[Fjord Carver] brings together an RGB LED and CdS Photoresistor to make a color sensor. Those Cadmium Sulfide lights sensors usually have a very wide swing of resistance when exposed to varying levels of light sensitivity. That makes for great resolution when reading them using the ADC of a microcontroller. The LED comes into play by shining known wavelengths of light on the surface being measured. Three separate readings are taken with each of the LED’s different colors, then used to extrapolate the RGB value of the test material. We saw the very same method used a couple of years back. This time around it’s an Arduino doing the measuring instead of a PIC.
So why isn’t that sensor shown in this picture? It’s because we appreciate the application which [Fjord] is using for this sensor. He built a lamp that shines the same color as the surface on which it is placed.