Often used as a sales argument (a little like processor speed for PCs), luminosity is not the miracle solution to all your problems. The main purpose of high luminosity is to be able to project an image of large size (above three meters) or view a video in a lighted room. If one of these cases applies to you, you'll need a projector with very high luminosity (2,500 to 3,000 lumens). It must also be capable of producing a high contrast level. Only top-of-the-line projectors can do this, and you can expect to pay around $6,000 for one!
If your priority is home cinema and your budget is more modest, projectors with a luminosity of 1,000 to 1,500 lumens are quite sufficient. Remember that the more luminous the image, the harder it will be to achieve a good contrast level. So it's better to darken the room than to increase the brightness. It's less costly and the quality is better. For a darkened room, 1,200 to 1,500 lumens will be sufficient. To view a film in a totally dark room, 800 lumens is enough.
Noise And Internal Scaler
This point is easy to neglect. Don't buy a projector if the noise level claimed by the manufacturer is above 31 dB! You should know that sound volume doubles every 3dB. You may not hear the projector during a machine-gun duel, but in more intimate scenes it can quickly become annoying. Today, the best projectors get noise levels down to around 27 dB, which is very reasonable.
The Internal Scaler
The scaler is what does the video processing of the signals coming from the DVD player, TV antenna, cable, VCR, game console, etc. The scaler is forgotten all too often in favor of luminosity or contrast, which can be a serious error. In fact, the scaler determines the quality of the projected image. Even with very good colorimetry, contrast, luminosity and noise characteristics, a projector can be catastrophic if its internal scaler is poor. Solarization, "dolly" effects, blurred contours, shadows around objects and people, jerky movement, stuttering on very rapid movements, trembling in light-colored image areas and motion sparkle can be the result and make movie-watching a painful experience.