Angles Of Reflection

There are plenty of very practical reasons to be excited about the capabilities and efficiencies of digital signage, even without considering how adept they are at attracting viewers’ interest.  The battle for this kind of attention is forever escalating, however, and as the once novel digital messages become commonplace, standing out will require new approaches.  One such approach that carries an undeniable allure is to use…

A Semi-Transparent “Holo” Screen

The simplest reason to take this route is that it offers a set of capabilities similar to yet quite separate from the established digital signage equipment.  Before we get to what this screen can do, we should establish what it is and what it is not. 

The Holo Screen is a passive display device comprised of a special diffusion coating applied to an acrylic substrate.  This is the same basic definition as any of our other rigid rear projection screens and, indeed, they are comparable in most respects.  As with any rear projection screen, the diffusion coating is designed to transmit and scatter light from a projector in order to render a bright, visible image.  Likewise, the substrate’s primary function is to provide a rigid structure to support the diffuser. 

Where the Holo Screen diverges from other rear projection materials is in the aforementioned “special diffusion coating”.  This coating is formulated with a particular emphasis on transmitting more incoming light and diffusing less than what is typically done.  To get a better idea of what that means, we should take a close look at a common diffuse surface and a transmissive medium: a thin sheet of white paper and a window. 

Most paper of this kind does a good job of evenly diffusing light that falls on it.  It provides a uniform, matte surface that is free of mirror like reflections, absorbs very little light and is largely – but not completely – opaque.  Holding the paper up to a light source will demonstrate this and probably also reveal obvious imperfections in its transmissive uniformity.  Light and dark patches will intertwine in unpredictable patterns corresponding with minor fluctuations in the density of the paper.  The thinner portions allow more light to pass through and, thus, appear brighter than the darker, denser portions. 

If we could refine the sheet to be the same density throughout and, in doing so, improve its uniformity when backlit, we would wind up with a decent version of a diffusion coating.  By affixing this makeshift coating to the interior side of a windowpane on a bright day, a rudimentary rear projection screen would be created.  In fact, many other screens of this sort are created using similar methods but there will be more to say on this topic later.

At this point, it should be easy to distinguish the paper from the surrounding glass window by virtue of the way these objects affect the light pouring in from outside.  Whereas the window simply lets most of the daylight into the room without much noticeable interference, the paper is scattering the light in all directions.  This difference is what makes the paper appear to be glowing white while the window is virtually invisible.  We have already established that the paper on the window is analogous to a rear projection screen, so the next step will be to turn it into a faux Holo Screen.

This will require making some changes to the paper so that it retains some of the diffusion properties it already has while adopting the transmission characteristics of the window.  An easy way to accomplish this is to dampen the paper with water or oil.  Once done, the paper will allow a more significant portion of incident light to pass without interference than before.  If the sheet is sufficiently thin, it should even be possible to see objects on the other side of the window through the paper.

Despite this change, the sheet would not be as transparent as the window and would still look very slightly white.  This is so because it would continue to diffuse some incident light, just as it did when it was a normal piece of paper.  Regardless, the transparency would be unmistakable and its balance between scattering and transmitting light would be roughly similar to that of a Holo Screen.  Of course, the actual coating on the Holo Screen is a lot more complicated than an oily sheet of paper.

Without digressing too far into the details, our diffusion coatings are sprayed onto substrates in a carefully controlled process not unlike powder coating or spray painting metal.  While the reasons for doing this are fairly practical and mundane, the benefits are much more interesting.  First, this method makes it impossible for the diffusion layer to peel away from the substrate, bubble or otherwise interfere with the image by way of losing adhesion.   Second, we are able to use a much thinner diffuser than many alternative methods, which improves the sharpness of the images it displays.

Now that we know what the Holo Screen is, we can explore how to take advantage of its properties.  The most compelling way to do this will be to utilize it in ways that would be impossible for a standard flat panel or projection display.  In short: make use of the transparency.

There are endless ways to do this.  Indeed, there would be far more than I could write down or even think of.  That is exactly why this is a valuable tool for digital signage: the possibilities have only begun to be explored.  Whether using a dim image to create a ghostly effect or bright text around a dark background to give the appearance of words floating in space, the techniques for engaging a viewer in unexpected ways are, to be sure, still unexpected. 

