Be careful, you may end up with TOO bright an image! An excerpt from a series of articles published in Practical Home Theater Guide.

Understanding Projector Screens

Andrew Ghigo | Practical Home Theater Guide

by Andrew Ghigo, Practical Home Theater Guide

Be careful, you may end up with TOO bright an image!

An excerpt from a series of articles published in
Practical Home Theater Guide.

A projection screen cannot be taken in isolation from the rest of the environment in which it will perform - matching the projection screen and in particular the screen gain with the ambient light and the projector output is critical if you want to enjoy the best results.

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, to ensure a correct level of image brightness.

Defining 'Screen Gain'

Prior to proceeding with our discussion on this subject, it is important that one has a full understanding of what the term 'screen gain' is all about. It may sound strange that we talk about 'gain' when in reality a projector screen is nothing more than a passive device. Yet there is - it is all a question of how the screen surface distributes the reflected light.

Screen gain is a measurement of the reflectivity of the screen surface. It measures the ability of the screen to direct incident light back to the audience.

The gain number represents the ratio between incident light and reflected light. A flat matte white surface has a gain of approximately 1. A gray screen has a gain less than 1 and therefore, it attenuates incident light. On the other hand, reflective screens have gains greater than 1; these screens direct more incident light back to the audience.

Gain is always measured in front and perpendicular to the screen at the point of maximum brightness; this represents peak screen gain and occurs at the zero viewing angle perpendicular to the screen surface.

For a screen with gain greater than one, the gain drops as one moves to the side and view the screen at an angle to the perpendicular drawn from the center of the screen. The angle at which screen gain falls to half the peak is referred to as 'half gain viewing angle'. A person viewing the screen from this angle will see the image half as bright as the person seated at the center. The more one moves further away from the center, the dimmer the projected image will be.

The higher the peak screen gain, the narrower the supported viewing angle. This is the price one has to pay for a higher screen gain - the higher projector screen gain at the center is in fact achieved by directing more light towards the center viewing positions rather than allowing for a uniform reflected light over a wider angle of view.

Projected Image Brightness:

To-day's compact yet powerful front video projectors may leave you with a too bright an image to watch! This being especially so if you intend to watch a projected image in a darkened room - which after all, is most often the case with a home theater setup.

Matching the projection screen gain with the ambient light and the video projector output is essential if you want to ensure that you will end up with a correct level of brightness for your projected image.

This is an extremely important issue; this has to be taken into account when selecting a projection screen. Do not forget that a basic requirement for a great movie experience is a comfortable environment. 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.

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 take away any visual distractions - rendering it easier for the viewer to get deeper 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 setup. The amount of ambient light present has 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. This in view that projected image brightness is a function not only of the projector output, but also of the screen gain, and image size.

The explanation that follows should help you get a better understanding of the relationship that exists between these three inter-related parameters.

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:

Luminance level = 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.

A useful reference here is 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.

One should note however that display brightness is in itself an entirely subjective term - it is all relative to the amount of ambient light falling on the screen surface.

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

  1. The contrast ratio between the projected image and the ambient light level falling on the screen should be at least 5:1. This is necessary for the eye to perceive a real impression of brightness. If this contrast ratio is not achieved, the projected image will not be considered of adequate brightness level.
  2. When viewing takes place under normal ambient light conditions, the luminance level should be close to 50 foot-Lamberts. This level of luminance is derived from the fact that SMPTE indicated a target level of image brightness for a CRT TV as 50 foot-Lamberts, this in view that a TV is normally viewed under normal ambient light. One may argue that TV is a totally different technology - this is true, but this target level for image brightness or luminance, still holds good for most circumstances.

Thus, if one is viewing an image in typical very low ambient light conditions (say under 2 foot-candles - equivalent to approximately 22 Lux), applying the first rule of thumb would imply that the minimum required image brightness illuminating the screen surface should be around 10 foot-candles.

For a matte white projector screen surface with a screen gain of one, this would translate to a minimum of 10 foot-Lamberts (approx. 110 Lux) 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 contrast level would result in a luminance level of 50 foot-Lamberts (approx. 540 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.

  • Light illuminating a surface is measured in lux (Lx = lumens/m2), or foot-candles.
  • Light reflected from an area (luminance) is measured in candelas/m2 (cd/m2) or foot-lamberts (fL).

To help you 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, lets consider as an example what will happen with different projector screen gains for say a given 1000 ANSI-Lumens projector and a 100" diagonal screen size.

Projector Screen Gain 1 1.5 2
Image Luminance in foot Lamberts 30 45 60

It is therefore clear that our 1000 ANSI-Lumens projector is too bright to view images in total darkness even when using a standard matte white projector screen with a gain of one. Similarly, use of this projector in conjunction with a projector screen having a screen gain of 2, may result in a too bright an image even when viewing takes place under normal ambient light conditions.

More information on home theater projector screens is available at Practical Home Theater Guide. Topics covered include step-by-step guides to selecting projector screen arrays and fabrics; you will also find a number of product reviews for a variety of projector screens and accessories available from major suppliers.

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