1. Brightening: This is the gerund or present participle form of "brighten" and acts as a noun, referring to the act of brightening.
Example: The brightening of the sky signaled the approaching dawn.
2. Brightness: This is a noun meaning the state or quality of being bright.
Example: The room was filled with brightness from the sun shining through the window.
3. Brightener: This is a noun referring to something that brightens, such as a substance or a person.
Example: Lemon is a natural brightener for your skin.
The best noun form to use depends on the specific meaning you want to convey.