Projection Theory

Lenses & Optics

A very important fact that many beginners do not yet understand is that there are a great many different types of lenses available. The differences are in the shape/curvature, focal length, material, and alignment. Not all lenses work the same way. We are most commonly used to a magnifying glass lens or camera lens, but it is important to understand that only particular types of lenses work for specific applications as in a DIY Projector.

First, Some Important Lens Terminology

• Diameter: The largest distance (usually represented in mm) from an outer edge of a lens to the opposing outer edge.
• Focal Length: This is a focal property of the lens. For our purposes, it determines how large our resulting image will be for a given distance.
• Triplet: This is a type of compound lens assembly, meaning that it ctually made up of multiple lenses that have been stacked in a particular fashion. In this case, three lenses are used inside of a common housing.
• Fresnel: This is a type of lens that is intended for shaping a beam of light. You are most familiar with this lens as a page magnifier. It is not good for video translation, and therefore is not good to use as a projection lens; however, it does serve a very important purpose in DIY projectors.

Why We Need a Fresnel Lens
In our DIY projectors, we use a fresnel lens between the LCD and light source to shape the light that goes through the LCD so that it will all end up on the back of our smaller projection lens. This diagram helps to better illustrate what a fresnel lens does.

Notice that in the diagram to the right, configuration 1 has no fresnel lens, and configuration 2 does. In configuration 1, only 2 units of light are entering the projection lens. In configuration 2, however, the fresnel lens causes approximately all of the incoming light to converge back onto the projection lens, so you get far more units of light entering the projection lens. This causes the desired performance in the case of configuration 2. If you try to build your projector without a fresnel lens, you will notice that your image is hardly visible. Also, you will only be able to see the part of the image that those 2 units of light are carrying from the center of the LCD. This is very undesirable, and will not permit you to use your projector at all.

Why We Use a Lens Triplet
As discussed earlier, a triplet lens is actually 3 lenses put together inside of one housing. We use a triplet lens instead of a singlet or meniscus lens (similar to a magnifying glass) because it gives the sharpest and truest representation of the source image. In short, lenses with fewer components do not allow us to project as high quality of an image as with a triplet. In fact, with naked eye comparison of projected images, there is practically no better lens than a triplet in terms of clarity. One caution though is that you must use a projection lens triplet - not just any lens triplet will automatically give a good picture. Another factor that needs to be considered is its focal length. 

Focal Length Explanation
The focal length of a lens basically determines its magnification. It seems logical that if we want the largest image possible, that we should use the smallest focal length lens possible (since smaller focal length results in higher magnification). In actuality this doesn't necessarily work. The reason is that shorter focal length lenses need to be closer to the source (LCD panel). If a lens has a 3" diameter and the LCD it is trying to magnify is 10" wide, then there is a specific distance range in  which the projection lens should be placed; otherwise, not all of the source image will be captured by the lens. The explanation for this phenomenon is not entirely straightforward, but the description so far is sufficient for our purposes.

It is important that the focal length of your upper fresnel lens be approximately similar to the focal length of your projection lens. This will ensure that the fresnel lens converges light to a small region where the projection lens will go. If the focal length of the projection lens is too short for the fresnel lens, then the light will want to converge at a point beyond the projection lens, which will cause a significant amount of light to pass around the projection lens entirely. The end result is a dim image that is sometimes discolored.

Screen Size and Focal Length
So the focal length of the projection lens determines the size of the projected image, but how can I know ahead of time how large to expect the image to be? There is a very simple equation that allows us to determine various properties of our projector by means of simple multiplication and division (if you don't want to do the math yourself, use our automatic projection calculator). Here are some simple equations that are desirable for optimizing your projector or estimating measurements ahead of time:

Click Here to Use Our Projection Calculator

© 2006 DIY For Life Inc