[simpits-tech] DLP Projector Image Issue

[Rob Hommel]

Fransisco:

The advantage of using a mirror is that the mirror will allow you to use a shorter distance to get a longer total distance. if your image is too small given the distance from projector to image try using a mirror. There are some problems inherent to using a mirror but they can all be mitigated. Every projection system has problems, optical systems in general are an exercise in compromises. 

Lets look at the system you are using first.

Simple projection 
The image is going to be dependent upon two factors the distance from the projector and the lens size times the focal length these two factors are what determines the size of the image. The focal length is what is referred to as the "f ratio". For example a twenty millimeter lens times a "f ratio of 150 gives a focal length of 300 mm.  I need a much longer distance than that you say, well the image maybe in focus at various distance according to the square of the distance there is however a loss of light at these distances this is what is referred to as the chi square law. The image is enlarged The square of the area and the light lost is diminished. Lets say that you are using a 1000 lumen lamp at three feet hypothetically ( you get the total 1000 lumens),  the available light at nine feet would be the the square root of 1000 lumen's. This equates to about 31.6 lumen's that is why you need a darker room and the greatest amount of light being projected from your projector. Remember the larger the number of lumen projector the brighter your image at a given distance. 

This says, if you are using front or back projection the larger the image the less total light per square foot of area. 

The only way to get a larger projection is too move the projector further away from the projected image or change the lens on the projector.

Rear Projection

Rear projection is the same thing the difference is that if you transmit light through a surface again there will be a loss of light as some of the light will be lost through reflection from the surface of your screen. Gene and I used a bed sheet to project through, alternatives to this could be a sheet of glass that has been prepared to act as a rear projection screen. Lapping a large sheet of plate glass with valve grinding compound might produce a superior surface the constraint would be how much time does it take to produce a surface that is 10 foot square. This will allow for a greater amount of the total light to be used than a bed sheet. The glass is much heavier than a bed sheet, another compromise. We have not found a plastic that transmits enough light to be usable at this time. That does not mean such a plastic surface does not exist we has simply not found one that is affordable. Television rear projection systems do make use of a plastic material to satisfy this need. The "Need for Speed" driving sim we built a few years ago uses just such a projection screen obtained from a salvaged rear projection television set. We built that sim in one day I might add. The 3X5 screen sits about 8 feet from the projector while in the television set the manufacture used a folded mirror system to get a longer focal length from a shorter distance. The use of a folded mirror system means the image will be distorted "trapezoidal" which you may correct for easily enough should you desire to use such a system. Again the amount of light will be dependent upon the distance from the projector and the projection surface.

Fresnel lens

Commercial flight sims use a monitor and fresnel lens system which means that you must place your head(eye)at the focal point of the fresnel lens. This gives the illusion of depth in the simulator while making minimal space used in the simulator and in focus. The monitors used are overdriven to increase the amount of light projected on the surface of the fresnel lens. While this shortens the life time of the monitor the projected image is bright. Remember the chi square law still applies to this system as well. This system makes use of the least amount of space possible there are further trade offs. The trade offs are as follows,
The pixal size is increased by the amount of enlargement this may mean large blocks, color distortion, fresnel lenses are typically a spherical lens system not all colors are at focus at a single plane. The last factor is what is called coma, coma means that the image maybe distorted at the edge of the lens. The coma is typically not much of a problem on commercial systems as they will use a fresnel lens system that ignores the edge of the lens. Think large lens with the edges cut off. A fresnel lens is just a flat lens. There are further compromises to make when specifying a fresnel lens amount of light transmitted versus image. The more lines in the lens the greater the image the less light so fewer lines means more light. 

While this does not satisfy your question Fransisco I hope you learned a bit about optical systems.

Keep 'em Flying
Rob Hommel

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