I have the same problem in lots of my LDs!

Greg's advice is great. Hand being a control 'lever' for the brightness.

I would like to add some other tricks too:

_You have a touch phone device in your pocket in LDs! Use it to control the characteristics of the dream.
_yell "light!", "light up", "let there be light!", etc.
_call your dream guide and tell him/her to light the dream up.
_take a flash pop from your pocket and throw it on the ground to explode and light up.
_burn the furniture! Use a fireball if you are confident enough you can (you always can, you just gotta expect success. Doubt kills it. In other words, believe that you can do it). Use a match or something, take it out of your pocket.
_put on night vision glasses (take it from your pocket. Lol gadget boy). Or tap on you head activating "night vision"!
_go out to the street because it's usually lit up.

Hope these work! But most importantly, don't forget to stay calm, stabilize and focus when you become lucid. Seriously give it a minute, not just a quick look at your hand. Trust me, it does wonders.