Lumens is important if you have ambient light. Less important in a dedicated home theater room. For comparison a candle generates 13 lumens and a 100W light bulb about 1,200.
Here's a nice article describing candlepower, candela, lumens, foot-Lamberts, etc but the short form is the brightness of the projector x shineyness of the screen / size of the screen gives you the final image brightness.
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