Simon, there are various things you could do. Which light meter are you using? Firstly it’s worth checking that you’re measuring light from the recommended position (centre of the seating area, at Eye height). If so then proceed to the next steps. Start by looking at the existing lamp. Check power output and if it’s near 100% try replacing with a new one (perhaps higher powered) and taking brightness levels again to see if there’s a noticeable difference. Then check cleanliness levels around the projector and port hole. It would likely also be worth contacting the projector manufacturer to get a quote for cleaning or replacing elements in the light engine. Failing that, depending on the type of screen you have, you could install a higher gain (more reflective) surface to up the light output. If you’re getting 6.8fL on an uncoated Matt White Screen you could almost double that with a high gain coated white screen such as a Perlux 180 from Harkness. Note that higher gain screens do compromise uniformity of image somewhat but it will eliminate your brightness issue if all else fails.