Issues of color rendering and perception by the human eye

At the recent Fourth International Conference on Solid-State Lighting, Yoshi Ohno of NIST spoke about the issues involved with using color rendering index (CRI), the only internationally recognized metric for the way in which light sources bring out the appearance of colored objects.

Unfortunately, there are many issues with using CRI - for example, the CRI value of a white LED source does not indicate how far the color coordinates are located from the blackbody locus.

Ohno also pointed out that there is a trade-off between color rendering and luminous efficacy, which are arguably the two most important parameters for light sources. Incandescent sources have (by definition) a CRI of 100 but the maximum luminous efficacy of radiation is 240 lm/W. For a green LED, the efficacy can reach 640 lm/W, since the eye is most sensitive at 550 nm, but with a green source all objects appear green, and the CRI value can be as low as -28.

Ohno has developed a simulation program that allows metrics such as CRI to be calculated for different LED types - for example by altering the wavelengths of the individual LEDs in an RGB combination. He demonstrated that a well designed RGB LED can give good color rendering, while a 4-chip device has excellent CRI. However, the deficiencies inherent in CRI mean that a new metric needs to be devised for the LED industry.

The human eye

In another talk, Kevin Dowling of Color Kinetics discussed how color is perceived by the human eye and the issues that are peculiar to using LEDs. The human eye has a huge dynamic range but also has a non-linear response to luminance. Small changes are very noticeable at low brightness levels, but much larger changes can be tolerated at high brightness.

One issue for LED lifetime, which is defined by lumen depreciation - the extent to which the lumen output of the device declines over time - is whether or not the change in lumen output can actually be observed by the user. Other sources also depreciate, but for these there tends to be different ways of defining lifetime.

Another issue is color consistency between LEDs: "Customers can perceive differences and they care," said Dowling, whose company manufactures LED-based lighting systems. "It is sometimes possible to perceive differences between LEDs in the same bin, and color mixing can be dramatically affected by improper binning."

Noting that the different LED manufacturers have different ways of binning their products, Dowling raised the question of the need for standardization in this area.

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This is an extract from a longer article originally published in the October 2004 issue of Compound Semiconductor magazine.

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