Ambient Light Reflective Screens and the Laws of Physics: Introducing Phantom™ HALR™

By Alan C. Brawn CTS, ISF, ISF-C

Ambient Light Reflective Screens and the Laws of Physics: Introducing Phantom™ HALR™

As we know, the new kid on the block in terms of projection screens falls under the heading of ambient light rejecting or ALR. The ultimate goal of an ALR screen is to give viewers brightness and picture quality that rivals or even surpasses what other display technologies can provide with the lights on or off. The key issue is the management of light falling on a screen and ultimately, what is reflected back to the eyes of the viewers.

Surfaces tend to reflect incoming light in all directions. You can take a look at a picture on a wall and from most angles it looks the same because the incoming light has been diffused or distributed evenly. These surfaces are called diffuse reflectors. In projection screen parlance, a matte white screen like the Stewart Filmscreen StudioTek™ 100 is a near perfect diffuser, spreading the incoming light at nearly 180 degrees for extremely wide viewing angels. From a purely technical point of view this would be called a Lambertian surface. Lambertian reflectance is the property that defines an ideal “matte” or diffusely reflecting surface. The apparent brightness of a Lambertian surface to an observer is the same regardless of the observer’s angle of view.

A “perfect” diffusion screen spreads the incoming light out evenly and for this reason, there is little natural ability to control the incoming ambient light that interacts with the projector’s illumination other than by some means of lighting control. Any unwanted ambient light falling on the screen surface will degrade the contrast and color saturation to some extent. In high ambient light situations this can render the images unacceptable.

Another type of surface that reflects light is known as a specular reflector.  Specular reflection is the mirror-like reflection of light from a surface.  Incoming light from a single direction is reflected into a single outgoing direction. Such behavior is described by the “law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal, thus the angle of incidence equals the angle of reflection.”

With specular reflective aka angular reflective screens, we have a degree of control over the general ambient light hitting the screen. In systems design, we can calculate where the incoming light is hitting the screen surface and then know that the reflection is going to equal that. The reflected light can then be taken into consideration such that it does not interfere with the viewing cone of the audience. Since angular reflective screens reflect light more in one direction than in others, it tends to make the image brighter for people sitting in a pre-determined area. These screens are typically higher gain where the incoming light is focused on a narrower viewing angle. The concept of controlling incoming light and reflecting it at predetermined angles is the basis for most ALR screens we see today

By selectively reflecting the projector’s light, you can position the screen in such a way that the projector’s light is directed towards the audience’s eyes.  The ambient light in the room, that can negatively affect the viewing experience, is absorbed to a degree and then reflected in some other direction away from the viewing cone. The solution is to match the light coming from other angles and to reflect as much away as possible, reducing image degradation. Keep in mind that ALR screens only work if the ambient light and the projector’s illumination are coming from different directions. Examples might be reflections from light colored walls, light from general illumination room lighting or task lights, or lights coming in through a doorway, or the sun coming through a window. Continue reading

4K, 8K, 16K-Stewart Filmscreen is There and Ready When You Are!

By Alan C. Brawn CTS, ISF, ISF-C

Just as we are beginning to embrace the 4K phenomenon in displays, we are already seeing the rumblings, dare I say emergence, of 8K and even 16K displays in the not-too-distant future. While some seem to be content with 1920 x 1080 or full HD, evidence shows 4K is here to stay – at least until the next evolution in resolution takes hold.

We have all seen articles with titles like “Is 4K Really Necessary?” and “Barriers to 4K.” Many of these articles talk about the availability of 4K source material and the bandwidth needed to stream 4K, while others mention that 4K is best viewed at close proximity and that there is little visible difference at longer viewing distances.  There is no argument that we are in the midst of an evolution of source material and available bandwidth before we can embrace full 4K adoption, but the issue of visible differences is one that begs for more discussion centered around the science of visual acuity.

Understanding Visual Acuity and Pixel Density

Let’s start with pixels. AV experts and video enthusiasts know: The more pixels there are, the smaller they become, resulting in more overall picture information that can be displayed on a screen. This is true from a technical standpoint, but too simplistic to truly explain the relevance of greater pixel count.  It ultimately boils down to what the human eye can resolve. So, just what can we see? Referring back to the Snellen eye chart to measure 20/20 human vison, we learn that the human eye resolves one arc minute of information. An arc minute is a subdivision of 1 arc degree. There are 360 arc degrees in a complete circle and 60 arc minutes in each arc degree.

