The mod was originally created by 4poc (in minecraft 1.6.4 and earlier). See here.
The default shortcut key to turn on/off light level overlay is F4. You can change it in “Options -> Controls”.
How to install:
- Download and install Minecraft Forge.
- Download the mod.
- Go to %appdata%.
- Go to .minecraft/mods folder.
- If the “mods” folder does not exist you can create one.
- Drag and drop the downloaded jar (zip) file into it.
- Enjoy the mod.