Vinay,
Every projector has fixed light output, in terms of lumens. Almost all the projector manufacturer's inflate the rated lumens, as it is very tough to reach those in normal viewing. Anyway, even if we take those nos, then projector lights up the screen with its light output. Now, the brightness of the picture will depend upon the screen area.
Let's take an example of 10 ft wide, 16:9 screen. Its area comes to 56.25 sq ft. If the projector is 1000 lumens, then the brightness is 17.77 ft-lambert. The cinema theaters are between 8~12 ft lamberts. It's still quite dim. For home viewing many people prefer 12~16 ft lambert. Add in the lamp dimming over time (lamp dims about 30% after first 100 hrs and then gradually dims) and you are already seeing less brightness. The problem with less brightness is picture will not look punchy, you will lose shadow details and as such.
In case of OP, the area of 12 ft wide screen is 81 sqft. For 15 ft wide, it jumps to 127 sqft. Its not a small increase. The projector will have to be atleast 150% more bright to begin with. I changed my setup to 16:9 with new projector and limited the screen width to 11ft wide (from earlier 12 ft wide) to factor in the brightness.
As for dual lamp, it has two lamps as name suggests. You can either use 1st lamp, 2nd lamp or both together to give the boost in brightness. As we know that lamps dim, many people use 1 fresh lamp for first 500 hrs. Then 2nd lamp (which is still new) for another 500 hrs. By this time both the lamps are quite dim. From this point on, they can choose to turn on both at the same time and regain that light output.
