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 used 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

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.

Solid-State Projection

Since 2010, Casio has offered a line of classroom projectors that use a hybrid LED and laser light source instead of a projection lamp. More recently, several manufacturers, including Casio, have begun offering projectors using an all-laser light source.

Either way, these new technologies promise a 20,000 hour expected lifetime without ever needing to change a lamp. Better still, they run cool, avoiding the high heat that lamps generate and that’s so destructive to electronics and to power supplies.

Running cool is a major benefit for any electronic product. For our LED/laser projectors, we believe it is one of the reasons that we have seen a remarkably low rate of repair.

Cooler operating temperatures also help make a 20,000 hour lifespan possible. Another reason is the dust resistant design that blocks off the optical engine. There are fans to cool the power supply and light source, but any dust coming through does not get into the optical system, maintaining brightness for the majority of the life of the projector. And 20,000 hours is a long time in any classroom, more than 18 years if the projectors are used six hours a day, 180 days each year.

I will pause a moment to make a plug for Casio. I said any laser or laser/LED projector should last 20,000 hours, but actually the LED/laser hybrid is the only one with a track record long enough to be sure it will. Casio introduced LED/laser hybrid projectors in 2010. We still have a large number of those projectors in use by our customers, with many that have reached the 20,000 hour mark and a few at almost 30,000 hours.

It’s important to note as well that Casio projectors contain no hazardous materials and actually use far less power than flat panels of comparable screen sizes. The Casio ultra-short-throw, in particular, provides all of the fundamental advantages of flat panels at a much lower cost for the larger sizes – with very high image quality.

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, 1/4 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.

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.