Although cameras have built-in filters that block UV and IR light, the IR portion is often weak and allows some light through. By adding a filter[1] that blocks

Although cameras have built-in filters that block UV and IR light, the IR portion is often weak and allows some light through. By adding a filter[1] that blocks visible light, there is often enough IR left to expose a photo[2]. With some caveats[3], it can be a cheap way to see the world in a different part of the spectrum. [1] Hoya R72 for example [2] A tripod is usually necessary since only a fraction of the light makes it through the filters. Even in bright sunlight a long exposure time may be required. [3] Not all lenses work well in IR. Some have internal reflections that lead to "hot spots" or white patches, and some have bad optics for IR that causes blur. Similarly, some cameras' IR cut filters are too strong to be useable.