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As far as I know, you can see an infrared (IR) beam by pointing a camera at its direction and take a photo or video of it.
As I understand it, external flashes like the Canon Speedlite 600EX RT have a IR beam to help the camera with focussing.
Can I use this beam as a sort of night vision device, such as the M3 carbine?
If you literally want to see the AF-assist beam:
P. 20 in the flash's manual states about the AF-assist beam:
When autofocus cannot achieve focus on the subject in low-light or when contrast is low, the built-in AF-assist beam activates automatically to help autofocus.
The AF-assist beam is sometimes referred to as "IR beam" - however, it really is just a red light (sometimes even ranging into near infrared) that helps the camera with focussing. Why red? Because it is the least distracting color.
Also, it is not intended to be always-on - i.e. there is no way to have it activated permanently.
If you want night vision, use a camcorder with that feature - even some of the cheapest ones have a IR LED built in for this purpose.