As you can see from the diagram below, we can also subtend 1 arc minute because someone with 20/20 vision can see that letter E on the eye chart. In short, that’s what our eyes actually detect, but let’s do a little bit more math and relate it to pixels.

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There are 10,800 arc minutes in 180 degrees of viewing. So the eye has a limited resolution of one arc minute and would require an image no less than 10,800 pixels wide. Achieving similar horizontal resolution with 1920 x 1080 projectors would require 10,800 divided by 1920 H pixels per projector, which is 5.6 projectors across edge to edge and 7000/1080 or 6.5 projectors vertically— and that’s without blending losses. So we would need about 6 x 7 or 42 of these HD projectors, minimum, to match human acuity. Clearly that is very expensive and not practical for most applications.  But the example gives us some idea of what the human eye can actually see. Now, those 4K, 8K, and 16K displays begin to make more sense, because human vision really can detect a difference.

Image Resolution: More than Just Pixel Density

If we turn our attention to the display devices that create what we see and keep in mind the “holy grail” of what the human eye can resolve, let’s explore resolution from another perspective.  Image resolution correlates directly to the amount of detail in an image. Higher resolution means greater image detail and more detail brings us closer to visual acuity. Continue reading

Matte White Screens, Gray Screens, and ALR Screens Are Different

Front projection screen materials have evolved over the years. In the days of filmstrip and 16MM projectors, we had glass beaded screens. For a long time, we have had matte white screens with both unity and higher gain, (to make up for lower light output projectors). Recently, while matte white screens are certainly still around, we have entered into the era of gray screens and ambient light rejection screens.

To pay proper respect, a traditional matte white screen like the Stewart Filmscreen SnoMatte 100 is a near perfect diffuser of light (a Lambertian surface), and is considered a reference standard of reflectance. This is the property that defines an ideal “matte” or diffusely reflecting surface. The reflected brightness of a Lambertian surface to an observer is the same regardless of the observer’s angle of view.  In basic terms this means that the screen reflects nearly identically across a 180° viewing angle, as produced by the projector. In a totally dark environment this type of reference screen is like a studio monitor in audio. It brings no added “coloration” to the equation and takes into consideration all angles of view. The illustration below shows the diffusion properties.

A Lambertian surface in the form of a matte white screen is great, but when ambient light falls on the surface, the image is degraded in terms of both contrast and color saturation.  One answer is to totally control the ambient light in a room and/or use a brighter projector to overcome that factor… or a gray screen can be used. A gray screen is designed to resist some of the elements of ambient light, and significantly improve contrast and the appearance of color saturation as a result. The key issue here is that gray screens are intended to be used in low level ambient light situations, with general illumination. While they do have a degree of ambient light rejection capability, they are not able to reject a significant amount of ambient light coming in from all angles such as windows, open spaces, and areas with little to no lighting control. This begs the question… what to do in those cases?

With limitations being accepted, and most lighting out of the control of the AV designer, the choice should be an ambient light rejection screen (ALR for short). An ALR is a screen engineered specifically to reject light coming in from obtuse angles. To understand better, there are two basic type of reflection; specular reflection (where the angle of incidence equals the angle of reflection) and diffuse reflection. See illustration below:

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ALR screens use a combination of optical dispersion techniques to accomplish their task.  Each ALR screen has characteristics that define it. There is both a horizontal and vertical viewing angle that is designed as the “sweet spot” for the viewer. Once again, we can’t break the laws of physics, so the techniques of rejecting ambient light will reduce the viewing angle for the audience to a varying degree. As with gain screens, going beyond the specified viewing angle will reduce the quality of the image on the screen and the ambient light rejection capabilities… but within the specified zone, the ambient light is directed (reflected) outside of the viewer’s eyesight. In this process, brightness, contrast, and color saturation are maintained in the presence of higher ambient light and in some cases even enhanced.

The point here is that there are really three general types of screens, and they should not be confused. A matte white screen is a near perfect Lambertian surface, ideal as a reference standard in dark environments. A gray screen is designed to enhance contrast and color saturation in the presence of a small amount of general ambient light. Finally, an ALR screen, designed to maintain image integrity for applications where there is a great deal of ambient light on the screen from numerous angles.  Knowing when to use what can save the day for an AV designer.

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