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Designing a custom projector can often be time-consuming and expensive. Here we offer a quick solution anyone can follow. Define your system, consult the chart to the right for the parts necessary, then fit it all together and your projector prototype is complete and fully functional.
STEP 1. DIVIDE YOUR SYSTEM INTO PARTS
Optical applications are many and varied, from simple magnifiers to laser beam conditioning. Most applications, however, can be broken down into modules that can be developed almost independently. Projector systems are prime examples of this modular concept. Projectors, like the classic slide projector, employ two main modules: a condenser lens system and projection lens system. The condenser lens system evenly illuminates a slide; the projector lens system projects an image of the slide onto a screen. Each module can be built with simple components and methods, beginning with the thin lens equations below. 25mm diameter optics will be used in this example because of their large aperture and wide range of focal lengths.
STEP 2. THE PROJECTION LENS SYSTEM
The projection lens system is governed by the desired magnification and throw distance of the reticle image. We recommend using two achromatic elements in symmetric alignment to yield the best image. To determine which achromats to use, you must first decide how far from the projected image the projector will be (throw distance). Next, decide what magnification you would like to achieve, then fill in the following equations. Remember, magnification is proportional to the ratio of the second focal length to the first, or :
or
M=I/O
(Image to Object Distance Ratio)
EXAMPLE:
For a 250mm throw (image distance) and 2.5 times magnification, the distance to reticle (object distance) calculates to:
F1 = 250mm /2.5 = 100mm
So, for this example you would choose a 25mm diameter achromatic element with a focal length of 100mm.
STEP 3. CONDENSER LENS SYSTEM
Condenser lens systems collect light from divergent illumination sources and redirect (condense) the light to flood the projector lens system. The classical condenser lens system consists of two plano-convex (PCX) elements mounted convex sides together. The first lens collects the divergent light cone from the illuminator (object or projector); the second lens outputs the light as a convergent cone (image).
If a light source is coupled with a 1/4" diameter fiber optic light guide, the condenser lens system must enlarge the cone by about four times to fill a 25mm diameter projector lens system. Implicitly, the projection lens solution defines part of the condenser lens system: the distance from the condenser lens system to the projector lens system is at least as far as the distance from the reticle to the projector lens system (F 2), which in the previous example calculated to be 100mm (see Diagram 1.0). So we will use a 125mm focal length plano-convex lens as the second condenser lens (F 2), and the solution becomes:
where M = 4; I = 125mm
which yields an object distance (O) of about 32mm.
STEP 4. COMPUTER OPTIMIZATION
Edmund Optics plano-convex and achromat lenses are available in 25mm diameter in the focal lengths estimated above. The next design step would be to optimize the distances and focal lengths using optical design software. Many programs include a library of our stock lens prescriptions and provide "closest-match" algorithms. Optimizing each system module will give the best case scenarios, for which appropriate mounting components can be chosen. (Computer optimization changed this system only nominally with respect to image and object distances).
STEP 5. CHOOSE THE PARTS
From the list of mounting components to the right, choose several barrel tubes for mounting your lens systems in the proper lengths. See Diagram 2.0 for our assembled example of component suggestions.