The important thing to keep in mind when devising content for a Holo Screen is that anything that appears black in the image will become transparent in the display.  This can be advantageous, as in the example of the white text in front of black mentioned above, but it can also be dangerous if not given proper consideration.  Whereas good, high contrast images are perfect for showing off the capabilities of most display systems in a variety of environments, that contrast is created when a part of the image is very dark.  A nice, deep black on a regular screen will become whatever happens to be behind a Holo Screen.  The options for accommodating this fact are to use bright images without dark areas, to use a dark background behind the screen so that black stays black or limit the use of black and dark gray to only areas that are intended to be transparent.

No matter how the screen is used, however, there will certainly be a projector involved.  As you might expect, the fact that the screen passes light nearly as easily as a window does will mean that the projector’s position will need some careful thought.  Keeping the lens from shining light directly into the eyes of viewers will be an important factor, as will controlling where that transmitted light falls.

Both of these conditions can be met by setting the projector either above or below the edges of the screen and aiming the lens down or up, respectively.  As a rule of thumb, we recommend placing the projector 18-35° either above or below the screen, though any position will be possible, so long as the projector is able to perform keystone correction and provide adequate heat ventilation when not laying flat.  Since most people do not carry a protractor with their projector installation equipment but do carry a tape measure, please refer to Figure 1 for the methods for determining where the projector should go.

The throw distance for the intended image size and appropriate lens should be calculated as indicated by the projector manufacturer.  Once the throw is known, simply multiply that distance by the sine of the projection angle (remember to use radians, not degrees) to find how much higher the projector should be placed.  For example, to project at 30° above normal from five meters away, multiply 5 by sin(30°) to get 2.5m.

For the horizontal axis, multiply the throw distance again, only this time with the cosine of the projection angle.  Using the formula: 5*cos(30°) = 4.3301m†.  If our projector were intended to be set up directly behind the top edge of a normal screen, five meters away, we would want the projector to be 2.5m above the top of the screen, and only about 4.33m from its surface.  This will give us the desired projection angle of 30°.

† Because calculating trigonometric functions is not always easy to do, consider using the chart in Figure 2 as a guide to make the process a bit simpler.

Incidentally, 30° is a good angle to choose.  We have taken Holo Screen gain measurements from a variety of vertical angles, beyond the standard 0° in use for all other materials.  Because so much light is going directly through the screen, its on-axis, peak gain measurement is somewhere around 400.  This is an astonishing number but virtually useless since anyone looking at a 400 gain screen will quickly look away.  Measurements taken from even a few degrees away from the center, thankfully, allow the gain to drop by a significant amount.  By the time we reach 30°, the screen is showing a much more reasonable 0.4 gain. 

Admittedly, 0.4 sounds like a low number – and it is.  While experience may suggest that such a gain is the result of absorption by gray pigmentation in the screen, the real cause is loss due to transmission.  Any screen that passes a large portion of light energy in one direction does so at the expense of all other directions.  When the area of peak gain is directed away from the viewer, what that viewer sees is the light that is scattered in those other directions.  The quantity that remains to be scattered is relatively small, hence the low gain. 

In practice, this gain will make it necessary to use a projector capable of generating approximately 2.5 times the lumen output needed for a comparable Matte White screen.  Other solutions are possible because as the projection angle increases or decreases, the lumen output must do the same.  The 30° angle we have been using until now is a good balance between keeping the peak gain out of the viewer’s eyes and maintaining a reasonable brightness requirement. 

With brightness accounted for, the next concern should be uniformity.  Fortunately, an enormous peak gain does not prevent all remaining light from being scattered in a uniform pattern.  What this means for the Holo Screen is that, while a lot of light is lost, what is left over is surprisingly uniform, thanks to the fact that the light is projected and viewed at an angle.  Specifically, the effective half angle is around 45° when the projector is set at 30°.  For reference, this is comparable to the half angle of a front projection surface with a gain of 1.3.  This does not change the fact that a bright projector is definitely called for to make a bright image on a Holo Screen but as far as uniformity is concerned, there should be no problem.

Actually, if there were a single phrase I would use to describe using a Holo Screen, it would be “no problem”.  Granted, there are some major differences between these screens and traditional displays and several of these differences require a little extra thought, preparation and design effort.  When the work is done right and there is well-designed content on the screen, though, the benefits are perfectly clear.

-- Adam Teevan
    ateevan@da-lite.com

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