Gamma not working

Put the summary of the bug you're having here

What I expected to happen was...:
Gamma to brighten caves and such so as to make them more visible as in previous versions of Minecraft

What actually happened was...:
Minecraft is dark

Steps to Reproduce:
1. Go into your .minecraft folder
2. Open options.txt
3. Increase your gamma significantly

4. Save, then open your game, and watch absolutely nothing change