Projector Screen and Material Selection

My Da-Lite Tab Tensioned Dual Masking Electrol Screen. Cinema Vision Material. 2.35:1/16:9. Gain 1.3

Introduction

It is very easy to forget the importance of your screen, and after having spent many thousands of dollars on your projector, it is a strong determining factor in just how good your image will look. Remember that your screen doesn’t just reflect the light back from the projector, it also reflects back any light striking it from the walls, ceiling, furniture and other room surfaces and light sources. All of which influence what you will eventually see, by impacting contrast and the colors of the image.

The ‘ideal’ HT room would be one where no light except the projector light is striking the screen. This, for 99.9% of us, is not practical, even if you have no windows in your HT and total control of the rooms light sources. Making a room totally light absorptive and covering all surfaces with non-reflective black, whilst making a technically ‘pure’ room from a light perspective, isn’t very pleasant to be in, attractive, and probably has zero WAF! Such a room would also have to be devoid of all equipment or at least have every LED and display covered up. No light means just that, NO LIGHT.

So for many of us it is about compromise and selecting the best screen for the environment in which we install the projector.

I am lucky to have a purpose built ‘bat cave’ HT in which I have total control of the light and have covered up most, but not all, of the little glowing equipment and speaker LED’s. I turn off all LCD displays prior to running a movie. With the advent of 4K HDR, and even with plain old HD or SD, you are striving for both high contrast ratios and colors that have not been ‘modified’ in some way. To meet the demands of 4K HDR its all about black levels, as no affordable home theater projectors have sufficient light output to achieve the required contrast ratios of 40,000:1 or more, without having blacks that are really black.

Many of us don’t have ‘bat caves’ so selecting the correct screen material and size of screen for your room becomes even more critical.

So, after controlling your light sources and minimizing room reflections, let’s look at the different screen types.

Selecting Your Screen

There are three basic types of screen installation. These are:

• Painted surface
• Fixed frame screen
• Dropped or raised screen

For those of us with a severally limited budget painting a suitably flat surface with one of the purpose manufactured paints can produce acceptable results, but their performance is certainly not optimal and will not be considered here.

It is also possible to make your own fixed screen, you just have to buy a suitable fabric and carefully stretch it over a flat frame or wooden panel. My first 80″ 16:9 screen was made by attaching button fasteners around the screen materials edge (to which I had machined a vinyl pocket, as shown in the images below) and clipping it to a wooden panel. It worked well for many years.

Fixed frame screens can be flat or curved and by their design are all tensioned. Dropped or raised screens may be tensioned or not. Fixed frame screens are the most popular and cost effective type especially when the room decor doesn’t require it to be hidden after use. Dropped screens may be attached to the wall, ceiling or even hidden in the ceiling (see image above), while raised screens may be installed into the top surface of a fixed piece of furniture. Electric screens are significantly more expensive than fixed screens and require AC power and often control (IR, GPI/O or switches) to activate them. They are therefore more complex in nature to install, especially if you want the screen to be dropped or retracted automatically when you activate your system.

Curved screens are not very popular but they can provide a slightly more immersive viewing experience, and do have the following two advantages:

1. Their curve helps reduce reflections from side walls.
2. They help minimize the geometrical distortions that Panamorph lenses create.

Ideally a screen should have black borders to visually enhance its contrast and allow for image spill, as rarely does it fit the screen perfectly (especially true if you are using a Panamorph lens). This is generally achieved with either a low reflectance black frame or black painted borders on the screen material. These black borders, or masking, are sometimes movable, either electro/mechanically or by hand. This allows the user to adjust the screens aspect ratio, typically between 16:9 and 2.35:1. Even though my own screen is dual masking, I rarely use the function (a waste of money).  Most films are produced in 2.35:1 Cinemascope and I use a Panamorph lens that enables me to use the full panel resolution and full light output of the projector without having to zoom it. If required, the lens may be moved out of the light path and the masking dropped to create a 16:9 screen, as shown below.

16:9 Masking Panels Partially Descended

The screen needs to be as flat as possible or the undulations or ripples in its surface can give rise to visible effects in the image. With fixed screens the material is tensioned as it is stretched and attached to its supporting frame. Dropped or raised screens may simply rely on the weight of the screen and/or bottom weighted slat. While the slat often produces a reasonably flat surface there is nothing to beat a tensioned screen whether it is by the use of side tabs or stretching the screen onto a frame.

Spring Frame Tensioning

Edge Frame Tensioning

Tab Tensioning

Some screen materials, in particular, high gain retroreflective types, are more tolerant of screen ripples.

Remember, biggest IS NOT always best when it comes to selecting your screen. It depends upon all the following factors:

• Room light control.
• Projector light output. (lumens)
• The age of the bulb. (not really applicable to lasers)
• Amount of screen gain. (see later section)
• Viewing angle/number of seats.
• Aspect ratio. (16:9 or 2:35:1)
• Use of a  Panamorph lens for 2.35:1 widescreen projection. (no black bars or zooming)
• Projector location. (above/below, on center and distance from screen)

All of the above will impact how bright your image is and whether it is ‘washed out’ by the rooms background/residual light. Resulting in poor contrast and unsaturated colors.

