Originally Posted by timatraw
I was reading a review on the Q750i and came upon this quote: ..."because of their findings as to the Q-750i's true performance, Runco doesn't recommend using the Q-750i with a screen larger than 108 inches diagonal"....
Could someone explain this quote to me? Is it because the lumens ( and hence brightness) will decrease with a larger screen?
I'm not familiar with that directive from Runco. Per my calcs shown previously, and with the defacto Stewart Studiotec 130 reference screen, a 110" will yield a 16ftL image, which is generally considered, just right.
Lumens don't change except for projectors with variable aperture (VA) lenses that stop down the light path. I believe the Q750's optics are Minolta constant aperture (CA) like what's in my 3 chip, but I have not verified this myself. For PJ's with VA lenses, they are brightest at their shortest (widest) throw.
Screen brightness is measured in
illuminance - this is the measure of light coming from the projector, using a illuminance
meter. Measured in foot candles fc (or lux)
luminance - this is the measure of light coming off of the screen if you will, using a luminance
meter. Measured in foot Lamberts ftL
The (il)luminance changes as you spread a given amount of lumens over a larger area. So as the screen gets bigger, for a given projector output, in lumens, the image will get dimmer (lower luminance)