In rear projection, the projector sits behind the projector screen fabric, as images are projected through the fabric and to the viewer. Since light must pass through a physical barrier, you must use a projector with a higher lumens output in order for the audience to see a bright and clear picture. If you are considering a rear projection setup, please keep in mind that you will need ample backstage room so you and your staff can walk behind the projector. Walking in front of the projector will cast shadows on your projection surface. The advantage of rear projection is that there is usually zero to little distortion in the projected image as the line of projection must be perpendicular to the projection surface.
Also, rear projector screen fabric can be painted on, so that it can blend in with the projection images from behind to create stunning and captivating backgrounds. Stage performers prefer rear projection over front projection because they can stand in between the screen and the audience without having to worry about blocking the image, which makes the production all the more realistic.
It does have some disadvantages. Since a higher output projector is often required to diffuse the image through the fabric, and also because the audience is essentially looking into the projector through the fabric, often noticeable is a large round hot spot in the center of the screen. Denser materials can be used to reduce hot spotting, however image quality will greatly be decreased for any viewer that is out of the recommended 60 to 110 degree off center viewing angle. The darker the surface, the smaller the off center viewing angle threshold.
There are many types of rear projection screen material. The type that you should use depends on the unique circumstances surrounding your projection setup. Consider these three most commonly used fabrics.
Light translucent fabric is an opaque light gray material used when there are high levels of competing ambient light, such as during an outdoor event. This fabric is able to transmit a high amount of projected light, it can successfully display clear and bright images amidst competing ambient light from the surrounding environment. However, the fabric’s lighter color makes it prone to showing hot spots.
Black rear projection material is the best type of fabric to used when you want to give your audience clear high quality images, or if you really want your projections to blend in will all your on stage elements. Even though there is only a six percent direct light transmission with this fabric, the superior contrast between dark and light colors makes your images look brighter regardless. This fabric is the preferred choice for opera and ballet shows, as reflections of floor follow spots are absorbed by the dark fabric. Take caution, however. Using black fabric greatly reduces your off center viewing angle threshold, to about sixty degrees to be exact. So, plan to have a smaller audience.
Medium grey projection material is a good compromise between black and white. You can effectively open up the off center viewing angle to 110 degrees to accommodate more viewers, while greatly reducing the noticeability of any hot spots. Plus, you will have brighter images that blend well with all your on stage elements