Is there a word that means "capable of being made brighter"? A word that means capable of being made stronger or more intense would work too. Brightenable would be perfect if it didn't provoke as much laughter as understanding. Intensifiable is about the best I can come up with.
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Although it's not directly relevant, you could use the word augmentable, to imply an increase in strength. Alternatively, you could consider illuminable, with a stronger reference to illumination.
"Variable intensity" is the customary phrase for describing a device that can be adjusted to be brighter or less bright, as suitable to the occasion.