This paper describes the construction and use of a color light mixer that uses different color LEDs. The idea was partly inspired by two papers.^1,2 The first one describes how a standard LED can be converted into a point-light source, and the second one explains how a Ping-Pong ball can be used to mix polarized color light from two lasers.

It was Sir Isaac Newton about 300 years ago who first proved that white light, such as sunlight, is really a mixture of colors that we perceive as white. We simplify the problem by saying: Let's pick three colors from the rainbow and see if we can get white light by adding them together (we may skip the discussion on what "adding the colors" means at this point). Then we show one of the many versions of the "R+G+B=W" experiment. If you ever tried to do this experiment, you know how difficult it is to obtain a white color that would convince the students. In principle, three flashlights and three colored filters should work, but in practice it is much harder to obtain the desired results than one would expect, because flashlights produce light spots of nonuniform intensity. Slide projectors do a good job producing a light spot with uniform brightness, but the experiment requires three slide projectors, careful choice of the filters, and fine adjustment of the color intensities by varying the distance from each projector to the screen.

Modern technology offers a simpler solution. The obvious choice today seems to be the use of red, green, and blue light-emitting diodes (LEDs). Though the idea is straightforward, it requires some tricks to make it work properly. Here are instructions on how to make a simple and reliable light mixer that you can carry in one hand and does not require complete darkness in the classroom to obtain a convincing result. In addition, this experiment is useful in introducing topics such as emission light spectra, filters, and human perception of colors.