What about putting the sheet of perspex in AND changing the bulbs to 47's Wotto . Great idea though . Anything that's gonna act as a heat barrier between the globes and the artwork has got to be a plus . If the perspex was to eventually crack etc. then at least it's a cheap and easy fix .

Only concern I would have is that you'd want to make sure the perspex sits nice and tight in the headbox . You don't want it to move with vibration etc. and rub on the artwork .