White Light from LEDs
Research for obtaining white light from light-emitting diodes (LED).
February 2004 by Ron Jeuch
This is an incomplete and ongoing project.
Light-emitting diodes (LED) have been available traditionally in discrete colors. The electronic mechanism responsible for the emitted light produces light of a fixed frequency for any given material. White light consists of a continuous spectrum of all visible colors. Most traditional light sources such as the sun also emit energy outside the visible range - ultraviolet and infrared.
Man-made lights include incandescent, fluorescent and gas-discharge bulbs. Fluorescent bulbs create an illusion of white light by emitting several bands of colors spread over the visible range. Gas-discharge fixtures often have a distinct nonwhite hue.
White LEDs are now available. Most of these LEDs actually contain a blue or ultraviolet light source and the white light is obtained by placing yellow phosphors between the emitter and the outside world. The light often is a very cold light, containing too much blue. So-called warm-white fluorescent phosphors have also been developed but offer a lower efficiency. The resulting light from white LEDs varies across the beam in the form of “blue holes” or “yellow rings”. “Friendly” is not an attribute applicable to white LED light. By the way, that is also the case with so many light fixtures we encounter everywhere.
The theory of colors tells us that we can produce white light by additive mixing of the three primary colors red, green and blue. This project will test that theory.
The idea is to take an array of 36 LEDs, 12 each of red, green and blue. Since their light efficiency differs and since the human eye’s different sensitivity over the visible light range changes, we cannot assume that the set of three LED colors will produce the desired result automatically.
The goal is to determine how many LEDs of each color will yield an acceptable white light. For testing purposes, we could vary the current through the LEDs. However, this will not lend itself to a quantitative analysis since the light output of an LED vs. its current is not linear. A better way is to drive all LEDs at their rated current but using current pulses where the pulse width can be changed (and measured exactly).