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Home Page - Home Theater Projector Screens - Projection Screens and Video Projector Output

Date: 28thJanuary 2010





A Quick Guide to Home Theater Design

How to research, design, and build your Home Theater.

Home Theater Design e-book

by Duncan McClelland
More information HERE

 
Accuscreen 800019 106-Inch HDTV Fixed Wall Sound Screen

Draper Accuscreen 800019
106-Inch Fixed Wall Sound Screen

An affordable acoustically transparent screen for the home theater 

Selecting your Front Projector Screen
Be careful, you may end up with TOO bright an image!

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:

=

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 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.


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:

Blue bullet

The ratio between the projected image brightness and the level of ambient light falling on the screen should be at least 5:1 - preferably 7:1 for color images.

Studies show that this minimum contrast is necessary for the eyes to perceive a real impression of brightness.
 

Blue bullet

When viewing takes place under normal ambient light conditions, the luminance level should be close to 50 foot-Lamberts, with 40 foot-Lamberts being considered comfortable in a dimly lit room.

This level of luminance is derived from an SMPTE recommendation that indicates a target level of image brightness of 50 ft-lamberts for CRT TVs when viewing under normal ambient light. It is true that a TV is a totally different technology, but this level for image brightness or luminance, still holds for most circumstances.

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.


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

Blue bullet Light illuminating a surface is measured in Lux (Lux = lumens/sq. m), or foot-candles (lumens/sq. ft); 1 foot-candle = 10.764 Lux
Blue bullet

Light reflected from an area (luminance) is measured in candelas/m2 (cd/m2) or foot-lamberts (fL).


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.)

Projector Screen Gain 1 1.5 2
Image Luminance in foot Lamberts 33.3 50 67

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.


Looking for an affordable Projector Screen?

Amazon offers a vast choice of projection screens - often at significantly reduced pricing. In addition, the various buying options available from the different retailers present on the amazon storefront make amazon one of the most comprehensive online sources in this respect. 

However, prior to any decision, we advise to do some research of your own. Look at what other customers had to say about their projection screen purchase especially with respect to product quality and manufacturer customer support. A good starting point is the customer feedback posted on amazon. 

You can search the amazon storefront without leaving our site by using the amazon search box below; your search results will appear here under.

 

 

 

Projection Screen Guides

Other articles under this section 

Selecting a home theater projection screen
A step-by-step guide to selecting the correct projection screen setup.

Home Theater Projector Screen Fabrics
Discussing the different projection screen fabrics and the criteria to take into account when selecting a projection screen surface.

Home Theater Screens Product Reviews:

Brands reviewed under this section include...

Da-Lite Screens

Draper Projection Screens

AccuScreens for the Home Theater on a Budget

Elite Screens

Stewart FilmScreen

Goo Systems Projection screen paints

 Home Theater Guides:

Recommended Technical Guides 

Projection TV - the center of attraction in the home theater
In search of BIG fun; look to the big screens of Projection TV. A complete guide to front and rear projection systems, and DIY projection television.

The LCD TV Guide:
LCDs are turning out sharper, bigger, better, and cheaper. Discover all you need to know about LCD TV sets in this series of articles on LCD TV.

Plasma Television
A complete guide to plasma TVs - from the features to look for when purchasing a plasma television, to installing and protecting your plasma TV investment.

Guide to Digital Satellite TV
Covering satellite TV basics - with a special emphasis on satellite TV decoder choice and satellite dish installation.

The HoloVision Rak Master Pull-out/Swivel AV RacksEquipment Racks:
There is more to equipment racks than a storage space for your system. More in this informative home theater guide.

Feel the Shake!
Bass Shakers, Buttkickers... Add the sensation of tactile sound and discover a whole new world in home entertainment.

Contrast Ratio - Playing with Numbers!
To what extent high contrast ratios affect the performance of a home theater projector or a video display device?

TV Viewing Distance
Buying a big screen TV? Care should be taken as you may easily fall into the trap of buying a too large TV.

1080p HDTV - What you need to know
Discover the strengths and weaknesses of the different HD formats - 720p, 1080i, and 1080p - in this HDTV Guide.

Dolby vs. DTS - Which delivers better sound?
We discuss the differences between these surround sound formats to help you understand why this Dolby vs. DTS controversy prevails.

Home Theater Design:
A series of articles covering all aspects of a home theater from design to realization.

Featured Home Theater Screens

Check out the latest reduced pricing on some of the most popular projector screens at amazon


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