Guide to Projector Screens
Projector Screen Gain, Ambient Light and Projector Output
Are these related, and if so how?
With today's compact yet high-output video projectors, there is a real risk that one ends up with a too bright an image in the home theater. Some may say that's OK but a bright image is tiring on the eyes, and is definitely not what you want in the darkened environment of the home theater.
The reality is that you cannot simply go and purchase a projection screen in isolation from your video projector, nor is it possible to buy a video projector and an accompanying home theater screen without taking into account the ambient light conditions in your room.
In this guide to projector screens, we discuss the importance of matching the projection screen gain with the projector output. We also explain how ambient light factors in the overall image projection equation; this is critical if you really want to enjoy the best image projections in your home theater.
One important criterion we highlighted in our guide to projection screen surfaces is projected image brightness. The implication here is that one cannot proceed with a projection screen purchase in isolation from your projector or ambient light levels.
Why? To-day's powerful front video projectors may leave you with a too bright an image, one that may prove stressful on your eyes especially in the darkened environment of the home theater.
Matching the projection screen gain with the video projector output is essential if you want to ensure you end up with a correct level of brightness for your projected image―one that would be safe to view for extended periods of time.
Yet, there is more to this than just a safety issue; a comfortable environment is a pre-requisite to a great movie experience. This means that the time one spends watching a movie should be as comfortable as possible. Exposure to a bright projected image is uncomfortable on the eyes, even when this is for just a few minutes, least imagine when watching a two-hour movie.
In this guide to home theater projection screens, we discuss image brightness and screen gain. We show you how to take into account the projector output and the ambient light conditions to arrive at a suitable value for the screen gain, one that ensures a correct level of image brightness that is both safe on your eyes and comfortable for extended viewing.
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A Tricky Matching Process!
Ambient Light Conditions, Image Brightness & Screen Gain
A projected image is at its best when viewed in total darkness as this helps produce the deepest blacks. Viewing in a totally darkened environment also helps take away any visual distractions, thus rendering it easier for the viewer to get better immersed into the movie action.
In the home, achieving a completely darkened room is almost impossible except in the case of the dedicated home theater room. The amount of ambient light present impacts the resultant projected image contrast. This calls for the need to increase the image brightness to maintain the correct level of image contrast.
The tricky issue is to arrive at the correct level of image brightness for a pre-set level of ambient light. The reason is that the projected image brightness is a function not only of the projector output, but also of the projector screen gain, and projected image size.
The explanation that follows should help you get a better understanding of the relationship that exists between these three parameters.
For the rest of this discussion to make sense, it is important that first you have an understanding of what we mean by projector screen gain.
Screen gain is a measurement of the reflectivity of a screen surface. It is the ratio of light reflected from the screen surface as compared to the light reflected from a reference standard white board; the reference used here is that of a flat expanse of magnesium oxide.
A screen with a gain of 1.0 will reflect the same amount of light as that from a reference white board. There is only one such surface that has a gain of 1; this is the most versatile 'Matte White' projector screen surface.
Instead, a projector screen surface with say a 1.8 gain will reflect 80% more light as that reflected from a white board. In a similar manner, a gray surface with a screen gain less than 1 will reflect less light than that reflected by the same reference white board.
The value given for screen gain by projector screen manufacturers refers to a very specific point on the projection screen. Screen gain is measured from the point where the projected image on the screen is at its brightest; this corresponds to a point directly in front and perpendicular to the screen i.e. at zero degrees viewing axis. This is known as the peak gain.
Screen gain varies with the viewing angle. A typical screen gain chart would look like the one shown below. The chart here refers to Draper's HiDef Grey projection screen surface; it indicates an almost flat response that is very close to that of a matte white surface, with a peak screen gain of 0.9 going down to around 0.7 at a viewing angle of 70-degrees.
An almost flat response for the projector screen gain is important as it helps ensure that the projected image brightness level would practically appear the same:
Irrespective of how close or far way the seating position is from the center of the screen.
Across the entire image width when viewed from the same seating position. This is important as light rays from the projector reaching the extreme ends of the projected image have a smaller angle of incidence, leading to an image that would appear dimmer at the far ends when dealing with very wide image projections over high-gain projector screen surfaces.
The point at which projected image brightness falls to 50% peak is known as the half-gain viewing angle. A person viewing the screen from this angle will see an image half as bright as the person seated at the center position. This also implies that a screen with a gain other than 1 will always exhibit some hot spotting; this becomes more visible with gains in excess of 1.3.
How is it possible that we talk of screen gain when the projector screen is just a passive optical surface?
A high gain projector screen surface reflects more of the projector light energy back towards the centerline of the projection path and less light as one moves away from the zero viewing position. Thus, while we get increased brightness at the zero viewing position, brightness falls off more rapidly and screen gain falls to a level less than 1, as one moves away from the projection path centerline. In other words, if you were to sum up the reflected energy levels from the projection screen surface over a full 180 degrees viewing angle, you would find that there is no gain in reflected overall light energy.
