A teacher’s guide to the technology behind today’s projectors
Projectors have been used in schools for decades. Go back a few years and you would see overhead projectors (OHPs) being pushed on trolleys from one classroom to the next. Teachers could draw something onto a piece of transparency film, place it on a sheet of glass with a light under it and through the miracles of mirrors the image would be projected onto a wall or screen. It was basic but effective.
Today, projectors – like the world around them – have gone digital. They can now display video, images or computer data and are smaller, lighter and more affordable than ever before.
Before investing in projectors for your school, it is a good idea to familiarize yourself with some of the acronyms and jargon you might come across.
Digital Light Processing (DLP)
In 1-chip Digital Light Processing systems, white lamp light is shone through red, green, blue and (usually) white parts of a rotating color wheel onto a DLP chip. The DLP chip is synchronized with the rotating motion of the color wheel. This creates thousands of pulses of colored light per second. These rapid light pulses are then reflected by a Digital Micromirror Device (DMD), a semiconductor chip made up of microscopically small mirrors, onto the screen. Each mirror represents one or more pixels in the projected image.
Given the speed at which this process takes place, our brains don’t usually pick out the individual flickers. Instead, they see a composite full-color image. Sometimes, however, an anomaly called the “rainbow effect” occurs with 1-chip DLP systems. Red, green and blue “shadows” are visible, by-products of the colors being projected sequentially by the color wheel. They are usually seen when the projected content features high contrast areas of moving bright or white objects on a mostly dark or black background, for example on the end credits of a film or TV program.
3LCD (Liquid Crystal Display)
3LCD systems work differently. They use a combination of dichroic mirrors that separate the white light (from the projector lamp) into red, green and blue light. Each of these three colors is then passed through its own LCD panel and recombined, using a prism, before being projected onto the screen.
Because 3LCD technology includes all three basic colors in each pixel of the projection the “rainbow effect” is avoided and the final image is rich and vibrant.
In conjunction with the above technologies, White Light Output (WLO) and Color Light Output (CLO) are key acronyms to understand.
White Light Output refers to how bright a projector is when showing a plain white image. This is obviously important, but it is not the full story. Color Light Output indicates how bright colors are when they are projected onto a screen. There is a significant difference between the color brightness of DLP and 3LCD projectors, with 3LCD being three times brighter than 1-chip DLP. When choosing a projector it is important to ask for both the WLO and CLO lumen specifications, with the ideal being equally high levels of both.
Other jargon terms used in projector specifications are Short Throw and Ultra Short Throw Projectors.
Short Throw and Ultra Short Throw simply refers to a projector designed to project a large, clear image in small tight spaces such as classrooms. A Short Throw or Ultra Short Throw projector can project the same size image of that of a long throw projector but at a much closer range, making it an ideal school projector.
Why is a shorter range important? The perks of having the projector closer to the projection space are twofold: 1) There’s less likelihood of obstacles such as furniture blocking the projection (especially in small classrooms), and 2) Teachers can be more interactive with the projected content without blocking the light.
Interactive projection is a newer feature the leading digital projector manufacturer, Epson, has included in some of its Ultra Short Throw projectors. It allows teachers to engage their class by changing the projected content while interacting with students, instead of frequently pausing to change content and thus fracturing the teaching flow, or talking to a computer screen. These Ultra Short Throw projectors are thus ideally designed for educational purposes.