The color of light can be represented using chromaticity coordinates to describe its hue and saturation. A pair of chromaticity coordinates corresponds to a unique color of light; two sources with the same chromaticity coordinates should theoretically appear the same. Chromaticity diagrams to represent the
different color space have been developed and standardized by the Commission Internationale de l’Eclairage (CIE). The most commonly used chromaticity diagrams are the CIE 1931 chromaticity diagram Yellow Blue Green Green Blue Red (a) (b) 7 using (x, y) coordinates to specify chromaticity and the CIE 1976 chromaticity diagram using (u’, v’) coordinates. Examples of the chromaticity diagrams are shown in Figure 1. The 1976 CIE diagram has been used more extensively by the LED industry in recent years to describe chromaticity changes with its advantage of describing a linear color space, which allows for a more intuitive determination of chromaticity differences or shifts (Δu’v’). Note: Δu’v’ describes the magnitude of a chromaticity change, but not the direction of shift; this is the value provided in the LM-80-08 reports. The newer LM-80-15 reports do require reporting of the individual chromaticity coordinates (u’ and v’) instead of the total shift (Δu’v’). LED package manufacturers have been shifting to this new LM-80-15 reporting requirement with newer LED product lines.
The importance of chromaticity stability varies by application. For example, a high degree of chromaticity stability is crucial for light sources in a museum or retail store, but less important for street lighting. Chromaticity stability of the lamp and luminaires is important where multiple lamps or luminaires are being used to wash a wall, or where objects are being evaluated based on color, such as in a hospital or factory. The chromaticity maintenance of LED lamps and luminaires varies among different products, and potentially for the same product used in different applications. Like many other metrics, there are no official standards limiting the amount of acceptable chromaticity shift.
Traditional Lighting Technologies
Many types of light sources have some chromaticity instability over time. The most pronounced is metal halide, but fluorescent and halogen lamps can also shift. The problem with this chromaticity instability is illustrated in Figure 2, where the lamps are creating a different visual appearance for the wall wash. A DOE study on the chromaticity maintenance of LED PAR38 lamps was performed to understand the current performance of LED products on the market. As part of this broader study on the chromaticity
Figure 2. A room lit with ceramic metal halide lamps shows the impact of poor chromaticity stability on the appearance of the wall wash. The varying color appearance of neighboring lamps would necessitate relamping.
8 shift in LED lamps, the chromaticity shift of various conventional lamp technologies has been measured.4 The average change in chromaticity for PAR38 lamp samples with different lighting source technology was measured and the results are shown in Figure 3. A high-level analysis shows that on average, the
LED PAR38 lamps had better chromaticity stability than any of the comparable conventional lighting benchmarks. Of the traditional lighting technologies, ceramic metal halide had the poorest chromaticity stability compared to halogen and compact fluorescent lamps, whereas the latter two are generally considered to have acceptably small levels of color stability.
Chromaticity stability can vary based on LED lamp or luminaire product design with several factors affecting the resulting performance. Ambient air temperature, drive current, and the design of the lamp or luminaire’s thermal management system can influence the junction temperature of the LED, which in
turn, can affect its output characteristics. Of greater concern for long-term chromaticity stability is the effect that high operating temperatures can have on certain package and optical materials. Depending on the design of the LED package, the phosphor layers may settle, curl, delaminate, or otherwise change
the number of photons that are converted to white. This behavior can occur even in the absence of high ambient temperatures. Likewise, other materials in the optical path, such as silicones or plastics may discolor over time. In addition, materials such as glues or chemicals may diffuse into the LED package and affect chromaticity stability. Temperature fluctuations during operation may also intensify degradation mechanisms for some LED products.
There are no official standards limiting the amount of acceptable chromaticity shift in LED lighting products, but different certifications have established requirements. For example, to qualify for the ENERGY STAR® label, nine out of 10 samples of an LED lamp must have a measured chromaticity shift
(Δu’v’) of less than 0.007 over the first 6,000 hours of operation. For applications that require high chromaticity stability, a specification may be established on a project-by-project basis.
Beyond the lack of agreement on acceptable levels of chromaticity shift, there is currently no standard methodology for projecting future chromaticity maintenance using standard test procedures like there is for projecting LED package lumen maintenance. Furthermore, there are no established methods for accelerated testing, leaving each manufacturer to develop their own testing methodologies and predictive modeling approaches. A consensus methodology for predicting chromaticity shift will be a challenge as different materials of construction and manufacturing processes can affect the results; however, an IES committee is working to come to accord on this pressing issue (TM-31).
Chromaticity stability should not be confused with chromaticity consistency — also referred to as color consistency. Chromaticity stability refers to the ability of a product to maintain a constant chromaticity point over its lifetime, whereas chromaticity consistency refers to the product-to-product variation within a lamp or luminaire type. This lamp-to-lamp consistency is important to provide uniform lighting within a room and building. In LED lighting, the chromaticity consistency from lamp to lamp depends on the consistency of the phosphor-converted LEDs. To counter variability that is inherent in the manufacturing process, white LEDs are binned based on chromaticity, lumen output, and forward voltage. This allows the manufacturers of LED lamps and luminaires to provide a more consistent
Figure 4 illustrates the problem of poor chromaticity consistency from lamp to lamp upon installation. Although this case represents a chromaticity consistency challenge, similar effects can be seen with varying chromaticity maintenance over time. Both factors, chromaticity consistency (at time = 0) and chromaticity stability (after thousands of hours in operation), are crucial for the customer
The LED package construction often drives the performance and long term behavior of the LED light source. The impact of LED package design and materials of construction on performance, color quality, lumen maintenance and chromaticity shift, have been investigated for a variety of LED packages under the DOE SSL Core Technology Research Project awarded to RTI International.6 One of the goals of this project is to determine failure modes for LED packages and develop software approaches to model failure rates in an effort to correlate package behavior to system reliability results.
