I have an outdoor event that takes place at night on the beach the client wants to project rear screen on 10.5x14 ft. will two 5,000 lumens lcd projectors stacked be sufficient. There is very little ambient light because its at night and no direct ligh on the screens.
Under normal and dark circumstances I would use only one 5000 AnsiLumen projector (correct rearprojection foil, no operafoil or other), so yes with two stacked 5000-projectors you are definitely on the save side.
The way I'd assess any such scenario is to calculate the 'Foot Lamberts' that (theoretically) falls onto the screen surface -
Surface area = 10.5x14ft = 147 sq. ft.
ANSI Lumens of Projectors = 5,000
with 2 projectors, I suppose that would be x 2 as an approximation, equvalent to 1 x 10K projector.
Therefore, 10,000/147 = 68 ft. lambert.
Compare this with the norm (by today's standards) as aLexx has advised, i.e. 5000/147 = 34 ft. lambert.
68ft lambert is therefore pretty bright, and if I may say so, more than satisfactory. I've used a 10K Christie Roadie in a darkend huge convention hall onto a 20x16 rear screen, for an audience of 3,000. That's 31.25 ft lambert, and there was not a whisper of a complaint.
The above is a simple calculation, but the principle is useful when one examines different screen sizes with other ANSI-rated projectors.
PS. to Bob, et al...If I'm wrong in the above, please say so. I'm still learning and would like to know the where, why and how.