Understanding Projector Screens

One important criteria we highlighted in our guide to projection screen surfaces is image brightness.

Why? To-day's powerful front video projectors may leave you with a too bright an image, one that may prove stressful on the 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 an extended period of time.

And this is not just an eye safety requirement - equally important is that 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 in order to arrive at a suitable value for the screen gain, this to ensure a correct level of image brightness - one that is both comfortable for extended viewing and safe on your eyes.

A Tricky Matching Process!

Ambient Light Conditions, Image Brightness & Screen Gain 

A projected image is at its best when viewed in total darkness - this helps produce the deepest blacks. And viewing in a totally darkened environment also helps take away any visual distractions - 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 a dedicated home theater room setup. The amount of ambient light present will have an impact on 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 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 relation that exists between these three parameters.

What is Screen Gain

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.

Gain is a measurement of the reflectivity of a screen surface. It represents a ratio of the light reflected from the screen as compared to the light reflected from a reference standard white (a flat expanse of magnesium oxide) board. Therefore, a screen with a gain of 1.0 will reflect the same amount of light as that from our reference white board. There is only one such surface that has a gain of 1 and this is the most versatile Matte White projector screen.

This means that a projector screen surface rated with say a 1.8 gain will reflect 80% more light as that 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.

Screen gain is measured from the point where the screen is at its brightest, which is directly in front and perpendicular to the screen i.e. at zero degrees viewing axis. This is known as the peak gain. 

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 - which 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 in itself a passive optical surface?

A high gain screen is constructed to reflect 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 while 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 - which 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 screen with a gain of one will diffuse light evenly in all directions.

Display Brightness and Screen Gain

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 - more specifically the Luminance level for a projected image (which in itself is a measure of the light reflected from the projector screen area), is defined as follows:

To arrive at the actual level of light reflected from the screen surface, the above result must be multiplied by the projector screen gain.

A guiding note here is a reference to what has been defined as 'sufficient brightness' by the Society of Motion Pictures and Television Engineers, also known as SMPTE. In standard 196M, the SMPTE determined sufficient brightness for showing motion pictures in a darkened room using a unity gain matte white projector screen, as 12 - 22 foot-Lamberts. In practice, the luminance target level is set to about 16 foot-Lamberts, while the average bright scene brightness level in a movie theater is typically 60% to 75% of this target value.

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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.

The following rules of thumb would surely come to some assistance in this respect:

Thus, if one is viewing an image in typical low ambient light conditions (say under 2 foot-candles - equivalent to approximately 21.53 Lux), applying the first rule of thumb would imply that the minimum required image brightness illuminating the screen surface should be around 10 to 14 foot-candles.

For a matte white projector screen surface with a screen gain of one, this would translate to a minimum of 10 to 14 foot-Lamberts in terms of light reflected from the screen for image brightness. This is close to the SMPTE typical requirement for average 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 8 - 10 foot-candles or 85 - 110 Lux), applying our first rule of thumb to achieve the required minimum contrast level would result in a required minimum luminance of 50 to 70 foot-Lamberts (approx. 540 -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.

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Lux, Luminance, Foot-Candles and Foot-Lamberts - Are things getting confusing?

	Light illuminating a surface is measured in Lux (Lux = lumens/sq. m), or foot-candles (lumens/sq. ft); 1 foot-candle = 10.764 Lux
	Light reflected from an area (luminance) is measured in candelas/m2 (cd/m2) or foot-lamberts (fL).

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To get a better understanding of the relationship between these different parameters, and in particular, the need to choose a projector screen with the appropriate screen gain, let's see what will happen with different projector screen gains with a 1000 ANSI-Lumens projector and a 30 sq. ft screen (approx. 100" diagonal 16:9 screen.)

From the above figures, it is clear that our 1000 ANSI-Lumens projector would produce an image that is a bit too bright to view in total darkness over a standard matte white projector screen with a screen gain of one.

On the other hand, use of this 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.

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 conditions in your home theater.

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