Legibility
The real issue with overhead projection, or any other kind of projection media, is whether students can see, read, and understand what you are putting on the screen, even if they are in the back of the class. If the projection facilities in auditoria were properly designed, this would not be as much of a problem as it is. The screen would be so large that the magnification of the projected image would take care of an original whose text was too small. However, projection is often the last item to be considered in auditorium design, and we have to work with projection screens that are too small. For example in my auditorium, which holds 400 students, according to generally accepted projection standards, the main projection screen should be at least thirteen feet wide. It is, in fact, ten feet wide, and the size of projected letters is thus almost one third smaller than it should be. What this means is that when preparing materials for overhead projection, either by copying printed material, or hand writing, the error should generally be on the conservative side, and the letters should be a bit larger than you think they need to be.
The following guidelines will get you started. If you take your finished overhead transparency, prop it up against a wall, and look at it from a distance of eight feet, that will give you a pretty good idea of what your students will see from the back of the auditorium. You will immediately see that if you copy a chart, spreadsheet, or text from an article or book directly to a transparency, using a copy machine, the students won't be able to read it. As a demonstration, prop this book up, stand eight times the height of the book away, and look at the type below: This is 11 point. This is 13 point. This is 17 point.This is 22 Point. What is the minimum size you can read? In practice, this means that lettering on the original that will be copied as a transparency needs to be at least 1/4 inch high. Typical 12 point type, which is the default size for most word processing programs, is about half the size needed for legibility when the paper original is copied to a transparency in a copy machine---. (As an aside, this passage is much more legible in print than it is on your screen).
