool Tones 8,000 7,000 HID Light Black body locus 6,000 Figure 4. The 1931 CIE color diagram. 5,000 Neutral Tones 4,000 3,000 Warm Tones 2,000 Halogen 1,000 Figure 3. Color temperatures in K (degrees Kelvin). is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all thermal motion ceases in the classical description of thermodynamics. The reference point that defines the Kelvin scale is the triple point of water at K (0.01 Celsius). The kelvin is defined as 1/ of the difference between these two reference points. The Kelvin scale is named after the Belfast-born engineer and physicist William Thomson, 1st Baron Kelvin ( ), who wrote of the need for an absolute thermometric scale. Unlike the degree Fahrenheit and degree Celsius, the kelvin is not referred to or typeset as a degree. The kelvin is the primary unit of measurement in the physical sciences, but is often used in conjunction with the degree Celsius, which has the same magnitude. Absolute zero at 0 kelvin is Celsius. While Kelvin was searching for a thermodynamic scale, a useful by-product was the ability to measure the temperature of fire. As most of us have learned from an early age, the blue part of a flame is the hottest part and the orange part is less hot. Ironically we refer to bluish light as cool and orange light as warm but suffice it to say the Kelvin scale is still used to this day to discuss color temperature of a white light source (Figure 3). As a point of reference our standard incandescent lamp is ~2850 Kelvin and a halogen source is ~3000 Kelvin. We now fast forward to 1931 where a group of scientists assembled in France and formed an International Commission on Illumination (CIE). Based on a body of research done in the 1920 s, the CIE formed what is known as the 1931 CIE color diagram (Figure 4). This diagram is still commonly used in the industry today and provides a frame of reference to discuss color and color point. Going back to the colors of the rainbow, each wavelength is represented going around the diagram while an Figure 5. David MacAdam s ellipses. X and Y grid is established to help define a particular color point. This diagram is usually shown with a line which runs the white portion of the color diagram. The line is known as the black body locus. Wikipedia defines this locus as: In physics and color science, the Planckian locus or black body locus is the path or locus that the color of an incandescent black body would take in a particular chromaticity space as the blackbody temperature changes. It goes from deep red at low temperatures through orange, yellowish white, white, and finally bluish white at very high temperatures. Very much like the flame above, the black body locus serves as a well defined reference point within the lighting community Another representation on many CIE diagrams is CCT lines. CCT is a correlated color temperature that is simply the correlation of the Kelvin temperature (K) superimposed onto the 1931 color diagram, using the black body locus as the zero point. What becomes important to know 14 Global LEDs/OLEDs Summer

4 is that CCT does not define color point but simply defines a color continuum, which is why the CCT is represented with a line rather than a point. Using 3000 CCT as a reference, one can follow the line north or upward on the vector and the color moves to the yellow region of the chart just as you can follow the line south or downward and the 3000 CCT becomes pink in shade. In both cases the color is defined as 3000 CCT although the white colors are very different in appearance. Approximately one decade later, with 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 try and determine when the human can see the difference in color. MacAdam carried out a series of experiments where he showed different color swatches to a group 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 and shows how the human eye has different sensitivities at different color points. For example, a significant change in X and Y color point is required within the green region prior to 50% of the observing population being able to see the difference in color while very little movement in X, Y in the purple region will yield the ability to recognize differences. While this particular metric is not commonly used, it is quite handy as a reference. It should be noted that MacAdam s data is under pressure for being valid as only 200 subjects were tested, and there was no real age, gender or ethnic considerations. Be that as it may, the definition serves as a point to discuss the ability to see the difference in color, which becomes the real issue when we begin to discuss white color differences with White LEDs. We have all probably experienced seeing differences in color from source to source within a built environment, especially within office buildings, when we can witness bluish or greenish or pinkish hues from fluorescent tubes. Picture 2 is of an installation using compact fluorescent lamps and obviously highlights the issues with color point consistency even within incumbent technology. Most lighting designers know to specify a particular lamp manufacturer to avoid this problem. Individual lamp manufacturers have controlled color point within tolerable specifications, but then again there are only a few big players in the conventional lamp market. Each large manufacturer would claim a portion of the color space specification, and as long as an individual built environment was populated with a single manufacturer, everything would be fine. After time, though, as alternative lamps become stocked and installed, the color consistency issue arises. This problem is exacerbated with LEDs due to the inherent variation within the LED process. Not only is there color variation between manufacturers, there is significant color variation within a single manufacturing run. With LEDs, the playing Blue Chip field is not limited to just a few major players but is occupied by hundreds of manufacturers making LED products. The factors affecting LEDs color differences arise from three main areas: the wave length of the LED, the formulation of the phosphor and the coating techniques. Of course white light can be created using RGB or RGBA arrays of LEDs, but the predominant method for creating white LEDs is with a blue LED and a phosphor coating (Figure 6). We will discuss the phosphor conversion approach relative to color issues. The blue LED, or the pump color, has a variation band of about +/- 5 nanometers. The phosphor has variation in the conversion characteristics. Some of these variables include particle size + 1 Phosphor Picture 2. Example of CFL color inconsistency. White light Phosphor InGaN Figure 6. White LEDs are created using a blue LED and a phosphor coating. LED Yellow Light Blue Light Picture 3. 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. Global LEDs/OLEDs Summer

