How big is big enough? How bright is bright enough? These questions can leave event managers scratching their heads as they ponder the projection systems for their for meetings and training sessions. Inconveniently, the answer to both is, it depends. Seeing the image on the screen is one thing; reading it is another.

Legibility is the result of a combination of factors, including contrast, brightness, angle of view, resolution of both the projector and source device, reflectivity or "gain" of the screen, distance to the viewer, and, often most significantly, character height. I've compiled several helpful rules of thumb that can help determine the right projection screen, given your audience size, room size, and the material being presented. (For this discussion, we'll consider only video or computer-generated images such as those from a laptop, VCR, or document camera.)

Get Out a Calculator l The ratio of the screen height to the throw (distance of the screen from the furthest viewer) will vary depending on your projection system. If you're projecting video images from a VCR and the material is general, not technical, then the ratio we use is 1:8; that is, the image's vertical dimension is one eighth of the distance to the furthest viewer. In other words, if you want someone 48 feet away to be able to see clearly, the image needs to be at least six feet high (48 divided by 8 equals 6). If, however, the projected image is PowerPoint or other presentation software, then the ratio is 1:6--you'll need a larger screen for same size room (48 divided by 6 equals 8).

l If the images to be projected include complex spreadsheets, technical CAD drawings, or similar high-density graphic material, even larger screen sizes are appropriate. Similarly, if a document camera is used with paper documents, an image height-to-throw ratio of 1:4 is typical.

l The distance from the screen to the nearest viewer is also important. The rule of thumb is a 1:2 ratio, that is, viewers should be no closer two times the height of the screen in order to see the whole image. For a six-foot-high screen, viewers should be at least 12 feet away.

I Can See Clearly Now l Keep in mind that the bottom of the image should be at least 3 feet above the floor to give all participants a chance to see the whole image.

l Need to keep the lights on while the presentation is running? Divide the area of the screen (in square feet) by the ANSI lumen output rating of the projector. That will give you the average brightness of the image, and you'll need at least 15 lumens/square foot to preserve your ability to keep the lights on in the room. Lower brightness levels may require darkening the room.

l The angle "off-axis" of the image also needs to be considered. Here we're talking about the sight lines for viewers seated on the far left and right of the screen. A rule of thumb is to make sure the audience is within a 90 to 120 degree included angle from the screen. Seat viewers within a 90 degree angle when a presentation is critical or detailed; the 120 angle is for less detailed presentations. All of these guidelines are based on the use of "low-gain" screens, which is my recommendation and preference.

The size of the screen and the brightness of the projector are directly related to the brightness of the image. Ideally, the largest, brightest image will provide the best legibility. However, the larger the screen, the more illumination required from the projector. And the brighter the projector, the more costly to rent. So, the most cost-effective approach is to provide the smallest acceptable screen according to guidelines explained here.