There would be several problems with using infra red heating lamps in a greenhouse. With the moisture present, the hot bulb could explode. Even the small units, 250-375 watts, have warnings to keep them at least 18 inches from any surface. And ,since most small, 120 volt lamps are only available in 250-375 watts, you would need several to heat even a small greenhouse in a warm location. They would probably be even worse to use in an even smaller cold frame.

With the moisture present, the hot bulb could explode.

Heat lamps, being designed for use in bathrooms, are required by law to withstand spray induced thermal shock. Mine explicitly state that they have been tested with 4C water after operating for some time. Otherwise I agree 100% - they would also be wasteful as a lot of heat would leak out in the form of light.

Probably not worth it.

nathanhurst,
Glad you agree 100% (minus moisture). My info on the water comes from the manufacturers warning statement on at least one ir lamp unit, link attached.

Here is a link that might be useful: uv lamp warning