As a result, high gain screens always have a relatively narrower half-gain viewing angle; this viewing angle gets narrower with increased gain. Low-gain screens have a wider half-gain viewing angle because low gain screens diffuse light more evenly over a wider angle of view. Instead, a projection screen with a gain of one diffuses light evenly in all directions.
The brightness output level of your projector is a measure of the projector output power in terms of light intensity. Yet, it is important to realize that the end projected image brightness is a different issue as the image brightness falls in proportion to the area of the projected image size. The bigger the projected image is, the dimmer it will look for a fixed level of projector brightness.
The projected image brightness, or Luminance level (which in itself is a measure of the light reflected from the projector screen area), is defined as follows:
ANSI-lumens of your projector
Square footage of Screen
Image brightness in foot-Lamberts
Where one foot-Lambert is equal to 1 ANSI lumen per square foot.
To arrive at the actual level of light reflected from the screen surface, the above result must be multiplied by the projector screen gain.
The ultimate aim in a video projection setup is to ensure a sufficient level of image brightness, more specifically known as screen luminance.
The Society of Motion Pictures and Television Engineers, also known as SMPTE, in standard SMPTE 196M-2003, indicated that screen luminance in cinema conditions (equivalent to a completely darkened room) using a unity gain matte white projector screen surface, should be a minimum of 12 foot-Lamberts and a maximum of 22, with a nominal 16 foot-Lamberts at the screen center; it also indicates that the minimum value of screen luminance at the corners and sides of the screen should not be less than 10 foot-Lamberts.
It is important to keep in mind that display brightness is in itself an entirely subjective term and is relative to the amount of ambient light falling on the projector screen surface.
Thus, if one is viewing an image in typical very low ambient light conditions (say under 2 foot-candles - equivalent to approximately 21.53 Lux), applying the first rule of thumb would lead to a minimum required image brightness illuminating the screen surface of 14 foot-candles.
For a matte white projector screen surface with a screen gain of one, this would translate to a minimum of 14 foot-Lamberts in terms of light reflected from the screen for image brightness. This is close to the SMPTE requirement for screen luminance when viewing pictures in a darkened room.
On the other hand, if viewing were to take place under normal ambient light room conditions (typically 10 foot-candles or 110 Lux), applying the first rule of thumb to achieve the required minimum contrast level would result in a required minimum luminance of 70 foot-Lamberts (approx. 750 Lux) for the same matte white projector screen surface. This is also in line with the target luminance level detailed in rule 2 for viewing under normal ambient light conditions.
Lux, Luminance, Foot-Candles and Foot-Lamberts
Are things getting confusing?
Light reflected from an area (luminance) is measured in candelas/m2 (cd/m2) or foot-lamberts (fL).
This is a somewhat heavy technical discussion; we therefore thought that the best way to conclude this article is by applying the principles discussed in this article in a typical home theater application. This should help you get a better understanding of the relationship between these different parameters, and in particular, of the need to choose a projector screen with the appropriate screen gain.
Let's see what happens when a projector of say 1000 ANSI-Lumens is set to project an image over a 100-inch diagonal (30 sq. ft) 16:9 projector screen, with varying screen gains:
|Projector Screen Gain||1||1.5||2|
|Image Luminance in foot Lamberts||33.3||50||67|
From the above figures, it is clear that a 1000 ANSI-Lumens projector would produce an image that is a bit too bright to view in total darkness if we want to remain within the 12 - 22 foot-Lamberts recommended by the SMPTE standard, when the projection screen has a standard matte white surface with a screen gain of one.
Use of this same projector in conjunction with a projector screen having a screen gain of 1.5 would result in a sufficiently bright 100-inch diagonal image when projected in a room under normal ambient light conditions.
On the other hand, this 1000 ANSI Lumens video projector would produce an image that is a bit too bright for viewing under normal ambient light conditions with a screen gain of 2.
The above goes to show that if you want to enjoy the best results from your front projection set-up, you cannot proceed with a projector screen purchase in isolation from your projector. In a similar manner, it is not possible to purchase a video projector and a projector screen without taking into account the ambient light conditions in your home theater.
This discussion would not be complete without mentioning something on 3D image projections - more specifically how the conclusions arrived in this article can be applied to 3D.
The fact that 3D projections make use of active 3D shutter glasses technology means that at any instant, the eye would see practically half the image brightness. The requirement here is to increase the projector output by approximate 60% to 70% over standard 2D viewing requirements for appropriate image brightness levels. This should lead to an adequate projected image brightness when watching 3D
We do not go at twice the amount for projector output as the eye sensitivity function is logarithmic in nature and an increase of around 60% should suffice.