Should be able to twist the outer housing that covers the fiber.

 

Just like that! Notall HHA sights have a rheostat so make sure you have one that does before twisting on the scope.

 

Just give it a turn like pic above, it's threaded.

 

OK, so now I feel really stupid!!! I purchased on of these sights a few weeks ago and (I guess not reading the instructions is a guy thing) I didn't realize until just now that the brightness could be adjusted. I'll have to go home and try that.

Automan

 

not all models

I am pretty sure only certain models can be adjusted so double check this before you twist too hard.

 

The OL5500/19 series is adjustable. Like others said twist the housing and push and pull at the same time.

 

The 55** series should all be adjustable due to the longer fiber.