Buyer’s guide to short-throw projectors
Posted on 16 Jun 2012 at 17:16
Tired of trying to get a working image in a standard classroom? George Cole has six short-throw projectors that will fit into any space
In an ideal world, projectors would always be used in a theatre-like environment. In reality, however, teachers often have to use them in less than ideal conditions, including rooms where space is at a premium. This is where short-throw projectors make a difference. They’re designed to display a large image when the projector is close to the screen. In fact, many models can produce a 70in image from less than a metre away, which makes large-screen projection possible in the smallest of classrooms.
It’s all about the throw ratio, or the distance between the screen and the projector, divided by the width of the image you’re trying to project. Most short-throw projectors have a throw ratio of between 0.38 and 0.75, meaning that to get an image with a 70in diagonal you’d need to have the projector between 68cm and 1.33m away. However, a new generation of ultra-short-throw projectors offers throw ratios of less than 0.38, making it possible to place the projector even closer to the screen and still get an extra-large image.
Using a short-throw projector means there’s less chance of shadows being cast across the screen, and teachers are less likely to be blinded by the projector light shining in their eyes – or trip over wires and cables. They work best when the projection screen is perfectly flat, but can be used on a wide range of surfaces including whiteboards, blackboards and green screens. Many even include menu settings to help get the optimum picture quality from different screen colours.
Short-throw projectors come in a variety of form factors, from highly portable to bulky versions designed for permanent or semi-permanent installations. The advantage of portable versions is their flexibility – they can be moved around a school with ease, and you have a choice of table-top or ceiling installation. The downside is that projectors are one of the biggest items on a burglar’s shopping list, and this portability makes them easier to steal. Large projectors are less likely to be stolen, but their fixed location makes them less flexible.
Types of projectors
There are two main types of projection technology used today: DLP and LCD. LCD projectors send light to three panels, which use filters to create red, green and blue light. Each LCD panel is composed of an array of pixels that open and close like shutters to create a wide range of colours and contrasts. DLP projectors shine light onto a chip composed of hundreds of thousands of tiny mirrors (representing the pixels) that reflect light onto a fast-moving colour wheel. LCD projectors generally offer better brightness and colour saturation, but images can suffer from pixellation and generally have a lower contrast ratio than those from a DLP projector. DLP images offer better contrast and don’t suffer from pixellation, but images are often less bright than those from an LCD projector, and can display what’s called a rainbow effect. Here, high-contrast elements, such as a white line on a black background, can produce perceptible coloured flashes, a phenomenon caused by the spinning colour wheel. Still, manufacturers have worked hard to minimise the issue, and most tend to build DLP products these days.
Installation Issues - Short-thow Advantage
I've come across numerous cases where a long throw projector has been installed directly on to the ceiling of the room instead of being 'normal' to the screen (presumably to save money or 'hide' the projector). Trapezoidal correction of the projected image has then to be used to match the screen/whiteboardg giving a different number of pixels for the top and bottem rows of the image on the screen.
The resulting image has a very low effective resolution for text due to the vertical lines crossing pixels.
A short-throw projector would eliminate this problem as it uses a special lens to optically make the correction.
By jcaiken on 2 Jan 2013
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