I would expect that your best bet is with a 16:10 screen of 1280x800 resolution. You get a lot of bang for the buck on that.
Your projection distance needs to be more flexible I expect as the throw distance is really far for most projectors. I would think about a 120" diagonal would be appropriate.
To get real brightness and to avoid RBE issues, I would recommend a LCD projector.
With the limitation of 18' (about 6m) to a 120" diagonal screen, you only get 3 results.http://www.projectorcentral.com/projectors.cfm?g=1&hide=0&st=1&mfg=&p=400&p=1100&w=&r=12&br=&ll=<g=&t=&db=&dt=1.0.0&c=&ar=&dvi=&wr=&pjl=&pjw=&pjh=&td=18&is=120&i=d&tr=&oop=1&sort=pop&sz=15
With over 40 square feet of screen space, I would say that 3000 lumens will not appear 'bright', but may be better considered to be minimally adequate. I would shoot for more brightness.
To do so, get rid of the throw distance and just put the projector wherever it fits and revise the list...http://www.projectorcentral.com/projectors.cfm?g=1&hide=0&st=1&mfg=&p=400&p=1100&w=&r=12&br=&ll=<g=&t=&db=&dt=1.0.0&c=&ar=&dvi=&wr=&pjl=&pjw=&pjh=&td=&is=&i=d&tr=&oop=1&sort=brt&sz=15
There are 46 projectors under $1,100 (things go on sale) which are 1280x800. The top two models are the ones I would look at first. The 4,500 lumen Sony and the 4,000 lumen Panasonic.
Both models need the lens to screen distance to be about 5m or so to work properly with a 120" screen.
LUMENS: Absolute minimum lumen rating is the square footage of the screen times 80. So, a 64"x102" screen (120") is just over 45 square feet. 45x80 = 3,600 lumens minimum. From there, you are increasing contrast to get a better image.
With 80 lumens (advertised) per square foot, you should expect about a 10:1 contrast ratio in a normally lit classroom.
Most people are using 3,000 lumen DLP projectors on 80" interactive white boards and they are barely acceptable in most cases. I would expect the Sony or Panasonic to do a much better job overall due to their design.
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