Is there a word that means capable of being made brighter?
Solution 1:
If it can be made brighter, perhaps it is attenuated?
Solution 2:
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.
Solution 3:
"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.