Angles Of Reflection

The front side of a projection screen is, arguably, the most important side.  It diffuses the light from the projector, it is the side the audience actually looks at and is, as a result, the side that carries all the important characteristics like gain and half-angle.  The reverse side of a front projection screen is also important, though much less is said about its properties and it garners little attention.  Let’s look at what goes on…

Behind the Screens

If you look at the back of a projection screen, you will probably see one of three things.  Either the back will look almost the same as the front surface, it will look like an untreated or uncoated version of the front or it will have a black coating. For the purposes of this discussion, the two screen variations without the black coating will collectively be called non-backed and the screens with the coating will be called black backed.  Compared to the numerous permutations found on the front side of projection screens, these few options for the back are not exactly robust.  I would argue that only one choice is really needed but we will get to that in due course.

First, we should establish a distinction between useful and unwanted light.  In essence, the projector will be responsible for supplying all of the useful light to the display system.  Any other light involved will be considered unwanted, at least as far as the display of a projected image is concerned.  The screen’s role is to make use of the projected light while minimizing the detrimental effects of the unwanted light.  Previous articles have shown how the front surface of a projection screen is able to contribute to this effort but what of the reverse side? 

For that, we often use a black backing, intended to help control unwanted light by absorbing any that would otherwise be transmitted through the screen.  While we do not recommend doing so, the obvious instances where black backing would be the most useful would be when the screen is installed in front of a window or some other source of illumination.  The logical question that would follow from this is:

What if there is no light source behind the screen?

Plenty of screens are installed directly in front of such things as walls, chalkboards and whiteboards, none of which count as particularly luminous surfaces.  In these circumstances, it would appear that a black backing may be, at best, superfluous.  Of course, that is assuming that no light is being reflected by whatever happens to be behind the screen.  Perhaps we could negate this possibility by ensuring that the projector is the only light source in the room but then, perhaps not.

Earlier, we established the basic assumption that the projector is the source of useful light for our purposes.  That remains true but it should not be implied that all projected light is useful.  Specifically, projected light that is allowed to pass through the screen and away from the audience can hardly be called useful and is, actually, in serious danger of becoming unwanted.

Any portion of that light that is reflected by an object behind the screen and then is allowed to pass back through its surface a second time is now very likely to interfere with the projected image.  The interference will most likely take the form of a drop in contrast as the display’s black level will rise towards gray as a result of the additional light. 

Does the backing have to be black?

If the main reason for the backing is to block light from being transmitted through the fabric, then it stands to reason that any adequately thick or dense material would serve that purpose regardless of its color.  While arguably true, it is important to remember that projection screens are usually designed to be as light and flexible as possible.  Increased density and the resultant rigidity and heaviness are not positive features for a screen.  Granted, some rigidity is needed to help the material to lie flat, especially in non-tensioned screens but too much would prevent a screen from being able to roll up easily.  Similarly, extra weight in the fabric puts stress on the roller or framework that supports it, making it increasingly difficult to maintain a flat surface, even if the screen is not intended to be rolled.

As a result, there is a limit to how thick or dense a screen surface can be before additional considerations are needed in its overall construction.  The alternative to blocking light via density is to absorb it using a dark color and, obviously, that is where black backing comes in.  It is able to absorb a good amount of light without significantly adding to the weight of the screen.  It is simply an efficient method of controlling unwanted light.

How much transmitted light is too much?

The safest thing to do is not to let any light through the screen at all.  A black backing all but ensures this in so many applications that it seems worth the effort of putting it there in the first place.  To answer the question at hand: the bare minimum contrast needed for an acceptable display is 10:1.  No one will mistake “acceptable” for “excellent” but as has been covered in this series earlier, this is the cut-off point beyond which lies disaster. 

To connect this contrast requirement to the issue of light transmission is a little difficult because of the number of variables involved.  Is the light from behind the screen the result of direct or indirect sunlight?  Is there a reflective surface behind the screen or a diffuse one?  How much light will that surface return towards the screen?  It would be possible to make some educated predictions for specific cases but, again, the easiest solution to all of these issues is to eliminate them with a black backing.

-- Adam Teevan
    ateevan@da-lite.com

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