5 Figure 7. Graphical representation of the chromaticity specification of SSL products in Table 1, on the CIE (x,y) chromaticity diagram. (Source: American National Standard Lighting Group) Nominal CCT 1 Target CCT and tolerance (K) Target D uv and tolerance 2700 K 2725 ± ± K 3045 ± ± K 3465 ± ± K 3985 ± ± K 4503 ± ± K 5028 ± ± K 5665 ± ± K 6530 ± ± Flexible CCT ( K) T 2 ± ΔT 3 4 D uv ± 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. 3 ΔT is given by ΔT = T T D uv is given by D uv = (1/T) (1/T) 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 characteristics 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 assembly 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 characteristics and put into a bin. When product is purchased, bin codes can be specified to assure you are receiving what you want. The big problem with this approach is the inherent nature of variation. The LED manufacturer cannot guarantee which parts are going to be coming off the end of the line, and the buyer does not have a predictable source of supply if extremely tight bin codes are required by the user. Because of this variation relative to color, NEMA, ANSI and the US DoE teamed up to create a standard for color binning LEDs. The standard is ANSI C (Figure 7 and Table 1). This standard basically uses the intersection point of a particular CCT vector with the black Figure 8. Typical binning structures from LED manufacturers. (Source: Philips, Osram, Cree, Nichia web-based data sheets.) body locus and draws a box around the intersection point. For 3000 CCT the box spans approximately 7 MacAdam Ellipses. The reasons for the specification are twofold: 1) The current CFL specification is a 7 step MacAdam Ellipse and 2) if the standard were any tighter the yields for the LED manufactures would fall and prices would potentially go up. Since the standard has been created, the major LED manufacturers have mostly followed the basic boxes set out by ANSI. Each of these boxes have then been subdivided into color bins. Figure 8 is a snap shot of typical binning structures from LED manufacturers. In these examples, Manufacturer A has chosen a single bin strategy of only using the ANSI standard. Manufacturer D on the other hand has subdivided each ANSI box into 18 different boxes. It would seem manufacturer D would be a good selection by simply specifying a single box where the color point is desired, but as mentioned before, it is difficult if not impossible for the LED manufacturer to guarantee steady supply of a specific bin selection throughout the lifecycle of a product. Another complexity arises if a bin per project strategy is employed. This strategy requires the ability to track exactly which color bin was used for the project 16 Global LEDs/OLEDs Summer

6 Figure 9. Each MacAdam ellipse around a particular center point can define the magnitude of noticeable color difference. Picture 4. Each of these light patterns is within the definitional confines of what is considered 3000 Kelvin or CCT by the ANSI standard. and any replacement products would have to be matched to the color bin if a replacement product is required. So why is all this color binning stuff 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 difference in color between the right and left beam in each picture. Using the 7-step picture as an example, the pattern on the left is running north on the 3000 CCT vector line towards the yellow region while the pattern on the right is running south towards the pink region of the 1931 CIE Diagram. If the color point were to run Northwest in the 3000 CCT ANSI box from the center point, the light would appear green in tone relative to a point in the Southeastern corner appearing reddish. As we would assume, the closer we approach to 1 MacAdam ellipse, the less color difference there is. The picture labeled 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 rendered against a white wall and less than 4 MacAdam Ellipses are acceptable when lighting a multi-colored scene such as produce 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, pathway and street lighting may be an example where a 7 MacAdam Ellipse tolerance could be accepted. As it is often said in lighting It is all about the application. Color consistency is no different. Everything discussed so far has been about color point consistency of a product coming out of the box. The other important attribute is how well the product maintains color overtime. So far, the LED lighting world has been worried about lumen maintenance, and a standard has been issued by IES known as LM-80. This standard lays out a test protocol to measure how well an LED holds the lumens over time but mentions nothing about holding color over time. If we buy into the original premise that light is about quantity and color, then our Figure 10. The mapping of color movement over time where each color grouping of points represents a device under test (DUT). 18 Global LEDs/OLEDs Summer

7 current lifetime standards are only addressing one attribute of the two required components, 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 limiting 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 staying mostly within the 1x2 MacAdam step box previously referenced and well within the 4 step MacAdam ellipse reference. In closing, color consistency and quality of color with LEDs are two critical components to the acceptance of the light source across all applications. This article 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 distributions, outlined the pros and cons of competing 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 successfully 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. Global LEDs/OLEDs Summer