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Title: Understanding Projector Screens: Be Careful, You May End Up With TOO Bright An Post by: Daniel Franklin on October 16, 2007, 05:05:01 PM Today'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.
Defining ‘Screen Gain’ Prior to proceeding with our discussion, 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. Reflective screen surfaces have gains greater than 1; these projection 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: 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 projected image brightness. This is an extremely important issue. 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 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. Please keep in mind however that the resultant projected image brightness is a different issue. 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, is a measure of the light reflected from the projector screen area. It is measured in foot-Lamberts, and is defined as: ANSI-lumens of your projector divided by the square footage of Screen. (Note that 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. Display brightness is in itself an entirely subjective term; what’s more, 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: 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. 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. How does all this translate in practical terms? If one is viewing an image in typical very low ambient light conditions (say less than 2 foot-candles - equivalent to approximately 22 Lux), then the minimum required light level ‘illuminating’ the screen surface should be around 10 foot-candles to achieve sufficient image contrast. For a matte white projector screen surface with a screen gain of one, this translates to a minimum of 10 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. Similarly, if viewing were to take place under normal ambient light room conditions (typically 10 foot-candles or 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. Note: 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). Now, let’s 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 clear that our 1000 ANSI-Lumens projector would produce a too bright an image when the latter is viewed in a darkened room, even when using a standard matte white projector screen with unity gain. Similarly, use of this projector in conjunction with a projector screen surface having a gain of 2 may result in a too bright an image even when viewing takes place under normal ambient light conditions. Andrew Ghigo – A Telecoms/Electronics engineer by profession, with specialization in digital switching and telecoms fraud management systems. Editor and publisher of http://www.practical-home-theater-guide.com - a site dedicated to all home theater enthusiasts with the scope of serving as a comprehensive home theater guide to home theater systems, product reviews and home theater design. This article is an excerpt from a series of guides appearing under the home theater screens section of the site. |