What Color Is That LED?

Measuring the spectrum is simple enough, but perception by the human eye is another matter.

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As I discovered while researching an upcoming article on phosphors for LED lighting, the question turns out to be surprisingly difficult to answer. Measuring the spectrum of a light source is straightforward, but determining how that spectrum will be perceived by the human eye is more challenging.

Under medium and high brightness conditions, the color perceived by the brain will depend on signals received from three types of cone cells in the eye. Each type is sensitive to a particular wavelength range, with some overlap between the types. For example, both the medium-wavelength and long-wavelength cones are excited by green light, and so green will be perceived as brighter than red light of the same objective intensity.

For this reason, LED lamps and LED components like phosphors are often specified relative to a “color space” such as the diagram in the figure. The edges of the shape are defined by “pure” monochromatic light of the specified wavelengths, while the area inside shows the results of mixing two or more such colors. For example, all possible blue LED + green phosphor combinations can be found along a line between the two sources. Blue + green + red combinations can be found within the triangle those three primary colors define.

Complicating matters further, there is no single absolute color space. Colors produced by pigments tend to be more muted than colors produced by light. The scientific understanding of human color perception has evolved over time, as have the primary colors available to lighting designers. The diagram shown here is based on a color space defined in 1931, at the dawn of the color film era. It is still widely used, but does not account for differences in perceived brightness as accurately as more recent models. The Wolfram Demonstrations Project hosts an interactive diagram showing the differences among color spaces and choices of primary source colors.

The color of an LED or LED phosphor, then, is typically specified as a pair of coordinates in a given color space. But even that information is not sufficient to determine whether the LED will be suitable for a given application. An individual’s color perception is both genetically and culturally determined: the same spectrum will be seen as more or less “natural” by different groups. Lighting design is inherently subjective; no single “white” is likely to satisfy the requirements of all potential applications.

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Figure: CIE 1931 color space chromaticity diagram. Image courtesy Wikimedia Commons.