Introduction to Projector Screens

Theater1PROLOGUE

In the last five years, more and more people are using front projection system and I’ve been asked one too many times on which projection screen to buy.  Just like you can not maximize your $2,000 dSLR camera body with a mere $500 lens, and just like you have to match the wine you’re drinking with the type of food you’re eating; the truth is that you cannot use a projector without a suitably matched projection screen.
A middle-of-the-road projector screened onto a perfectly matched screen can easily beat a high-end projector projected directly onto a wall.  It is really a complex marriage between projector, screen, ambient lighting, room colour and viewing distance.  It is best to just hire an independent THX/CEDIA certified home theatre designer (read: a designer that is NOT working for any store to avoid bias) but it’s always good to have some basic knowledge of the components in the mixture so you don’t end up being blindly guided.
VIEWING DISTANCE
First you have to determine your viewing distance.  In a home theatre setting, it is not recommended to put your seats all the way back against the wall.  You want the rear speakers to be behind you to give an enveloping soundfield.  Secondly, projectors have cooling fan.  In order to minimize the sound of the cooling fan from being heard, you’ll need the projector to be located at least one foot behind your seat.  Don’t forget that the projector itself will require a depth of about another foot.  So to establish your furthest viewing distance in any given room, measure the depth of the room and deduct at least two feet, although five feet will be the most ideal in most cases.
SCREEN SIZE
Choosing the right screen size and the right aspect ratio will have a huge impact on your long term enjoyment of your theatre. So give these matters serious thought. When it comes to a successful home theatre design, these factors are much more important than the projector you choose.
After determining the viewing distance, you can now easily determine the screen size.  There is no right or wrong size although there are two common screen size recommendations.  One is as per SMPTE recommendation, another is as per THX recommendation which recommends a larger screen size to give a more enveloping sense of vision.
As my personal general rule of thumb, the most comfortable viewing distance from my eyes to a projection screen is approximately 1.5 times the width of 16:9 screen; or in the case of Constant Image Height (CIH), equals the width of a 21:9 (2.35:1) screen.  Sitting closer than that, unless you will only be viewing Blu-ray Discs with top notch transfer quality, you’ll just be torturing yourself because the more visible and intolerable various artefacts will become.
Also bear in mind that just because your wall is (say) 10 feet wide does not mean using a 10-foot-wide screen is recommended.  Give the screen some “room to breathe”.  If you are using regular speakers, usually you will need at least one foot space on each side of the wall to put the speakers although two feet each side will be more ideal.  Even if you install the speakers behind the screen, you still want to have at least one foot room to breathe on each side (again, two feet will be ideal) to avoid light reflecting from the screen to the side walls because it can give an illusion of lower contrast (read: bad black level) or at the very minimum, distracting.
My suggestion is to be conservative in the size of screen you choose because most projectors below the $10,000 mark tend to be maximized for approximately 12-foot-lambert to 22-foot-lambert brightness at around 120” diagonal (at 16:9 in 2D) or 96” diagonal (at 16:9 in 3D mode) as per SMPTE Standard 196M Luminance Level. (Side note: THX specify 16-foot-lambert to be the minimum Luminance Level)
viewing-distance-03
ASPECT RATIO
Aspect ratio is the ratio of the width of the screen to the height of the screen. Essentially, it describes the shape of the rectangle. Today the most popular aspect ratio for consumer video display is 16:9, which is the standard HDTV format.  If you are going to use a flat panel HDTV for your home theatre, you are stuck with the 16:9 aspect ratio.  However, for projection you can choose either 16:9 or the ultrawide 21:9 (also known as Cinemascope or 2.4:1 aspect ratio).
With a 16:9 screen, any movie presented in 21:9 will result in two black bars making the picture to appear smaller.  This is what being referred to as Constant Image Width; meaning, regardless of the aspect ratio, the width of the screen remain the same.  This is NOT, however, the intention of wider aspect ratio of 21:9.  Cinemascope aspect ratio is intended to be shown WIDER than movies with 16:9 aspect ratio in order to give a sense of envelopment of every scene in the movie.  With projection system, just like the commercial theatres, this is achieved by using 21:9 screen so movies, regardless of aspect ratio, will have Constant Image Height.
My personal recommendation is to choose a 21:9 screen as opposed to 16:9 aspect ratio.  This is simply because from my personal movie collection of (approximately) 500 LaserDisc, 1,000 DVDs, and 800 Blu-ray, easily 90% of the movies are in 21:9 ratio.  The rest is a mish-mash of 16:9 and 4:3 ratio.
Below are examples of various aspect ratios shown on a 16:9 screen in Constant Image Width and the same examples shown on a 21:9 screen in Constant Image Height

Constant Image Height

CIH

Constant Image Width

CIW

DROP-DOWN, FIXED (FLAT) SCREEN, or (FIXED) CURVED SCREEN
If you don’t have a dedicated theatre room you may want to use a drop-down screen to de-clutter the room when you’re not watching a movie.  However, a drop-down screen, in my opinion, is the worst type of screen because they tend to wrinkle after a while (it can start to wrinkle anywhere from a year to five years depending on your room’s temperature and humidity changes) which results in uneven surface… which negates the function of a screen.  The wrinkle-free drop down screens use tension system will decrease the chance of  wrinkling but aesthetically they tend to look very ugly.
DropDown
For most instances when you can set the projector far back (read: long-throw / closer to minimum zoom) I recommend a fixed flat screen.  These screens are very affordable and usually will not wrinkle with time since every sides and corners are tensioned equally behind the frame.
 Fixed Flat

