For the past several years, I've been carrying around an LED light in my photo bag. I actually own several, the lest expensive I've found has come from Gadget Brando. These are wonderful little lights that have a good amount of power to them. One nice feature is that these lights have an adjustable knob on them which allows you to dim light without shifting the White Balance.
I used one of these lights in the photo above. My assistant John was in the back seat in this mining tug next to my subject. John simply held the light as we were driving through the dark mine. I thought it to be a very useful light source as it provide a means for me to focus as well as enough power to shoot with. DT