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.
More important than the technology are specifications, and there are three key areas. Resolution determines image sharpness, and most education-focused projectors offer a XGA (1,024 x 768) or WXGA (1,280 x 800) native resolution. The latter has a widescreen aspect ratio (16:10) and is more suitable for interactive whiteboards. Brightness is measured in lumens: the brighter the image, the easier it is to see in a well-lit classroom. The best projectors offer at least 2,500 lumens brightness. The contrast ratio determines how light or dark images appear; whether an object looks a deep black or a washed-out grey. As a rule of thumb, the higher the contrast ratio, the better the image quality.
Connectivity is important because it will determine the number and type of devices that can be connected to your projector. If the projector is destined for a fixed location (such as a ceiling mount), it’s important that you don’t have to keep plugging in and unplugging devices. Virtually all projectors offer D-SUB in and out, S-Video and composite video as standard, but if you want to use a Blu-ray player with your projector, say, then an HDMI port is useful. Some projectors also include USB ports for displaying images stored on flash drives.
Short-throw projectors command a price premium, although prices are falling and some models are now around £400. But while price is important, so are running costs. LCD projectors use dust filters that must be regularly cleaned, and eventually replaced. All projectors will need their lamp replacing at some time, and it pays to compare the stated lamp life. Bear in mind, though, that manufacturers’ lifetime ratings are often shorter in the real world. Eco and economy modes can extend lamp life, but check how much these modes affect image brightness.
All projectors use a fan, and you should ensure that the noise level isn’t intrusive. Also look for security features such as password and PIN protection – and use them! Finally, check what warranty comes with your projector and what it covers. Some manufacturers offer extended warranties for schools, and these can bring extra peace of mind.
Getting 3D ready
A growing number of projectors now advertise themselves as “3D Ready”, but what does this actually mean? All it guarantees is that the projector will accept a 3D signal in at least one of the four most widely used formats – frame sequential, checkerboard, side by side or frame packing – and display it, provided you have the right equipment. The equipment needed will vary from brand to brand, but in most cases you’ll be looking at a 3D adapter unit, which synchronises the left- and right-eye frames being projected with the left- and right-eye shutters in the 3D glasses, or an adapter – such as Nvidia’s 3D Vision system – that plugs into your PC. As you’ll also need to buy a number of compatible 3D glasses, often costing upwards of £50 each, getting into 3D can be an expensive business.
The other part of the puzzle is 3D content. This is coming online, and anyone who attended BETT this year couldn’t help but be impressed by the quality of 3D anatomical models and 3D maps being exhibited, all visualising subjects in a way that 2D graphics can’t easily match. Seeing a diagram of the human lung is one thing, but watching the bronchioles swell and shrink in 3D as your view moves through the body is quite another.
Companies such as Designmate, Amazing Interactives, Xpand and Cyber-Anatomy are pioneering here, but there’s a definite bias towards science over other areas of the curriculum, and it’s questionable whether there’s enough material out there yet to justify a move into 3D now. However, early studies look promising, with teachers in pilot projects praising the impact that 3D has on students, and the way it seems to get them talking and help them retain knowledge. Don’t write off 3D as a gimmick quite yet: it could one day become an essential classroom tool.
Author: George Cole
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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