However, if you are using a short-throw projector, or need to set the projector nearer to maximum zoom or use an anamorphic lens, you will need a curved screen.  This is because the higher the zoom ratio (and the use of anamorphic lens) makes the distance travelled by the light from the centre point of the projector to the middle of the screen to be visibly shorter then the sides of the screen which creates a visually distracting bowing of image and/or out of focus on the sides of the screen.  This type of screen is obviously more expensive, more difficult to assemble and due to the nature of anything curved, all the viewers should be inside the width of the screen in order to view the presentation properly
flatcurve
WHAT ABOUT “ACOUSTICALLY TRANSPARENT” SCREENS?
First of all, “Acoustically Transparent” is a misnomer.  Once you put something, anything, in front of your speakers, it can never be truly acoustically transparent.  More like acoustically “translucent”.  The question is how transparent is the acoustic translucence of the screen.  First of all, “acoustically transparent” screens tend to be more expensive than a regular flat screen.  The most affordable ones are made of micro perforated vinyl with a density around 200 holes per square inch.  This type of screen will result in drop off starting at around 100 Hz and tend to cliff down at around 1 kHz.  I’m strongly against this type of screen because the end result is extremely detrimental in the audio quality.  Furthermore, these types of micro-perforated screens tend to create moiré when coupled with digital projectors, require a minimum viewing distance of approximately 12-feet and the speakers behind the screen can not be installed closer than 12-inch away.
The second type of “acoustically transparent” is the woven-material screen.  They have perforation-pitch of anywhere between 500 holes per square inch to 1,500 holes per square inch.  The higher the number, the closer it is to be a truly “acoustically transparent” screen.  Also with the higher perforation-pitch number, the closer you can sit to the screen and the closer the speakers can be located behind the screen.  At 1,500 holes per square inch, you can sit as close as 8-feet without being able to see any perforation (even if you try very hard) and you can install the speakers as close as 1-inch behind the screen without creating any ripple on the screen.  Also at 1,500 holes per square inch, the frequency roll-off starts at around 4 kHz and only noticeable at frequencies higher than 14 kHz which is approximately the roll-off frequency of most adult hearing frequency curve anyway making the roll-off undetectable.
ATS
SCREEN GAIN
Every screen on the market has a “gain” rating/factor. Some screens are “negative gain” and some are “positive gain.” The gain factor number represents a ratio of the light that is reflected from the screen as compared to the light reflected from a standard white (Magnesium Oxide) board. Therefore, a screen with a gain factor of 1.0 will reflect the same amount of light as that from a white board. A screen rated at 1.5 gain factor will reflect 50% more light as that from a white board, whereas a gray screen with a 0.8 gain factor will reflect 80% of the light from a white board.
A screen is a passive device, so how can they have “gain”?  Screen with positive gain factor reflect more of the light incident on them toward the centre of the viewing area producing a dimmer image the more it goes off to the sides.  The higher the gain factor, the more apparent the difference in brightness between its middle area and its sides.  The most ideal home theatre screens are neutral matte white screen, ie. having a gain factor of 1.0. To enhance black level, some of today’s screens are gray with a negative gain as low as 0.8. On the other hand, one vendor is marketing screens with gain factors that they claim to be higher than 2.0. However, unless you have gone shopping for a screen before you are not likely to know what any of these numbers mean.
The higher the gain, however, have the negative side-effect of having a smaller viewing angle.  With a 1.0 gain screen you will achieve equal brightness and the most accurate colour virtually wherever you sit.  The higher the gain factor, the smaller the viewing angle and the more chance of hot-spotting (That is, when viewing the screen from a center position, the middle portion of the image will appear brighter than the edges) within the screen that can be an annoyance.  Furthermore, screens with higher gain factors tend to generate color shifts in the image that are noticeable as the viewer move around the screen viewing it from different angles.  This anomaly happens because high gain-factor screens do not typically reflect colours equally. So once again, the image looks different to each viewer depending on where they are seated.
I strongly recommend screen gain factors between 1.0 to 1.3, beyond that I personally can easily detect hot-spotting and at 2.0 gain factor or higher, I find the hot-spots to be unbearable and bothersome.
graph
WHAT ABOUT BARE WALL or SCREEN GOO?
If you’re asking whether you can use bare wall or painting it using a “special” paint called “Screen Goo” (or its variants), yes you can.  Will it have a good result?  No.
First of all you will have to be able to make be wall itself to be 100% flat which is a virtually an impossible task to do (and very time consuming too).  Second, if you use “Screen Goo” or even any kind of paint, you will need to be able to have the exact thickness of paint coats in order to give an equally reflective surface which again a very time consuming task.  If you absolutely need to save money for a screen, at the very least, just get the cheapest stretched vinyl screen.  They can be purchased for around $100.
build4
After all, you wouldn’t drink a Vintage 1998 Dom Pérignon Rosé out of a paper cup, would you?
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