Myth 1: Moving the projector closer to the screen will brighten the projected image.
Frankly, the idea that the distance between projector and screen will influence brightness makes quite a bit of sense. Most people will be familiar enough with a candle, flashlight or torch to know that there is a certain range where the illumination from those sources is most useful. All of them are more than adequate for the close work of navigating an otherwise dark room or staying on a path through the forest, yet they generally fail to light up distant objects very effectively.

Projectors give off light much like flashlights do, so it stands to reason that they would follow a similar pattern. However, there is a critical difference that contradicts this expectation. To preface this difference, I think it is worthwhile to look at the way light is measured.

The diagram below shows a flashlight with a familiar looking cone of light being emitted from it. The top and bottom of the beam are obviously not parallel and we can say that the light is coming out at a measurable angle. Since the light is emitted in a three dimensional, conical shape, rather than the two dimensional wedge shown in the illustration, the angle we measure will describe the edges of the beam all around the cone, not just the top and bottom.

This 3D angle is what is known as a solid angle. When we shine this sold angle of light onto an object, it will illuminate a roundish area on it. If we were to hold the light inside a 1 ft radius sphere and light up a 1 ft² area while the light is originating from the center of the sphere, then we could say that we are shining a unit solid angle (in SI units: 1 Steradian).

The amount of light energy that is being emitted inside that unit solid angle is measured in a unit called “lumens”. An increase in energy will mean an increase in lumens and we will see this as a brighter light. Alternatively, leaving the lumen output constant and enlarging the sphere will decrease the brightness of any 1 ft² relative to what it was at the smaller size.

To find the reasons for this, let’s leave the sphere and return to the flashlight in Figure 1. Line segments a and b signify two planes onto which the beam of light might shine. At a, the diameter of the beam is 2 inches, and at b, the diameter is 3 inches. Figure 2 shows how this minor increase in diameter will more than double the surface area that the beam covers. The same number of lumens in a will be essentially spread out more thinly on a larger area in b. As you might suspect, the brightness of each square inch at b will be reduced because of this.

Another way to look at this is to leave line a at 2 inches high and move it towards b. It would grow dimmer to match b’s brightness as it approached b’s position. This is so because as a moves away from the light, each 1 in² occupies less and less of the total area of the beam and so uses fewer and fewer of the available lumens. Moving the line back towards the light will cause it to regain its brightness.

The careful reader will now recall that the original question was whether or not moving the projector closer to the screen will increase the brightness and it would seem that we have just confirmed that it will. At last, however, we come to the important difference that separates a projector and a flashlight.

To compensate for changes in distance between projector and screen, the projector is designed to alter the sizeof the beam it emits and focus the same number of lumens into a wider or narrower solid angle. This can allow an identical number of lumens to fall on a 1 ft² area from a distance of 1 ft as on 1 ft² from 10 ft. It is still possible for the projector to act more like a flashlight and give a brighter image at a shorter distance, but only if the image is allowed to decrease in size along with the distance.

As soon as the lens is adjusted and the image returns to its original size, it will be almost exactly as bright as it was before. All other things being equal, the important factor for overall brightness, in other words, is the size of the image on the screen.

Myth 2: Light sources between the projector and screen will directly interfere with the projected light
Instead of light, think of the projector as sending out a jet of water in roughly the same conical shape as the beam of light it actually does emit. For the sake of the analogy, imagine that the water pressure is sufficient that the water reaches a screen several feet away without succumbing to gravity. Between the projector and the screen, add in some light fixtures which will act like waterfalls, each dumping a dense volume of water directly through the projector’s stream.

Needless to say, the projector would have to send water out with enough force to penetrate the waterfalls or else there would be little more than a fine mist reaching the screen. If we went further and added colored dye to the projector to simulate the different colored wavelengths in the light beam, there would surely be some dye lost amid the falling water on its way to the screen. Does a real projector need to be bright enough to pierce through the lights in the room? Does mixing the projected light with the other lights dilute the color saturation of the image?

Well, not really. Ambient light absolutely can compete with the projected light in ways that could suggest that the kind of interference described above is occurring. This does not, however, take place when the streams are crossed.

The color dilution question should be easy enough to demonstrate by using two flashlights, a colored filter and a dark room. By shining one light through the filter, note the color of the light where it hits a solid surface (a screen, perhaps). With the second flashlight, intercept the red beam at an angle near perpendicular so that the second light is not falling in the same place as the red light (the wall will suffice). So arranged, these lights should effectively show that whether the white light is on or off it will have little bearing on the color of the red beam.

Admittedly, it is possible for there to be some desaturation of the colored beam in this experiment but this leads us back to the acknowledgement that ambient light does have an impact. The important distinction here is that the impact is indirect. When the white light shines through the red light, both beams continue on without direct interaction. However, the white light will eventually reach a solid object and when it does, some of that light will be absorbed, reflected and diffused. The light that is not absorbed will travel away from the object and some of it could possibly fall on the same area onto which the red beam is shining. When that happens, the white light is essentially added to the red light and what reaches your eyes is a combination of these, giving the appearance of a somewhat diluted red light.

This is the real justification for wanting to control ambient light. The issue is not that it blocks or washes out the light from the projector directly but that it finds its way onto the screen. Keeping these additional lights under control is, therefore, important and the source of another myth.

Myth 3: A completely light-controlled environment is a completely darkened environment.
It is generally the case that an illuminated display will look better in the absence of competing sources of illumination. As mentioned in the previous section, any light that falls on the screen and makes its way back to the eyes of a viewer will degrade the image to some extent.

The safest thing to do, therefore, would be to avoid this possibility by eliminating all light sources other than the projector. This does not actually eliminate the issue and, of course, there are situations where doing this is simply not practical. Some light may be needed for safety or note taking or simply to be able to see who is speaking in a lecture hall or meeting room.

Fortunately, it is possible to leave these lights on and still maintain a light-controlled environment. The key here is to keep the light from shining on the screen directly and to keep the intensity of the light low enough that very little is shining on the screen indirectly.

Restricting direct light simply means ensuring that whatever light sources are lit inside the room do not send rays at the screen. This can generally be accomplished by covering the windows and by not installing lights close to the front of the screen. Placing them several feet away from the screen, to the sides or, when possible, even behind the screen may be perfectly acceptable. The goal here is to keep the screen surface dark when the projector is off.

The indirect light is a bit trickier to control and, frankly, it is virtually impossible to stop it entirely. There is, after all, a large screen bouncing light from a projector back towards the audience and that light will continue to bounce off of other objects in the room. Again, we don’t need to stop it; we just need to control it. In most cases, dark colored walls and surfaces that are not reflective or glossy will help minimize these secondary reflections.

As far as the screen itself is concerned, adding contrast will work to negate some of ambient light’s effects. The darker surface material will appear blacker than a pure white screen will in environments where some additional light is reaching the screen. Another approach is to use retro-reflective materials for their ability to reflect light back towards the source of illumination. When those sources are the walls or table tops, the majority of the light those objects reflect will be returned to them and not to the audience.

So long as the majority of the light the audience sees is coming from the projector and it is possible to allow only a minimal amount to come from other sources, we can say that the ambient light is controlled.

We can also say that one more AV myth has fallen.