If projectors are so bright now, why do we need to use Fresnel screens?

There are two basic reasons for using Fresnel elements in screens. The first is the evenness of image brightness – avoiding the infamous ‘hot-spot’. The second – and secondary – reason is image brightness.

What is ‘hot spot’? Hot spot is that bright spot one sees in the centre of the image. It seems to follow you around the room because it is actually the direct line between your eye and the projector lens. It is an artefact associated with diffusion screens, and produced by the screen’s inability to redirect the light. Why? Because the light from the projector is angled away from the viewer in the corners, and because such screens do not have Fresnel lenses to redirect the light perpendicular (i.e. square) to the screen, the corners are darker in consequence.

The problem of hot spot is worse with wide-angle projector lenses (e.g. less than 1.8:1 projection ratio) and often extreme in the case of the lenses used most commonly in rearpro applications (0.8:1 – 1.2:1). (These figures are called the ‘throw ratio’ - they relate image width to projection distance.)

The second reason for using a Fresnel within a screen is still brightness. It’s true that projectors are much brighter now. But it is also true that customer expectations have increased as well – not just in the brightness of image they demand, but also in the levels of ambient light in which they need to operate.

But the last word must remain with the market. The manufacturers of commercial retro displays, videowall cubes, rearpro TVs etc, operate in a fiercely competitive environment. If they save pennies they can win large competitive advantage. They could save many pounds by using diffusion screens. So why don’t they? Because the Fresnel is vital and without it such units cannot meet minimum standards demanded by the market. (An uncharacteristically aggressive training slide by DNP demonstrates that it is possible to get a brighter image using a cheaper projector with an optical screen, rather than using a more expensive, brighter projector using a diffusion screen.)

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Greg Jeffreys is Director of Paradigm Audio Visual Ltd

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