The width of the screen is usually based on a target horizontal viewing angle, room size and the number of seats you want to accommodate. THX recommends a 36 degree horizontal viewing angle for a 16:9 aspect ratio screen, or a 45 degree horizontal viewing angle for a 2:35:1 aspect ratio screen. These viewing angles correspond to the following number of pixels per degree.

• 16:9 – 36 degrees – 53 pixels/degree (HD), 107 pixels/degree (UHD)
• 2.35:1 – 45 degrees – 43 pixels/degree (HD), 85 pixels/degree (UHD)

Research has shown that the limit of visual acuity is approximately 310 pixels/degree.

Viewing Distance And Angle Calculation

For a room where the viewer is seated 15 feet from the screen a 16:9 screen would be approximately 9.75 feet wide and a 2.35:1 screen would be approximately 12.42 feet wide.

• 16:9 width= 2x(viewing distance (ft)x0.325)
• 2.35:1 width=2x(viewing distance (ft)x0.414)

These viewing distances will drop for a given screen size when you switch to 4K if you want to take full advantage of the increased resolution, due to the smaller pixel size.

Effect of Resolution on Viewing Distance

The optimal seating distances based upon visual acuity are:

• HD – viewing distance = 1.7*screen width
• UHD – viewing distance = 0.8*screen width

The optimal vertical viewing angle should be set so that the viewers do not have an angle greater than 15 degrees from either the top or bottom of the screen.

Optimal Screen Viewing Height

There are two different types of screen reflecting material:

1. Angular reflective materials reflect the light back at the opposite and equal angle to what it strikes the screen at. (the majority of low gain materials)
2. Retro-reflective materials reflect the light directly back to light source. (for optimal performance these screens generally require the projector to be within + -5 degrees of the viewing height)

Angular Reflective Screen Materials

Retro-Reflective Screen Materials

All screens have a parameter called gain. It is the ability of the screen to focus or attenuate the light that strikes it before reflection back to the viewer(s). High gain screens will always have significantly narrower viewing angles before the light falls off relative to low gain screens. A screens gain is totally dependent upon its surface finish or coating.

Depending upon the screens surface finish both material types can provide some ambient light rejection (ALR). This ability not to reflect the rooms ambient light back to the viewers can be very important for those of you who do not have the luxury of a dedicated HT or total light control. The amount of ALR generally increases with the ability of the screen to focus the incoming light into a narrower reflection angel. Plain white screens generally have a low ALR, where as those screens with narrow viewing angles will tend to have high ALR. However, care must be taken with these high gain screens as the projector location significantly effects the screens performance, and the angle over which the improvement occurs can be quite limited. Screens with narrow viewing angles may also suffer from ‘hot spotting’ . This is where the center of the screen is notably brighter than its edges. HT enthusiasts will generally use plain white, or low gain screens as they produce the best technical performance if you have a bright enough projector and a light controlled room.

Screen Brightness & Gain

There are very specific minimum requirements that are set out by a number of professional bodies like SMPTE (SMPTE 196M) and others bodies like Digital Cinema Initiatives (DCI) that define the required brightness of a screen in order to achieve a good image. These standards were development for commercial theaters and are now applied to home theaters.

So how is screen brightness measured? In short, as Foot-lamberts (fL). It is simply calculated by dividing the lumens output from your projector by the screens area in square feet and multiplying the result by your screens gain. So a 1000 lumen projector on a unity gain screen that is 4′ high and 6′ wide would be 41.67 Foot-lamberts (fL); (1000/(4×6)) x1.

Image brightness in fL= (projector lumens / Screen Area in sqft)xScreen Gain

SMPTE 196M requires a minimum luminance target of 16fL, DCI has adopted 14fL.

NOTE: The fL is a the USA measurement of luminace. The SI unit is the candela or nit. Where 1fL=3.426 candela per square meter. The current version of SMPTE 196M has been revised to show a luminance target of 55 candela per square meter (nits).

Levels below 9fL, even in a dark room, are insufficient for satisfactory viewing, and levels of at least 16-18fL should be sought after. Remember that your bulbs output will drop by as much as 50% over several thousand hours of use, and continually running it on high power will significantly reduce its life. Correspondingly, too high luminance levels, in excess of 20fL, can start to produce eye fatigue over long periods of viewing, especially for very large screens. For rooms with significant ambient lighting, levels as high as 40 to 60 fL are typically required. Remember that luminance figures for flat panel displays are around 40-60fL for Plasmas and 80-120fL for LCDs.

