I've had head torches that can do that for over a decade. ?Dimmer switch? Or the shitty click button where they jump up and down brightness in stages? I'm guessing the latter.I'm on my second LED Lensor H7 head torch must be close to 10years ago I got my first, I'll put a video up of it working if I sort my Photobucket out, no clicking through proper dimmer switch
http://rs1274.pbsrc.com/albums/y425/swift100/Mobile%20Uploads/222E9E03-08B7-4A14-ABCC-CB9F3555B583_zpsbt0xdrmh.mp4?w=160&h=160&fit=clip