Flat Panels, Projectors and the Classroom

A new look at an old problem – and its solution
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There’s no escaping the 1/4 Rule.

IT managers may not know about it, but many AV and media people do. It states that the minimum height of a classroom display, or any display used to show small text, must be at least 1/4 the distance to the farthest viewer. If it’s not, text on the screen won’t be readable to those in the back. It’s an update of the older 1/6 rule, used to size screens for the large text in PowerPoint presentations.

If you think about it, you will realize that the 1/4 Rule pretty much rules out the use of a flat-panel display as the main viewing screen in a classroom. Why? If the typical American classroom is at least 20’ deep (or 240”), then the minimum screen height must be 60” or the height of a 113” diagonal display. While it’s true that you can buy a flat panel that large, it’s far too expensive for most classrooms.

Sometimes you see this screen-sizing rule expressed as the Four-Six-Eight Rule. The farthest seat can be no farther than 8X the screen height if you’re showing only video, 6X if you’re showing large text from PowerPoint, or 4X if you’re showing small text from a spreadsheet, document camera or web page.

In this light, a 100 – 120” classroom projection screen makes a lot of sense. Yet many IT managers don’t like projectors, because traditionally they require a lot of maintenance and break down more often than flat panels. They know, too, that LED-lit flat panels are ‘greener’ than projectors using mercury-vapor lamps that must be disposed of properly and can release hazardous materials into a classroom.

Is there a way to combine large screen sizes with high reliability, concern for the environment and high image quality at an affordable price? As the strategic planning manager of Casio America, I happen to know there is.

Measure your classrooms

Does the 1/4 Rule apply to you? It’s easy to find out.

First, just walk through your school and talk to your teachers. What are they projecting? Big text from PowerPoint or smaller text from websites, diagrams, drawings, or documents? Then choose a rule of thumb: 1/6 for larger text, ¼ for smaller.

Next, measure your classrooms. How far is it from the screen to the farthest seat, in inches?

Finally, apply the math. You may find this ratio helpful: the diagonal of a 16:10 display (1280x800 or 1920x1200) is about 1.9 times its height. (So if you find you need an image 60” tall, the diagonal is 60 x 1.9 = 114”.)

Now you have what you need to make a good decision. You know the minimum screen size that will be readable, and you understand the key differences between traditional, bulb-based projectors and LampFree.


For a detailed discussion of the ¼, 1/6, and 1/8 rules, see




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