The Stewart FireHawk Projection Screen: How it Works

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White Projection Screens vs. the Stewart FireHawk

Judging from many of the calls we receive, there seems to be much confusion about when to use a grey “high contrast” screen vs. when to use a white screen. To add to the confusion, many projector manufacturers are recommending white screens in their instruction manuals, claiming that the contrast levels are so good in current projectors that grey “high contrast” screens are no longer necessary. But what the projector manufacturers do not address is the dramatic effect room lighting or having light colored walls, ceiling or furnishings can have on a front projection system. They seem to be basing their recommendation on the assumption that their projector is going to be placed in a completely darkened dedicated theater room – or as we call it, a “cave” with black walls, ceiling, and furnishings. After five years of helping people design their front projection home theaters, we can say with some authority that is absolutely the exception rather than the rule. Almost every single customer we talk to is trying to incorporate a front projection system into a family or entertainment room rather than a dedicated home theater. Not only do they want to occasionally run the system with the lights up, most of the time the rooms also have light colored walls, ceilings and furnishings.

Choosing the right screen material for both the projector and the room you intend to put it in can be just as important as the projector choice itself. “Light control” as it pertains to front projection does not just mean being able to pull down the shades and turn off the lights. A truly light controlled room is one where not only is there an absence of light, but one where the walls, ceiling, and furnishings are a dark color (optimally flat black, but in the real world that almost never happens). Putting any projector on a white screen in a room with white walls and ceiling will cause the picture to wash out almost as much as a room with lights on or window shades open. What happens is that the light from the projector bounces off the screen, then off the side walls, ceiling and floor, and then back on to the screen to wash out the image. This is easily demonstrated – next time you are in a totally darkened home theater with light colored walls, notice how much the walls glow and are lit up by the light reflected off the screen. All of this “light pollution” comes back to wash out the image on the screen and destroy contrast and black levels.

Since contrast is one of the prime determiners of what defines a good picture vs. a bad one, figuring out a way to maintain contrast is essential to getting all of the performance you paid for when you bought your new “high contrast” front projection system. Contrast – simply put – is the relationship between the depth of black in the image vs. the brightness of the white. The greater the difference between black and white, the greater the contrast ratio, and the richer the image. Systems with high contrast ratios display far greater image depth and three dimensionality, plus the picture just seems to “pop” off the screen. The problem in front projection systems is that black is determined by the absence of light on the screen. When there are lights up in the room or when washout from light colored walls reflects back onto the screen, the blacks get destroyed. When that happens, you also greatly reduce the contrast ratio of the image, no matter what the stated contrast ratio of the projector happens to be.

This is why the Stewart FireHawk was created, and to some degree, other grey screens as well. A properly designed grey screen can help overcome room and lighting issues, as you will see below. Unfortunately, there seems to be a common perception out there that grey screens “dull” the image. This is not necessarily true. A neutral grey screen is just as capable of displaying vivid colors as a white screen. In fact, the only difference between black, white, and grey is brightness. They are all the same “color” (or lack of such). In the screen industry, the overall brightness of the image on the screen – in other words, the amount of light reflected – is referred to as gain, with a reference point of 1.0 (neutral) gain. Where the FireHawk differs from most other grey screens is that most other greys have a negative gain (less than 1.0), thereby dimming (but not dulling) the image compared to a neutral 1.0 gain white screen. Negative gain screens absorb light, while high gain screens reflect it. The difference here is that the FireHawk happens to be a high gain grey – in fact, it has the very same gain as the reference screen for the industry, the Stewart StudioTek 130. The gain of both screens is 1.3 (or 30% higher than the 1.0 gain, neutral reference). The Stewart FireHawk has a negative gain grey backing but has a high gain coating placed on top to get the overall gain up to 1.3.

(Another important fact to keep in mind pertaining to the gain of a screen. Screens with a gain of 1.0 or less tend to reflect light evenly in all directions. As the gain of the screen rises over 1.0, the more the light is focused back along the path it came from. The higher the gain, the narrower the beam of light being reflected back to the seating area. This is important to the explanation that follows)

I personally used to believe that the FireHawk used the grey backing to deepen blacks while the high gain coating boosted whites, giving you an overall contrast boost.

This is NOT true.

The grey backing is used to absorb reflections, NOT to deepen blacks. As mentioned above, the gain on the FireHawk G3 and the Studiotek 130 are both 1.3. If they have the same gain characteristics, they should produce an almost identical picture in terms of brightness, contrast and black level. Yet when you compare samples of both materials in most rooms, obviously they do not. The FireHawk will almost always produce deeper blacks, which most people attribute to the grey backing lowering the light level thereby deepening the blacks. But this is not how it works. And that can be confirmed by a simple test, provided you have one of those "perfect theater rooms" with dark walls, ceiling, furnishings, etc.

When you place a sample of FireHawk and Studiotek 130 side by side in a TOTALLY light controlled room (with dark walls, ceiling, carpet, furnishings, etc) and sit in your theater's sweet spot, they are almost indistinguishable. The black levels are almost exactly the same! Because you have eliminated light reflections from the equation, the StudioTek is now reproducing the same deep level of black of the FireHawk. If the grey backing of the FireHawk is what deepened the blacks, then the FireHawk should still have deeper blacks than the Studiotek even in a totally darkened room. But it does not.

This is because the FireHawk uses the grey backing to absorb side and ceiling room reflections, while the high gain coating on top of the grey backing focuses more light back to the viewing area, and much less light onto the side walls and ceiling. The StudioTek, on the other hand, reflects light in an almost 180 degree pattern, thereby spilling much more light onto the side walls and ceiling, which comes back to wash out the blacks. Here is how it breaks out:

StudioTek 130: 1.0 gain matte white backing + .3 gain coating = 1.3 net gain

FireHawk: .5 gain grey backing + .8 gain coating = 1.3 net gain

The higher the gain the narrower the viewing angle, which also translates into less light spilled onto the side walls and ceiling. Since the FireHawk has a .8 gain coating vs. the .3 of the Studiotek, it does not light up the room nearly as much as the Studiotek. That means less reflections and less washout from the walls and ceiling. Now the grey backing absorbs even the limited amount of reflections left over. Since you have the same net gain in the main viewing cone, however, from the main home theater viewing area the brightness and overall picture look the same between the two materials. Of course, if you get more than 30 - 45 degrees off axis, the StudioTek will remain nice and bright while the FireHawk starts to fall off. But the tradeoff is better performance in ambient light, and far less washout from room reflections.

Another way to test this - take a piece of Studiotek and place it on a FireHawk with the projector on. Watch your side walls - they will light up MUCH more than when you did not have the Studiotek sample there. Again, this illustrates how the Studiotek, despite having the same overall gain characteristics, spills light much more dramatically onto the walls and ceiling where the FireHawk does not.

So, when do you choose a white screen like the StudioTek 130 over the FireHawk? When you DO have a dedicated, darkened theater room with dark colored walls, ceiling, and furnishings. A white screen in that situation will give you a wider viewing angle and less hotspotting than a high gain grey like the FireHawk (hotspotting is when the middle of the image is brighter than the sides, a phenomenon very obvious with most rear projection systems). On the other hand, even if you do have a totally darkened theater room yet want to watch much of the time with the lights up, the FireHawk may be the better choice.