When setting up a front projection system the screen is as much a part of the system as the room, acoustics, sound system and the projector. Deciding on a screen is an iterative process between screen properties, screen features, projector cost, projector light output, room furnishing, screen cost, seating location and the location of the projector. Projector and screen manufacturers also complicate this by frequently overstating what their products are capable of. This article addresses the issues with 2D image projection. 3D adds additional levels of complexity.
Actual projector light output (lumens), screen surface area and the installed screen gain will determine how bright the image is. The desired light output from the screen will vary based on the application. The lowest light level I would target is 9 foot Lamberts if you must push the limit with screen size. For a typical system I would target between 14 and 18 fL as the starting light output from the screen. This will allow for some light loss with time of the product which can be very substantial. The lower the initial light level the shorter the usable lamp life will be. If the projection application is for a room with normal light levels like a bar I would target 30 to 50 fL. Any room that is illuminated to a significant level will reduce the quality of the image dramatically. Some screen materials can help with ambient light, but nothing is better than a dark room with dark furnishings.
There is a common perception that the darker my room is the dimmer I can run the image. That perception is false. For a completely dark room around 12 to 15 fL maximum will give the average person the best image for film reproduction. At normal light levels the human eye operates in the photopic region where we see color normally. When things are very dark the eye cannot see color and operates in scotopic vision. Between photopic and scotopic we are in mesopic vision where we do not see color as well as photopic, but not yet colorless. With a maximum light level around 15 fL images will look bright and vivid in a dark room like they are supposed to when you drop down below 12 fL maximum from the screen things start looking less colorful. In a dark room above 18 fL the light from the screen can start being fatiguing. I have been in many theaters where the projector was insufficient and operated from 3 to 6 fL. These theaters fall far short of those with 12 to 15 fL no matter what the surroundings are.
Projector lumens are a huge factor in the process of screen selection. When you are comparing projectors be sure to find out how many lumens are actually being achieved for this unit in a color accurate mode. This is frequently as little as half of what is advertised. Some projectors offer manual irises which can be very useful in adjusting the light output to match the screen. Lamp power adjustments can also be available. High lamp mode can be a problem with noise level if the machine is mounted close to the seating area. High lamp mode will also shorten the lamp life by as much as 50 percent. The lumens available will also depend on the relative lens zoom used. The closer you are to the maximum throw ratio for a projector’s lens the lower the light output. This can lower the light output another 25 percent. Throw ratio is the ratio of distance from the lens to the width of a 16:9 screen for a 16:9 projector.
The first decision to make on the screen is will it be fixed or retractable. The room will frequently determine this, but you should be aware of the consequences involved with this decision. Retractable screens have more problems with mechanical failures, wrinkles, bugs getting squashed on the screen, deformation of the projection surface and black edging that does not hide the light spill. The edges of retractable screens are not usually as straight as fixed ones resulting in the image not properly fitting into the projection area. Some retractable screens use wire tensioners to help remove wrinkles. Others like Stewart Filmscreen use heavy bars at the bottom to help keep them flat. The larger a retractable screen is the harder it is to keep flat. It is very rare for a screen to perfectly match the image size from a projector. Because of this some light (1 to 3 percent of the image) will either spill onto the black edge or the image will have a gap between the black edge and the projection surface. Many fixed screens use a velvet material that will hide these errors very well if you spill a little of the image on to the frame.
The size of the screen is usually based on a target 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 screen. If you are installing a 2:35 aspect ratio screen a 45 degree horizontal viewing angle is recommended. This is the same as you will find 2/3 of the way back from a quality commercial theatrical screen showing a 2:35 movie. The closer you sit the smaller the surface area will be and the more likely it is a lower cost projector will work well with your screen choice.
Actual screen gain is one of the most important factors in screen selection. A value of 1.0 means the screen will reflect all of the light back to the viewer from the projector. A value of 2.0 means that you will see an image twice as bright as a piece of printer paper would look. This happens because the light from a 1.0 gain surface is reflected uniformly while that from a 2.0 screen is reflected more toward the viewer than the sides. You should also try and find out what the actual screen gain is. Like projectors the actual screen gain is frequently overstated. There are some negatives to using a screen with gain. These include image artifacts like sparkles and grain, uneven image brightness and color errors. Screen gain also will fall off as you move away from the center of the screen. The wider the seating is compared to the screen width the more difficult it will be to take advantage of screen gain. This effect is also stronger the stronger the screen gain is. Screen gain can also help with reflections from the side walls, floor and ceiling washing out the image. If you have a room with light colored furnishings a product like a Stewart Firehawk will help reduce these reflections.
The next decision to make is will the material be acoustically transparent or not. Acoustically transparent surfaces allow you to position the speakers behind the screen. The larger the screen the more difficult it is to locate the front speakers such that they do not obstruct the image and are unobstructed for all of the viewers. This is especially true when multiple rows of seating are used. Generally 2:35 aspect ratio screens over 9 ft in width are where acoustically transparent materials become more important. These materials either have small holes in them or are some type of woven material. The ones with holes are available with higher gain materials and are generally usable at 15 feet or more. The weaves are generally 1.0 gain or less and several are usable at 11 feet or more. Most weaves will impact the sound less than a perforated screen. These materials can also present complex false patterns from interference with the material and the display pixel structure. Some vendors can tilt the product to reduce this problem. Test patterns can be used with screen samples to ensure your system will not have these problems.
Curved screens are an option that is available for 2:35 aspect ratio screens. These can offset some of the distortion from using an anamorphic lens. Curved screens will also reduce the impact of side wall reflections which tend to be more of a problem with 2:35 screens because the proximity of the screen to the sidewalls is closer than most 16:9 systems.
Masking systems can be used to mask off the unused areas of the screen. This helps render the image with crisp dark boarders even when the image aspect ratio does not match the screen. These systems are available motorized in the horizontal and vertical planes. They can also be removable panels to change a 2:35 screen to 16:9.
Additional resources for screen materials can be found on my screen material test report. This calculator from Accupel is a great resource to determine viewing angles, screen luminance and seating height to determine if the front row will obstruct the back row. Links to various quality manufacturers can be found on the left side of this blog as well. There are also many options to make your own screen or masking system.
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