References

1. G. T. Gillies, "Altered light-emitting diode point source emitter," Am. J. Phys. 48, 418–419 (May 1980). first citation in article
   
2. A. A. Middleton and S. Sampere, "Color mixing via polarization," Phys. Teach. 39, 123–124 (Feb. 2001). first citation in article
   
3. I used the following LEDs from Kingbright : red L-53SEC-H, green L-53VGC-E, and yellow L-53SYC-H, and blue NSPB500S from Nichia. Kingbright: http://www.kingbright-led.com/ Nichia: http://www.nichia.co.jp/index_eng.html. first citation in article
   
4. You can get one for about $50 U.S. from Educational Innovations (see http://www.teachersource.com). first citation in article
   

1. The Ping-Pong Ball Color Mixer Revisited
   Chris Chiaverina, Phys. Teach. 45, 120 (2007)