system
Given the following post, respond as if you're commenting on Reddit:
human
What exactly controls brightness? Is it just a denser concentration of photons, or are the photons more or less "active"?
gpt
Both. Brightness (or more accurately, luminosity) is a measurement of energy per second. Fewer more energetic photons can have the same energy output as more photons with less energy.

In most cases though, an energy source radiates photons at fixed energy levels and brightness equates to more photons.