A cold mirror is a dichroic surface that reflects visible light, while passing through most infrared light and UV radiation. While they would be ideal to use with DIY projection, they are quite expensive and mostly a custom order item, far out of reach of the average DIYer. They also need to be large enough to pass the whole beam at a 45 degree angle.
HotMirrors, by contrast, and be put directly in the beam (90 degrees) near the bulb when it's small.
