Brightness is lowered while in an area with low light level

What I expected to happen was...:
In previous versions of the game, setting the gamma option to 10 would light up all areas fully, regardless of light level (similar to night vision potion)

What actually happened was...:
The screen slowly darkens to the point where nothing is visible

Steps to Reproduce:
1. in options.txt set gamma option to 10.0
2. enter a cave/area with low light level
3. screen will slowly darken to the point where you can no longer see