What is correlated color temperature?
Correlated color temperature (CCT) measures the color of a lighting source, from “warm” color temperatures (reddish, yellow hues) to “cool” color temperatures (blueish hues), and “pure white” in between.
In technical terms, the color temperature of a light is equivalent to the temperature of an ideal black body radiator that radiates light of a similar color to the light source. Consequently CCT is measured in kelvins (K), the unit of absolute temperature.
The chart below is from EagleLight’s LED University. It provides a good overview of different color temperature measures, and examples of each:
In the LED industry, references to “cool white” or “warm white” light bulbs reflect the color temperature of the light emitted from the LED bulb.
Why is it important for lighting?
Light affects the body’s circadian cycles, i.e. the sleep and wake cycles of everyday life. This is due to light’s effects on melatonin, a hormone secreted by humans to regulate their circadian rhythms. Blue light, or blue light wavelengths in broad-spectrum light suppresses melatonin production. This is why bright midday sunlight, composed of more short blue wavelengths, encourages wakefulness, while a dark room encourages the production of melatonin, and therefore sleep.
Artificial lighting, done poorly, can disrupt the body’s natural circadian rhythms and aggravate sleep disorders by suppressing melatonin production. Too much blue light at night is a reason why many people do not sleep enough, and too little sleep can increase the risks of depression, diabetes, and cardiovascular problems. In addition, previous research suggests that prolonged exposure to blue light may cause retinal damage.
On the flip side, artificial lighting that mimics broad-spectrum sunlight and natural circadian rhythms can help contribute to well being and productivity. For instance, daytime office lighting with a high correlated colour temperature can improve alertness and workplace productivity. A 2007 report analyzed the effects of replacing 2900k flourescent lights with 17000k lights on shift workers, and found that the employees experienced a 26.9% drop in fatigue, 28.2% increase in alertness, and 19.4% increase in work performance.
In addition to the effects of blue light on health and well-being, color temperature can have pronounced aesthetic effects. If a “white light” is too yellow or blue, it will cast a yellowish or blue tint on the objects underneath it, and will not produce a saturated range of colours found under a broad-spectrum white light. This makes lighting with too high or low of a color temperature unsuitable for photography, film, art installations, and certain residential markets.
LEDs & color temperature control
White LED light sources are composed of more of blue light wavelengths than incandescents, and even flourescent bulbs. This is because most white LED bulbs user highly efficient blue InGaN LEDs to produce the light. The chart below, taken from patent US8253336, showcases the wavelengths of light emitted from these three devices (Curve B – incandescent, Curve C – flourescent, Curve D – white LED).
The light output from the first few generations of white LEDs also had less than appealing aesthetic qualities. In order to produce the white light from these blue LEDs, manufacturere would coat the blue LED die with a yellow phosphor layer. These original YAG phosphors would produce a dim, yellow light.
Luckily, new phosphor compounds, remote phosphors, and advanced drivers and dimmers offer improved methods of controlling the color temperature of LEDs, with resulting benefits in aesthetics and human health.
For instance, patent application US20110084614, “Led Lighting System” from Summalux, describes a system for generating light “using spectral characteristics that resembles sunlight.” Through a system of user-inputted data on geography and time of day, sensors, white and color LEDs, and network controllers, the LED lighting system is able to display light from 1800K to 6500K – the range of color temperatures of daytime sunlight. In addition, the patent application discloses methods for adjusting the blue component of the light from 1-100%, even when color temperature maintains steady, to facilitate circadian responses.
Issued patent US8253336, “LED lamp for producing biologically-corrected light” from Biological Illumination, LLC, describes a system for generating broad spectrum light that minimizes melatonin suppression. This is accomplished through the use of color filters that subtract certain wavelengths of light. One of the claims, for instance, describes the use of color filter Roscolux #87 Pale Yellow Green with a 2700K color temperature blue-pumped white LED.
Intematix’s issued patent US8203260, “Color temperature tunable white light source,” describes another means by which users can adjust the color temperature of a white LED. Here, the color temperature of the light source is controlled by altering the magnitude of the drive currents behind two led arrays of different color temperatures. A remote phosphor is attached to at least one LED of one of the arrays, and the varying magnitude of the drive current affects the wavelengths of light emitted by the remote phosphor. Another method for tuning the light involves controlling a duty cycle of the drive current, thereby controlling the relative proportion of time each LED emits light.
Manufacturers and installers should push for the production and installation of higher quality, tunable white LED lights. Given the growth of the LED market, and individual and government demand for more energy-efficient, environmentally-friendly lighting solutions, it is tempting to push for immediate and mass development and installation of LEDs – and worry about health and aesthetic concerns later.
This, however, is a bad idea for consumers and the LED industry in the long term. We only have to look at previous consumer backlash to a new lighting solution – compact flourescents. Despite CFL’s energy saving property, many people baulked at putting them in their homes and offices due to their harsh lighting and poor aesthetic qualities. The emerging concern over the health effects of mercury found in CFLs prompted a second backlash against this new lighting technology.
In order for LEDs to stay in the market, manufacturers and installers need to prioritize broad-spectrum, biologically appropriate lighting sources to satisfy current and future consumer concerns, rather than focusing on just being the lowest-cost provider. This will be a win-win for LED makers and end-users alike.
After all, at the end of the day, we all need to be able to turn off our lights and get a good night’s sleep.