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 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. This in view 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 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.  

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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 that 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 here that display brightness is in itself an entirely subjective term - it is all 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:

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. It is true that TV is a totally different technology, but this target level for image brightness or luminance, still holds good for most circumstances.

Thus, if one is viewing an image in typical 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 required 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 what will happen with different projector screen gains for say a given 1000 ANSI-Lumens projector and a 100" diagonal screen size.

From the above figures, it is 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.

The above goes to show that if you want to enjoy the best results from your video projector screen set-up, you cannot proceed with a projector screen purchase in isolation from your projector; nor it is possible to purchase a video projector and a projector screen without taking into account the ambient conditions in your home theater.