Developing Color Tolerance Criteria for White LEDs
A study participant judges white light color differences between two compartments in the display cabinet designed for the experiment.
The color of white light can vary greatly, from a warm yellow-white to a cool blue-white. Most phosphor-based, high-power white LEDs on the market today show significant color variations from one white LED to another. In side-by-side applications, such as wall washers or in an LED array, this color variation may be noticeable and unpleasing. As white LEDs make their way into the general illumination market, color consistency becomes critical for acceptance.
Rather than changing LED manufacturing processes, color binning (i.e., sorting by color appearance) is a more cost-effective option for creating white LED arrays with consistent color. But then the question becomes: How much color variation can there be between similar white LEDs? The American National Standards Institute (ANSI) specifies a 4-step MacAdam ellipse—a region plotted on a color space diagram showing where colors are perceived to be the same—as the color-tolerance criterion for certain types of fluorescent lamps. Though color tolerance is well-known for light sources such as fluorescent, researchers have not investigated color tolerance for LEDs.
In this study, LRC researchers conducted a human factors experiment to develop color-tolerance criteria for white LEDs. This criteria defines at what points humans observe a just-noticeable color difference between LED light sources. The study also investigated the impact of light level, spectrum, correlated color temperature (CCT), and visual complexity of the illuminated scene on the color tolerance range. This information was then used to establish a color-binning criteria for white LEDs.
LRC researchers built a display cabinet with two side-by-side compartments. Using RGB LED panels, MR16 halogen lamps with RGB filters, and a variety of white and multicolored backgrounds, subjects compared the color of white light in one compartment against the other. The left-side reference compartment held a constant white light color at a specific x, y chromaticity value. This same value was used as the starting point for the right-side test compartment light source. The test compartment then changed color systematically in incremental steps. At each step, subjects were asked whether they saw a color match or noticed a color difference. If subjects saw a color difference, they commented on how different the two compartments appeared.
The type of visual background has a major impact on the color tolerance range. Complex visual backgrounds using different colors allow for greater variations in white light color; however, white light color differences are more noticeable against a plain white background. Light source spectrum and CCT have very little influence on the color tolerance range. Light levels in the range used for this experiment (300-900 lux) have a small but insignificant impact on the color tolerance range.
Based on the results, the following two criteria were proposed for color binning white LEDs:
- 2-step MacAdam ellipse — For applications where the white LEDs (or white LED fixtures) are placed side-by-side and are directly visible, or when these fixtures are used to illuminate an achromatic (white) scene. Accent lighting a white wall and lighting a white cove are some examples. (The chromaticity values of presently available T8F32 linear fluorescent lamps operated on electronic ballasts fall within 2-step MacAdam ellipses.)
- 4-step MacAdam ellipse — For applications where the white LEDs (or white LED fixtures) are not directly visible, or when these fixtures are used to illuminate a visually complex, multicolored scene. Lighting a display case and accent lighting multicolored objects or paintings are some examples.
Final Report: Developing Color Tolerance Criteria for White LEDs