Four main LED package platforms have emerged as light sources for LED luminaires:
High-power packages (1 to 5 W) typically used in products requiring small optical source size (e.g., directional lamps) or high reliability (e.g., street lights)
Mid-power packages (0.1 to 0.5 W) typically used in products requiring multiple light sources for diffuse emission (e.g., troffers, A-type lamps)
Chip-on-board (COB) packages typically used in products needing high luminous fluxes from a small optical source or extremely high luminous flux density (e.g., high-bay lighting)
Chip scale packages (CSPs), also called package-free LEDs or white chips, which have gained attention as a compact, low cost alternative to the high-power and mid-power platforms.
Representative packages from these major LED package platforms are illustrated in Figure 5
To begin the modeling work of the lumen maintenance and chromaticity shift of LEDs, a methodology was developed to analyze LM-80 data across multiple LED manufacturers to provide new insights into LED-level factors impacting lifetime. Data from more than 200 different LED data sets was analyzed using this methodology combined with TM-21 projections and supplemented with experimental data. This process calculates a decay rate constant () that provides a measure of the rate of luminous flux change. Higher values indicate faster lumen depreciation whereas small values indicate longer lumen maintenance times. The analysis provided a detailed look at lumen maintenance and chromaticity shift behavior for a range of LED packages with different designs and materials of construction from multiple manufacturers and found that the materials of construction have a direct impact on long-term performance of LEDs.
Figure 6. Summary of the LM-80 report records by year and LED platform type.
The different LED package platforms have different intrinsic characteristics based on materials of construction and manufacturing processes, which impact their lumen depreciation and chromaticity point stability. Figure 7 shows the decay rate constants as a function of LED junction temperature for different package platforms. Mid-power LEDs can often exhibit more rapid lumen degradation than high-power LEDs or chip on board LEDs; this faster decay of luminous flux is largely due to degradation of the plastic resin body used in the mid-power LED compared to the more stable ceramic substrate used in the high-power LED. The plastic material most commonly employed in mid-power LED packages is polyphthalamide (PPA), a thermoplastic resin. At high temperatures and long operating times, the materials in the package can discolor, crack, or delaminate, leading to lumen depreciation and chromaticity shift.
Different types of plastic resin, however, have different lumen degradation behavior. Improved plastic resins such as epoxy molding compound (EMC) can reduce the thermal constraints associated with conventional mid-power commodity packages. Mid-power LEDs based on EMC resin are more resistant to degradation than PPA and compatible with higher operating temperatures. Figure 8 compares the
Figure 7. Decay rate constants for high-power, mid-power, and COB LEDs as a function of junction temperature. These were calculated using LM-80 and TM-21 projections combined with the new analysis methodology described.
Figure 8. Lumen degradation performance of mid-power packages (PPA and EMC plastic resins) operating at 150 mA and a high-power package (ceramic substrates) operating at 1 A drive current.
lumen degradation performance of mid-power packages using PPA and EMC plastic resins to high power packages using ceramic substrates. While the quality of mid-power packages can vary between LED manufacturers, one commonly seen trend is that EMC-based LED packages can achieve high lumen maintenance at higher temperatures and drive currents than PPA-based LED packages. In addition, it is commonly observed that the ceramic substrates in high power LEDs provide improved heat dissipation and thus result in higher lumen maintenance behavior, especially at high currents and temperatures.
Though the various package materials of construction have different lumen maintenance performance,
both PPA and EMC mid-power packages can achieve excellent lumen maintenance performance (e.g., better than 50,000 hours in some cases), as long as their drive currents are kept low enough to stay in a ‘safe operating zone’ (below where that particular resin material discolors and breaks down over time). While lumen maintenance is important, other forms of parametric failure for LED packages must not be overlooked. Chromaticity shift, for example, may be more detrimental than lumen depreciation for some applications; however, this is sometimes difficult to know in advance. To date, the industry generally quantifies chromaticity shift using Δu’v’, which describes the magnitude of chromaticity shift, but it does not capture the direction of the shift. (The actual chromaticity coordinates u’ and v’ are required to know the direction of the chromaticity shift.) The point at which a chromaticity shift becomes noticeable and results in parametric failure will depend on the lighting application. If the chromaticity change occurs slowly over a very long period (e.g., 25,000 hours), it may not be objectionable in the case where the light sources shift by the same magnitude and in the same direction (unlikely in practice).
Factors impacting chromaticity point stability in LEDs include aging-induced changes in the emitter, phosphor, encapsulant materials, and plastic resin. Emitters can exhibit decreases in radiant flux over time; phosphors can experience decreases in quantum efficiency or shifts in emission spectrum due to oxidation; encapsulants can exhibit cracking, oxidation and yellowing, or changes in index of refraction; and resins can discolor and absorb photons. Higher temperatures will accelerate these degradation mechanisms leading to greater color shift, though the magnitude of the color shift as a function of temperature will vary with packaging materials and manufacturing processes. As with lumen maintenance behavior, if the LEDs are operated at low drive currents and lower than normal operating temperatures, these materials changes leading to chromaticity shift will be very slow to develop, if they occur at all
Figure 9. 1976 CIE chromaticity diagram (u’, v’) illustration the white chromaticity region (denoted by the black circle) and the common directions of chromaticity shift in LED packages. The right figure is an enlargement of the black circle, showing the white chromaticity bins.