An easy way is to check the viewing angle.
If your projector is floor or coffee table mounted, you should see noticeable a brightness increase while seated rather than standing. If your projector is ceiling mounted, it should appear brighter when you're standing.
You could also do a side-to-side test. Moving 6' or so to either side should reduce brightness to about what you would get on a white painted wall.
You can also try raising your screen to half the height. When in your viewing position, does the screen half appear brighter than the wall half?
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Tom L.
NEC LT150 FAQ
My stuff, pictures, HTPC, and screen tests
If your projector is floor or coffee table mounted, you should see noticeable a brightness increase while seated rather than standing. If your projector is ceiling mounted, it should appear brighter when you're standing.
You could also do a side-to-side test. Moving 6' or so to either side should reduce brightness to about what you would get on a white painted wall.
You can also try raising your screen to half the height. When in your viewing position, does the screen half appear brighter than the wall half?
------------------
Tom L.
NEC LT150 FAQ
My stuff, pictures, HTPC, and screen tests