Large Projection screen warping, twisting and bending.

Over the years we have discussed bending and bowing issues of projection screen frames, in particular larger sizes.

These frame distortions are created by gravity, material weight and the tension of the projection surfaces and they have been dealt with by choosing the right balance between profile strength, enigeering and counter tension.

However one aspect is never really discussed. It is frame twisting and warping.

In the past, using projectors with ‘normal’ or ‘long’ focal length lenses, this effect had, apart from looking unsightly if looked at sidewise to the frame, no real effect. These type of lenses, would not distort much, even if the screen surface was a bit out of wack.

But with the increasing use of Short Throw and Ultra Short Throw projectors we need to consider this apsect.

You will have noticed, that these projectors create massive distortions if only slightly angled from the perpendicular or the ‘designed’ optimal projection angle of the unit. Any distance difference between one point of the screen and another is sufficient to change the image size notably.

This is difficult to correct if the frame is warped or twisted. Even corner adjustments at the projector may not be able to correct this.

Hardware blending units, such as the Geobox or VIOSO may allow you to ‘push’ the screen into the correct location, but there will be inherent pixel distortions or pixel loss. Your usual keystome correction will not deal with it.

Warping and twisting can, in the extreme, lead to focus loss as well.

One could argue that this may not be relevant for general audiences, since they would not even note this.

True. But lot of presentations and installations deal with precise data, such as architectural drawings, graphics, spreadsheet or scientific or medical imaging or simulations.

In such cases the wrong presentation of the projected information can be misleading to the audience and ‘over correction’ in the projector set-up, can change the proportion of pixels and lines.

How to solve this?

Lets take a long sheet of metal and try to keep it flat and straight. Almost impossible, unless you support it. But as more you support it, as heavier it becomes. Earlier we learnt that weight creates downward bending of the screen horizontals.

As longer a projection screen is, as larger the deviation from the perfect straight line. How to solve this?

Secondly, in most projection screens the surface is attached to the front and stretched. This creates an additional force on the metal tubing of the frame.

This is why ‘normal’ rectangular or square tubes, such as 32×32 or 32×64 will twist if the screen gets large.

This twisting and warping changes the flatness of your projection surfaces and with it, the distance to the projector at various points. Usually towards the edges, assuming the projector is set-up perfectly perpendicular.

ST and UST projectors will respond disproportional to this distance change with the image being larger at the point where the screen frame bends away.

So how to solve it?
1) Shoring your frame up by add braces. But extra metal adds to weight.
2) Use a frame which places the surfaces in the centre of the profile. But this may not exist.
3) Place a projection surfaces on both sides of the frame, so that the tension from the surface cancels each other out. But this addes weight and makes set-up difficult.
4) Select a frame profile which is designed to twist and warp less.

Warp and twist less. Ultimately all frames which have a surface attached to one side may warp and twist to some extend. Even borderless screens, which wrap the surface over the frame and thus reduce some of the problem.

So next time you struggle to make your corners fit, twisting and warping may be the reason in addition to any bending and bowing of the screen. You may have to reconsider which screen to use of your application.