"Focused on target" vs "focused on the target"
Solution 1:
Based on how definite articles work in English, if you're saying the light is focused on "the" target, you've got a specific target in mind (in the context of focusing a light fixture, that probably means something you could point to in person, for example). So that seems like what you're after here.
I understand "on target" to be a colloquial expression, so in your example that's like saying it's focused pointed purposefully or intentionally, or it's "on point" (in the colloquial use of the phrase "on point").