Testing new PJ before getting a screen-what "gain" was my off-white drywall?

Request sample screen material from several manufactures and put it up on your wall. A screen, even a $!00 portable can look better than the wall but the difference may not be worth the cost depending on your needs. The samples are free, so give it a shot. You could always watch on the wall for a few months and when you loose the WOW feeling buy the screen ;-)

You could probably get the answer in the DIY screen thread. I would guess it is close to 1.0. Did the same thing with my bare walls when I first got the PJ except that I had primer for bare drywall already on the wall. (that point in the theater construction) I can tell you that the primed wall matched exactly the 1.4 Carada BW screen swatch I had taped to the wall and was slightly brighter than the 1.0 swatches on the wall.

Most of the screen mfg's will send you screen samples to test with. Da-lite and Carada were excellent with this. Just contact them on their site.

It is amazing how good it looks just projected onto a wall - looks even better on a screen

My very light gray wall was about .85 (guessing from looking at samples of "known" value taped on it)

I was a little disappointed that the "1.4" was so close to a 1.0 (not to mention that it looked the same as my drywall ) as I new I wanted a bright punchy pic.

I think I can offer some pertinant experience here.

I projected on to various materials before my final screen was put up. This included the Da Lite High Power material, Carada BW material, dry-wall with primer, and finally the Stewart Studiotek ST-130 material in a fixed screen.

To cut to the chase: The primer ended up being almost exactly the same gain as the ST-130 material, which is 1.3 gain, so above neutral white. (BTW, sometimes two samples of screen material that are slightly different gain can look almost identical if you are projecting images on them. It's hard to get exactly the same
image on both if you are using movie images. A sure-fire way to see the difference is to put up a solid color screen, for instance a full blue screen - some projectors default to this color, like my JVC, when it is projecting no image. If one piece of material has even slightly more gain you'll likely see it pretty easily on a full blue screen).

Once I was projecting on the primer the image was so bright and punchy I was starting to think "who needs a screen?"

Once I put up the ST-130 the differences were: The image was equally bright and punchy, but everything seemed more refined - a perfectly smooth surface seemed to render detail better, contrast, especially intra-scene contrast seemed to improve (probably because the ST-130's gain is directional which would slightly help reducing side-wall light reflections), and there even seemed to be more subtle color detail. Basically, it just seemed a step up image quality wise in every way. I was sort of surprised by the difference, frankly, but was certainly glad in the end I bought a professional quality commercial screen.

I now have my JVC RS20 projector (calibrated by umr) projecting on to the ST-130 screen and it's mind-blowing, perhaps the best projected image I've seen.

I agree with what Rich is saying. That is one of the reasons I went with a High Power over a standard screen. (I do not categorize the Stewart as a standard screen) There was not going to be a major mind blowing difference between the drywall and a standard screen. They both look great so I thought I would go with something that did make a huge difference. If I ended up not liking it - Hey I always have the drywall.

But, seriously, the hi power is not for everyone. If you can get your set up right to maximise the gain. Projector at eye level and have your seating within the screen so most have a similar PQ - you have a good start. I love the bright punchy picture. A while back I was starting to wonder if I was missing out on lower black levels. I was able to get a 1.0 screen to watch a few movies with and it helped me appreciate the high power image. It is easy to forget what a standard image looks like.

If you do go hi power be sure to have your back wall not be reflective. It is amazing to me how the screen reflects any light coming off the back wall. Mine is all fabric but I had some step lights that I had to redo as they affected the screen even though I could have sworn there was NO way the light was getting to the screen.

W-
You should be fine. Continuous sources of light, such as step lights, are far more serious, because they are present at full strength during dark scenes where they are most likely to be visible on screen. Reflected screen light from walls follows the on-screen brightness, and is thus less noticeable unless the reflective areas are really bad (e.g., large and light colored).