I had this posted earlier but deleted it when I was a bit upset and thinking about leaving the forum. I removed a lot of posts that day, but this is one that should have stayed up. I wanted to include it in the index so I have to repost it, so here it is...
... A quick screen Primer.fL- (foot Lamberts)
Foot lamberts relates to how bright the screen actually is.
To calculate fL, take the number of ANSI Lumens of your projector and divide it by the screen size in square feet (area), then multiply that by the screen's gain. For example a projector with an output of 400 ANSI lumens matched with a 100" screen (60" by 80" which is 33.34 square feet) with a gain of 1.3 will produce an image with a brightness of 15.6 fL.
Here is the math formula:
ANSI Lumens of projector
------------------------- X screen gain = foot lamberts
Square footage of screen
(SDTV measures between 25-35 fL)Screen TypesDiffusion Screens
When the projector light hits the screen it scatters in all directions evenly. This allows a very wide viewing angle with the least amount of hot spotting, but it also tends to be the dimmest type of screen because the light that hits the screen is scattered evenly and not directed back towards your eyes. Most diffusion style screens will have a gain of around 1. One of the big advantage of a diffusion screen is it allows the largest viewing angle. The image is usually equally bright across the screen for most seating positions. Painted screens fall into the diffusion screen category unless special reflective properties were added to a top coating to change the type of reflectivity of the screen.Reflective Screens
Reflective screens contain a special top layer and texture which increases the reflective properties of the screen. This style of screen is suitable for rooms with some ambient light or for projectors with lower light output. These screens reflect light in the opposite direction of the projector (the incident angle equals the reflected angle, like a pool ball bouncing off the rail) and are best when used with ceiling mounted projectors. Reflective screens have a slightly smaller viewing angle than diffusion style screens, but the viewing angle is still quite good. Most commercial screens for LCD's and DLP projectors are reflective screens.Retroreflective Screens
Retroreflective screens reflect the light primarily in the direction of the projector, like how a traffic sign reflects your car headlights back towards you (otherwise the signs wouldn't work, or at least be as bright) The recommended projector setup for a retroreflective screen is either a floor or table setup. Retroreflective screens tend to have a high gain value (2+). The viewing angle of a retroreflective screen is the smallest. Some may exhibit hot spotting as well, especially with higher lumen projectors. There are screens with less than a 2.0 gain that are retroreflective. The Stewart FireHawk is one example of a lower gain retroreflective screen.
Hot spotting is being mentioned, so for anyone new that really isn't sure what it is, here it is:
This is an extreme example but it demonstrates what hot spotting is quite well.
Hot spotting is when the center of the image will be brighter than the edges. Again this is not a problem with low gain screens but can become a problem for screens with a higher gain. Hot spotting is caused by insufficient light diffusion, or excessively high lumens hitting a reflective screen with a gain usually greater than 1.0.
It is important to note that DIY isn't solely susceptible to this problem, people encounter it all the time with commercial screens too.
There are a few ways to combat hot spotting if it is an issue. The first thing to try is to lower the projector brightness some, but this may not be acceptable because it will also decrease the overall image brightness. Another way is through the use of inexpensive ND filters, but really the issue in the previous posts is to identify why it is happening before jumping on making alterations and adding filters.
It is possible this is what's happening with some setups that are shooting straight on: With the projector more at a straight on angle hitting a reflective surface, the incident angle is greatly reduced, causing more light to be reflected back on the axis and on center of the projected image. This would make sense, but it is only a working theory at this point.
The same setup with a diffusion type screen surface would look a little different as far as the reflected light pattern. Light would still tend to be brighter in the center of the screen, but since it is being defused in more directions it is not as intense or noticeable. Again, this is only a working theory, but based on some of the previously reported setups and results it does seem to make sense.
If this is what is happening, I think it is a perfect example of how everyone's setup and situation is different. What works wonders for one person may not work at all for someone else, or at best yield inferior results. I wouldn't say though that it means the painted screen with diffusion properties is better than a reflective material like a laminate, or that laminates are better... they are just different and that is something everyone should keep in mind.
If after everything else fails, or it is impossible to reposition the projector, another great benefit of laminates is that they are not only extremely tough, but flexible. This makes them perfect candidates for a curved screen. (Curved screens aren't all bad and can even look exotic to some)
The purpose of a curved screen is to direct all the light that is projected to the screen back to the viewer. With a flat screen light bounces off the screen and around the room. With a curved screen most of the light is bounced back to the viewer which results in a very bright image. Curved screens tend to have a very high gain value, i.e. a gain of 13 is common. A curved screen can get away with such a high gain because it essentially turns the entire screen into a giant hot spot so there is no visible hot spot. Laminates do not have gain anywhere near that high, but a slight curve would have the same effect.
If hot spotting is an issue but other than that the picture is fine, a slight curve could resolve the problem without the need of ND filters, repositioning the projector, or going with a different screen. It also should be mentioned that as pointed out, all projectors are dimmer towards the edges of the screen than in the center. Sometimes in very bright scenes and for some projectors this can be slightly noticeable-- but that is not hot spotting, it is just an inherent characteristic of projectors that some may perceive as hot spotting because some scenes look brighter on center than the edges. True hot spotting is pretty easy to recognize.