Here are a bunch of things to think about.
Projector brightness is only part of the projection puzzle. You have helped us a
bit by telling us how large an
image that you are hoping to achieve (as stated with the
inverse square law, the smaller your
image from this size, the brighter your
image will be, so that might be an option for you).
We can assume that you have a new
projector with a new lamp. This will be the brightest that you will ever get. So, if you can't make it work now, as the lamp/
projector ages, you will get poorer results and be less happy. Lamps will have a noticeable brightness
drop as they age. Usually, you can get close to full brightness for the first 200 hours or so. After that, the
lumen hill starts getting steep and you will lose brightness faster the older the lamp. For the purposes of my show, I replace the lamp at 2/3rds its stated life as it gets too dim for my purpose.
Apparent brightness from a
projector (not measured) will have a few variables. As humans are most sensitive to contrast (bright to dark ratio), how bright your
projector is will only be a part of this. Most
projector manufacturers will state the highest contrast achievable by the
projector, which isn't really helpful. For one, this is telling you the difference between full white and full black (which can be helpful to know how bright "video black" is). You need to know what the
ANSI contrast is (as measured by a checkerboard of 16 white and black squares). This is helpful in determining what the contrast of your content will be. Tiny amounts of ambient light will greatly impact this ratio. In addition, the surface that you are projecting on will add into the equation. With a traditional projection
screen, we know what the
gain (reflectance variable) will be. Without knowing, we generally derate the surface to a
gain of 0.5 (scattering the light more or potentially absorbing it) which usually works for an off-white
cyc. The apparent brightness can be affected by the type of optics used to generate the
image. A 3-panel
LCD will appear brighter than a single-chip
DLP (which will have roughly the same price
point) when you are showing anything besides a full white
screen. The lamp technology can
play into this as well, but you probably just have the standard lamp based
projector, so this will be less noticeable. Since a 3-chip
DLP will generally have a better
ANSI contrast ratio than the same brightness 3-panel
LCD, the
DLP projector may seem brighter as well (this can depend on the surface and ambient light). Finally, the content that you choose will make a difference in how bright a
projector seems. If you have brighter colors covering a greater amount of your
image (and have good contrast), then your
image will be more acceptable.
If you intend to rig the
projector to one of your battens, please use a manufactured assembly, preferably by the manufacturer, but there are a number of secondary manufacturers (Chief, Premier Mounts,
etc.) that also make quality equipment. These will either have pre-determined mounting points which match to the bottom of the
projector or will be adjustable to fit a variety of projectors. The biggest challenge to rigging to a
batten is that your
image may move as the
batten will likely
swing as you move other scenery (including
soft goods) or as air currents change.