A short explanation of color temperature

Why would someone want to learn about color temperature? Because knowing how color temperature affects the output hue is a key element in designing light sources with LEDs. Binary colors such as Red, Green, and Blue are expressed in wavelengths as nanometers. White colors are expressed in temperatures.

Color temperature is a method of describing and measuring the color characteristics of light. The scale used is the Kelvin Color Temperature Scale, which was named after William Lord Kelvin of Ireland, who wrote of the need for an “absolute thermometric scale” of temperatures. Temperatures in Kelvin are not referred to nor expressed as degrees, such as in the Fahrenheit or Celsius scales. The definition implies that absolute zero (0 K) is equivalent to -273.15⁰C, or -449.67⁰F.

Color temperature is based upon the principle of the black body radiator; this black body will emit light whose color depends upon the temperature of the radiator. As an object is heated, it will begin to glow, and the color will move from deep reds (such as seen in a wood fire), through oranges and yellows, all the way to white hot. Light sources that glow this way are called incandescent radiators, and the advantage to them is that they emit a continuous spectrum, radiating light energy at all wavelengths of their spectrum. Because the spectrum of light is wider than our ability to see it, values falling below the visible part of the spectrum are referred to as infrared, and values above the spectrum are ultraviolet.

Light emitting diodes are not incandescent radiators because each color die emits light at a slightly different band. These types of radiators have what is referred to as a Correlated Color Temperature (CCT), which is an indicator of their ability to render all colors of their spectrum. While the theoretical black body radiator has a CCT of 100, LED’s approach this with a 70-90+ rating. There are variations on this standard, but this is a good start to understanding the relationship between color temperature and color hue. It is important to keep in mind that lower temperature reds are considered to be “warm” while higher temperature blues are described as “cool”. In this instance, the warmer and cooler refer to the hue, not the temperature.

Pixent LEDs are offered in three temperatures of white: Blue White (5700-7500 K), Natural White (3800-4500 K), and Warm White (2700-3200 K). The ranges here represent the entire range of temperature output from any particular die series; in the factory the dies are ‘binned’, or selected; to be within 200-300 K of each other on any particular lot of parts. Thus any order of LEDs will be only of a single bin, keeping the light temperature very consistent across their design life, and can be depended upon to do so. There is some drift that correlates with the temperature of the LED, but it is generally very linear within a single bin.

But what if we want to create differing temperatures of white at different times with the same LED? To do this, we use the RGB LED and vary the grayscale intensity of each of the three dies to produce a emitted light product which is the particular hue of white we want to achieve. Using the Pixent led-exec software and the PixelDrive board, a desired hue can be designed and implemented very simply. Because the entire string of LEDs will display the hue being selected, the decision on hue can be made in real time and under real conditions. Below is an example of how the color selection slider can be used to create a hue with RGB LEDs.

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