Tuning RGB LEDs for Reduced Color Variation

The first light bulbs were invented in the 1800s, and since then, lighting technologies have continued to grow and improve in order to provide better flexibility and energy efficiency. Light-emitting diodes (LEDs), which use semiconductors to convert electricity to light, are considered to be one of the fastest developing technologies in lighting today.

Image Credit: Jaros/Shutterstock.com

LEDs are indeed one of the most energy efficient lights available. They offer extended lifetimes when compared to other light sources while being more physically robust and smaller. LEDs are available in a range of intensities, colors and sizes.

Human-Centric Lighting

Human-centric lighting draws upon the production of LEDs and is the most recent trend in lighting. Human-centric lighting employs precisely controlled brightness, direction and colors of light to offer both visual effects and psychological or physiological benefits. Since the 1990s, the effects of light on humans have been researched by scientists who began exploring the effects of light therapy on patients with seasonal affective disorder.

Research has since then demonstrated that light helps to maintain daily sleep/wake cycles and controlled indoor lighting systems could help in sleeping better and waking up in a more refreshed manner. Other potential benefits of human-centric lighting systems include improved mood, visual acuity, health, productivity, perception and performance.^1-3

Variations in LEDs Reduce Visual Appeal and Benefits

The design of human-centric lighting systems requires a  precise control of the color and brightness of light sources.

Although LEDs are available in a wide range of intensities and colors, making them superior candidates for human-centric lighting systems, LED light sources frequently show differences in brightness and/or color due to tiny variations in material structure, composition and/or thickness resulting from mass production processes.

For instance, two LEDs from the same production batch may show brightness and colour differences that can be easily seen by the human eye. Additionally, brightness and color differences can take place over time as the LED is operated due to small variations in voltages applied.

Variations in color and brightness reduce the quality of the light, the visual appeal and may possibly reduce any intended physiological or psychological benefits.³

Tuning LEDs to Reduce Color Variations

Variation in LEDs is frequently reduced by using ‘binning,' where LEDs are arranged into groups to keep variations of brightness and color to a minimum. However, small tolerances in differences and small ‘bins’ can drive up production costs. Additionally, differences in brightness and color may still accumulate over time during operation.^3,4

The best way to decrease variation between LEDs is to calibrate them. LED calibration can be executed in two ways: gamut calibration or white point calibration. White point calibration, as the name itself explains, deals with moving the white point of each LED in order to match a target.

However, this can still bring about variations in other colors within the spectrum of the LED. Thus, gamut calibration is considered to be the best option for RGB LEDs.

A color gamut with a white point marked. Image Credit: Admesy

Gamut calibration involves initially calibrating the white point and then tuning the maximum red, blue and green values so that each individual LED has exactly the same output.

Spectroradiometers are employed for measuring a light source's spectrum (from the individual R, B and G LEDs) and uses the three-color points in order to determine the color gamut. It is then possible to calibrate the gamut by adjusting the RGB values of the input to new RGB values.

Accurate spectrometers are needed for performing white point adjustment and gamut calibration. Admesy offers a range of color measuring equipment including spectroradiometers perfect for LED color gamut calibration including the highly accurate Rhea and all-round Hera series.

Image Credit: Admesy

Conclusion

White point adjustment and gamut calibration of RGB LEDs are capable of overcoming the need for highly accurate binning and is the only solution that helps to obtain exactly the same light output for LED light sources.

Proper calibration explains the fact that manufacturers are no longer limited to the quality level of LED binning, and customers can in fact design their perfect lighting solutions with optimum visual appeal and potentially beneficial effects on performance and health. Admesy provides a range of spectroradiometers perfect for LED calibration.

References and Further Reading

1. Lighting Europe ‘Quantified benefits of Human Centric Lighting’ 2014, available from https://www.zumtobel.com/com-en/index.html Accessed October 10^th, 2017.
2. Schlangen, L ‘The effect of light on our sleep/wake cycle,' available from http://www.lighting.philips.com/b-dam/b2b-li/en_AA/Experience/Topics/Education/Lighting_Academy/how-to-beat/Daily-sleep-wake-cycles-whitepaper-FINAL.pdf Accessed October 10^th, 2017.
3. Bouten R, ‘Avoiding Brightness and Color Mismatch with Proper RGB Gamut Calibration,' Luger Research EU, Issue 62, 2017.
4. CREE ‘LED Color Mixing: Basics and Background’ 2016, available from https://www.cree-led.com/ Accessed October 10^th, 2017.

This information has been sourced, reviewed and adapted from materials provided by Admesy.

For more information on this source, please visit Admesy.