There are lots of types of projectors, and the overhead projector is one of the most common one in our life.  The projectors have different surfaces that the screens can be made of, and all the surfaces have benefits and downsides.  The purpose of the projection screen is to display the images from the projector, and the quality of the images is usually decided by the projectors and the screen.  And the projection screens can be made by different materials, like optical and high gain materials. 

The optical materials usually help you to focus the images since they are able to absorb the light from the environment to make the color brighter.  It is always a good way to increase the contrast of the projected images.  On the other hand, the high gain materials can create brighter images because they do not have a large viewing area, so normally the images can only be seen clearly when people stand in front of the screen.  Compare to other materials, the optical made projection screen the images show more clearly, and this advantage makes it costs the highest among the others.

As for the projectors, the overhead projector is commonly used for educators.  The reason why is that the equipment facilitates can help the educators to create an easy low-cost interactive teaching environment.  How can educators apply the projector?  You can pre-print your teaching materials on plastic sheets because the transparency can be used repetitively, and you can use a washable color marking pen as well.  This would save you lots of efforts on preparing teaching materials manually before class.  Besides, the enlarging features of the projector allow the educator to write comfortably in a small script while other traditional ways may cause some muscle sore problems because you have to hold your arm out in midair to write on the blackboard and erase the writing constantly.