To achieve a CIE color coordinate target with LEDs requires mixing accurate light amounts from the LED sources and knowledge of the precise color that an LED will generate under the given excitation and ambient conditions.
The requirements for accurate color illumination with LEDs are derived from the sensitivity of human color vision. Human vision translates color by the ratio of red, green, and blue cone simulation. The red, green, and blue cones do not only respond to red, green, and blue light. There is continuous overlap in cone excitation. The red cone is excited at some level from deep red through blue, the green cone is stimulated by red-orange through blue-violet, and the blue cone is stimulated by yellow-green to deep violet. The total of all three cone stimulation amounts follows a smooth bell curve that peaks at green. The sensitivity of the eye to color differences depends on how much the cone stimulation ratio changes in that area of color. The eye is most sensitive around the area of warm whites and least sensitive in the saturated green area. A green mix that has a 5% error in the amount of light coming from the green source may not look off target but a warm white mix with that amount of error would be obviously off target.
Human brightness perception is also helpful to understand. Like many other senses brightness perception is not linear. For a perceived doubling of light intensity the number light light photons coming from the source must increase by much more than double. An easy approximation mathematically is the use the square so that a perceived doubling requires a four times increase in the source photon emission. If this is translated into dimming, achieving a perceived 10% dimming level from full brightness requires the source to emit 1% of the photons that it does at full intensity.
To design a precise color illumination system it is necessary to understand how an LED behaves. LEDs do not make light photons at any current level. There is a minimum current at which the LED will make photons and around this level the color of the light of the LED changes a lot making it difficult to compensate for. This level is typically below 1% of the maximum LED current. When operated above the minimum current level the LED’s color shifts with the current and operating temperature an amount that is dependent on the LED’s color. Some red LEDs change in intensity by a factor of two as the operating temperature changes from minimum to maximum. Some Green LEDs change in color several perceptible steps as the operating temperature changes. All LEDs decrease in efficiency as the current increases.
To generate an accurate color illuminant from LED sources an LED drive level is calculated based upon the LEDs’ intensity and color characteristics over the operating temperature and current levels. Some of these characteristics like color drift are consistent enough for a particular LED that the same data can be used for all fixtures if the color accuracy requirement isn’t too great. Other characteristics like LED intensity can change more between parts or lots of parts. This data would need to be collected for each fixture produced as a calibration step in the production process. If the color accuracy requirement is sufficiently loose calibration wouldn’t be necessary.