Ultra short throw
Because of this we saw an upswing of ultra short throw projectors in the last couple of years. As the name suggests, these devices can be placed even closer to the screen or wall than a regular short throw projector. Ultra short throw models are even mostly used to be placed (nearly) entirely against the projection wall, for example on an audio-video cabinet or dresser that is placed against the wall.
Ultra short throw projectors have a different construction that other projectors: they project the image via a mirror towards the screen. The lens system is actually backwards in the projector. The light shines to the back, or towards the viewer. The lens light is caught by a convex mirror at the back of the projector, which sends it upwards to the screen. This allows the system to project a very big image at very short distance. The models that we tested project an image more than a meter wide if you place them directly against the wall; if you pull them from the wall the image quickly becomes even bigger.
Ultra short throw projectors were primarily introduced for the business and educational market, because it allows them to be very close to the projection screen without casting a shadow. Ideal for teachers and other people giving a presentation. In the business market, short throw projectors are usually mounted to a very short arm that is directly connector with the screen.
In the meantime ultra short throw projectors also found their way to the regular market, although the offer is limited. The advantages are clear for home use: you no longer have the problem of people walking through the image, the projector can easily be placed on a dresser where it is not in the way and can be connected to other devices with short cables.
However, ultra short throw projectors also have disadvantages. The most important one being the flexibility in terms of placement. Affordable ultra short throw models do not offer options such as lens shift or zoom. The projector has to be placed in exactly the right way when it comes to height and distance in order to have the image in the right place.
Another problem is the geometry of the image. Because the lens system has to magnify the image, the standard problem of wide-angle lenses quickly appears: image distortion. While the models we tested manage to keep this to a minimum, all three ultra short throw models show some form of barrel distortion. This means that the edges of the image are not entirely straight, but expand slightly in the middle.
Aside from that it is difficult for some models to show a sharp image from top to bottom. Once again an effect of the lens system and the fact that the bottom of the image – with standing projection – is a lot closer to the lens than the top.