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Objects at any temperature greater than absolute zero will emit radiation. The peak frequency of this radiation is proportional to the object's absolute temperature - the hotter the object, the higher the frequency of the peak radiation. At room temperature, a lamp filament radiates mostly in the infrared portion of the electromagnetic spectrum. This radiation is invisible to the naked eye, but can be captured by thermal imaging cameras (these are commonly used to spot heat leaks in a home's walls, ceilings, doors and windows). As the filament is heated up, it emits more radiation at all frequencies, and the peak of that radiation moves into the visible part of the spectrum.