Hi Strong and Mech!
I'm seizing the opportunity to say hello through this thread
If the screen is correctly built and projector basically calibrated, it might then be the projector itself. It is sometimes possible to have some brightness and color non-uniformity which might lead to this. But you should not give it too much importance if the variation is slight and as far as it is not noticeable with program material....no worries (who watches tests patterns?
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Have a nice time