Figure 1: A very simple, cheeeep, IR detector for hobby photography
A lot of hobby photographers are using IR cameras, or retrofitting common cameras to do IR photography. If the camera is a retrofit, then the photo hobbyist may not know the level of IR before a shot is taken.
This easy junk box build is a way to discover at least a ball-park guestimate of how much IR is present before a shot is taken.
The nice thing about this detector is that it uses a photodiode that reaches all the way down to 1.7 um (micro-meters) in the Near IR / Mid IR boundary area. Some newer cameras may be able to take photographs in that band.