The screens coating will effect the way, and how much of the light, is reflected by the screens surface. Some screens have high contrast coatings. These typically gray colored screens absorb the light and reflect back less than what strikes them and are said to have a gain of less than 1, and may be a low as 0.5. They are used to enhance perceived contrast ratio. Unity gain screens reflect all the light back that strikes them. A screen gain of 0.5 only reflecting 50% of the light back. Generally, screens with a gain of one or less reflect the light back over a fairly uniform broad arc of up to 180 degrees.

High Quality Unity Gain Screen Angular Dispersion Characteristic

Screen gains may be as high as 4.  Four meaning that you see a brightness four times greater than that being projected to the screen. Remember that the screen is NOT amplifying the light level. These screens ‘focus’ the projected light back into a smaller arc (like a magnifying glass) so as to increase the overall luminance level, while at the same time rejecting more ambient room light than a unity gain screen. There is no such thing as a ‘free meal ticket’, and this increase in luminance is not without some impact on the image and viewing angle.

The distance your projector is from the screen will also impact the images luminance due to the amount of zoom the lens system has to supply. The further away from the screen the projector is, the lower the projectors output. However, generally the upside to this is that the black levels and contrast ratio also improve. This reduction in light output can be as much as 25%. 3D users need to be especially careful as it can really reduce the available lumens output by as much as 50% from your projector.

NOTE: When reviewing your projectors lumens output do not rely upon the manufacturers specifications.  These idealistic values are always for a perfect projector and rarely for one that has been correctly calbrated to REC709 or REC2020. Look for reviews of you projector model to deteremine what its calibrated output is likely to be.

Screen Coatings

There are a very large number of screen coatings, each vendor providing their own ‘secret sauce’, each coating tailored to a specific environment/use/specification. These coatings have very distinct properties that impacts the reflected light. Ideally you want to reflect all the light back to the viewers in a uniform manner, rejecting light sources that are not from the projector so as not to impact the projectors color calibration or contrast ratio. For which you may have paid a highly skilled ISF technician to calibrate!

These coating may be split into three broad groups:

• Low Gain – 1 or less. Wide viewing angle up to 180 degrees with the lowest number of visual artifacts.
• Medium Gain – 1 to 1.5. Viewing angles up to about 120 degrees. Increase in visual artifacts but generally should not be too intrusive.
• High Gain – 1.5 and higher. Viewing angles up to about 90 degrees. Visual artifacts very dependent upon the vendors coating. High gain retro-reflective screens tend to create the minimum number of visual artifacts but require careful projector placement. (see above)

Rough Comparison of Several Da-Lite Screen Materials Angular Gain Characteristics

Remember, as the gain increase all the artifacts described below generally become more exaggerated and your viewing angle reduces. There is no substitute for a really dark light controlled room, a correctly sized unity gain screen and a bright (but not too bright) projector.

Screen Resolution

Image contrast is extremely important and is the primary way we see details in an image. So assuming that you have a projector with excellent optics with minimal chromatic aberration, plus good panel convergence (not relevant to single panel DLP projectors), each pixels sharpness and contrast ratio will have a profound impact on the images resolution.

Ignoring the number of pixels, SD, HD or UHD, the final resolution is all about the screens surface finish or how smooth it is. Why? Remember that the image is made of up of tiny pixels. Depending upon the size of the screen these pixels can be anything from 1mm to less than 0.5mm square. If the screens surface texture is sufficiently optically and physically rough to disturb those pixels by distorting them or causing their edges to be light diffracted, the ability of the screen to resolve fine detail is diminished. This is especially true now for native 4K where the pixel is only a quarter the size of that for HD. Hence the large number of very smooth screen surfaces that are now available.

Besides the screens surface finish, the gain of the screen also impacts its resolution. Screens with gain have coatings that can create very undesirable properties while at the same time reducing the viewing angle and improving ALR. These special coatings can produce a number of distracting effects to include:

• Hot spots – This is where the center of the screen in notably brighter than its edges. This can be very distracting if it is excessive.
• Sheen – Areas of the screen have a shiny appearance. Mostly seen in bright areas on screens with gain.
• Poor color uniformity – The coating applied can have a non uniform absorption/reflection across the color range. After having your projector ISF calibrated you don’t want the screen to alter the projectors color temperature.
• Sparkle/screen noise/speckling – These are very tiny bright/glistening spots that become more apparent the brighter the scene becomes. In bright areas they can look like digital noise.
• ‘Window effect’ – The screen can glisten as if covered by a translucent film. This static effect looks as if you are viewing the movie through a dirty window and can be very distracting on bright scenes as the film appears to move/pan behind the effect.

When possible try to get large samples of your selected screen materials and try them out before spending what may be thousands of dollars on your screen. Most screen vendors sell, or provide free samples, depending upon its size.

A brief review of my Da-Lite screen and its recent material upgrade.

Check this link for a detailed Projection Screen Material report from AccuCal AV by Jeff Meier.

Projection Screen Calculators.

Visit Da-Lite Screens here.

Visit Stewart Screens here.

Visit Elite screens here.

My JVC DLA-RS640 projector review.