All systems Goo
Published: WORSHIP
Kevin Nute is one of the original creators of Screen Goo projection and optical coatings, and one of the founders of Goo Systems and Goo Systems Global Distribution. He knows Screen Goo better than almost anyone else and has been responsible for the acceptance of the product on a worldwide basis. Here he begins a series of ‘e-seminars’ on the very nature of video projection and the surfaces we use for it…
With flat panel HDTVs continuing to decrease in price, why would anyone buy a projection system for commercial use?
The first reason is simple: size matters. A 60-inch, 16:9 display represents about 11sqft of picture size whereas even a relatively small projected image at 92-inch diagonal is about 25sqft or, if you prefer, better than twice as much picture!
So why is size important? The more believable our sense of being immersed in the picture ‘space’ is, the more engaging the whole experience becomes. The corollary to this is that the more the picture fills our field of vision the more we feel like we’re part of the action. One of the keys to achieving this sensation is to make sure that the image is wide enough so that we have to move our eyes to focus on the far edges of the picture.
This changes how our brain processes visual information; you could say that it forces us to actively watch as opposed to simply seeing. Our eyes are particularly sensitive to motion on the peripheries of our vision and any activity in those areas really gets the old adrenaline going! You would have to sit awfully close to a 60-inch display to trigger this sense of immersion and it would only be available to one, maybe two viewers at a time.
In a commercial setting the benefits of image size are simpler to understand: it’s possible to catch and convey your content to way more eyeballs for the same investment if your image is 50sqft – 130-inch diagonal – or larger.
What is the role of the screen in a projection-based video system?
In simplest terms a screen should provide the most suitable reflective surface for the light produced by a projector. This is straightforward enough until we start to look a bit more deeply at what’s meant by ‘suitable’. Going back to the earliest days of motion pictures, the job of a screen was, in some ways, more difficult but at the same time far more predictable. Early film projectors produced relatively small quantities of light, meaning that movies could only be projected in very dark auditoriums and screens had to return a very high percentage of the light produced by the projectors.
The good news is that these limitations meant that early screen makers knew rather precisely the circumstances their screens were going to be used in. To simplify matters even further, colour wasn’t an issue: all of the content was monochrome. Finally, there were only a handful of projector manufacturers producing a very narrow selection of models. So while the job of a screen was difficult it was largely predictable, and one type of screen worked well in virtually all types of applications.
Well, things have changed! There are dozens of projector brands representing hundreds of models. Projection is used in a huge variety of applications, representing different circumstances and settings and almost every conceivable level of ambient light. But this hasn’t changed the role of the projection screen: it still needs to reflect the content accurately and in a manner that allows for contrast appropriate to the circumstances it finds itself used in.
Why aren’t all screens white?
The short answer is because all projectors aren’t used in rooms with complete light control. Let’s consider two crucial but largely misunderstood aspects of video projection. The first is what’s meant when we say ‘black’. In casual conversation, and in many fashion circles, black is often referred to as a colour. Technically, this is incorrect: black is more properly defined as the absence of light and colour. In video terms black is literally nothing.
If we look at the input signal of a video in electrical terms, what we see in the areas of the signal representing black in the image is: nothing! And this is what the projector is being asked to produce when the image calls for black: nothing; no light and no colour.
This brings us to our second common misperception: that projectors ‘produce’ black. We’ve all seen projector reviews and promotional literature where a projector is referred to as producing “excellent blacks”. As we’ve seen with our definition of black, this claim just isn’t accurate. All a projector can really do is maintain black; it can’t produce it.
Projectors have been getting better at controlling stray light, which has improved their measured contrast ratio specifications and their image quality. Yet using a projector with a stratospheric contrast ratio spec does not mean that the user can expect to see believable black level reproduction in all circumstances. Irrespective of whether a manufacturer claims a contrast ratio of 1000:1 or 100,000:1, and recalling that black is the absence of light, any light falling on the screen compromises the maintenance of true black.
Consider this: imagine yourself in a reasonably well lit room. You are looking at a white projection screen. It appears white to you, the observer, because it’s reflecting the white components of the light in the room.
Stick with me here for a brief digression: if you were to gradually lower the levels of light in the room to complete darkness, the observer’s perception of the white screen’s colour would move through progressively darker shades of grey until, when the room reached complete darkness, the screen becomes invisible or ‘black’. Fire up a projector under these circumstances, absorb the light from the projector being reflected from the screen with dark, non-reflective walls and ceiling and you can expect to perceive a reasonable sense of black in the projected image. However, bring the lights back up with the projector off. The screen starts to look white again.
Well, given that we’ve established that projectors can’t produce black, it’s reasonable to assume that they also can’t ‘throw’ black across the room. What this means is that the perceived lightness or darkness of the screen is in fact the actual black level of the projected image. There is no possible way for the projector to throw black across a room and ‘darken’ a white screen. So what do you do if you want a reasonable approximation of black and, therefore, decent contrast in a projected image in a room with ambient light?
Look to the screen for help: a grey screen starts off darker than a white screen and, assuming it is colour correct (more on this next month), in terms of maintaining black is functionally the same as dimming the lights. So the answer to the question ‘why aren’t all screens white’ is ‘because not all rooms are fully dark’.
More from Kevin Nute in next month’s newsletter