There is nothing quite like the majesty and splender of a huge vivid rainbow on a bright day after a storm. Unfortunately rainbows seem to be more than a little uncooperative when trying to coax them indoors. All I wanted was nature's splash of multicolored light on the ceiling. But it turned to more difficult than anticipated.
I tried the traditional prism approach but the projected spectrum was way too small. I looked at commercial offerings - the one that seemed to give good results was battery powered and didn't project the spectrum at all but 'faked' a rainbow by projecting individual colored LEDs. I considered purchasing diffraction grating but wasn't sure what I'd get. I read about mirrors in trays of water and splitting open DVD-RW disks and pealing off metallization with packaging tape. But ultimately I ended up with something much simpler... I ended up with this Instructable!
My Rainbow projector is based upon what I had around the house. The main component is a LED PAR (spotlight) bulb and an unused old CD-R disk. I wanted to make the projector smaller, but the physical size was based on the LED lamp which I already had so unless I wanted to spend more on a new lamp I was stuck with something a bit larger than ideal, but still very nice. The projector is about 12-1/2" tall and about 5" square. The on/off switch is hidden inside the ventilation opening at the bottom.