I went on a couple of blackwater dives recently, and was very impressed with Inon LF800-N dive light. It’s got a Fresnel lens that tightens the beam to some 5 degrees and completely eliminates falloff. The guy who was using it was swinging it around, and it looked like a lightsabre - just this long, solid beam of light, cutting through all the particulate matter in the water, finding the critters to photograph without blinding the user with backscatter from a diffuse beam. I am not, however, impressed with its $250 price tag. Are there any cheaper lights that follow the same design pattern?
Based on your requirements, you may be interested in xtar D26 1100, max output 1100lm, spot light angle 5°:
“https://www.xtar.cc/product/XTAR-WHALE-D26-L2-U3-(Switch-upgrade)~~Diving-Flashlight-38.html”:https://www.xtar.cc/product/XTAR-WHALE-D26-L2-U3~~(Switch-upgrade)-Diving-Flashlight-38.html
Thank you, that model does look interesting, although checking reviews, it does appear to have significant sidelobes on its main beam. The primary feature of the Inon LF800-N is that in addition to a reflector, it uses a condenser lens in front of the LED, producing a sharply defined beam with nothing along its sides. The fall-off produced by more common pure reflector designs can illuminate a lot of particles when diving in black water, which will obscure targets picked up by the core beam.
That’s a reflector flashlight, it has neither a tight beam nor a sharp cutoff.
OP is looking for a lens/TIR dive light.