I agree, screen brightness was not that of crt from rear projection, owned a Kloss model one from 1980? Watched it using a wall --- original curved screen not vailable-these took up a lot of space. Corner convergence was bad due to this. Produced a beautiful picture and worked for many years.
Then I owned a 3 CRT zenith similar projector from late eighties. Was designed to shoot on flat wall, had much more brightness--can be watched with room lights on-- showing 10 foot screen image. The convergence to tweak right was a bear , proper convergence was only acheived right before I sold it. Still a great set-- had to buy special cabinet for unit to be at right height and parallel to projection wall. These units being expensive usually people only sold them if they had major issues. These were generally hard to find in good used shape.