HI!

Could anyone give an accurate/scientific account of the angles and positioning of the various components in a DIY projector, and how it affects the projection, in relation to an UNSPLIT setup? My question :-

1. How far theoretically should the Fresnels (unsplit) be placed away from the Centre of Light? (looking for magic no.)

2. How far theoretically should the Fresnel be placed away from the LCD? (looking for magic no.)

3. Should the Fresnel be placed perpendicular (90deg) to the LCD? or at an ANGLE?, such that it 'helps' project at a higher trajectory? (The ASK Projectors do so, additionally angling the light source as a method of "shifting lens").

4. Should the Light Source be PARALLEL to the LCD and Triplet, or just the LCD has to be Parallel to Triplet?

5. Anything else, anyone?


I believe that with the scientific evidence/data available to all forum-ers, we could all be building near perfect projectors flawlessly without the need to time-consuming, cost-eating trial and errors!

Now, I'm just wondering if anyone will (bother to) try this...

First of all, there are no "magic" numbers. Ideally, you would want to be able to adjust all your internal components for tweaking. That being said...

1) The rear fresnel is placed at it's focal length. If the rear fresnel has a FL of 220mm, then you place it 220mm away from the lamp's arc. You ideally want to have about an inch forward and back for adjustments.

2) A standard LCD/fresnel gap is 20mm. You want to leave enough room between the LCD and fresnel to allow for cool air to pass between them in order to cool your LCD.

3) I think you mean parallel. See the Un-Split Optics section of the DIY Projector Guide.

4) I don't know if you mean perpendicular, here. You should have a straight line from the center of your lamp's arc, through the centers of your fresnels, LCD and triplet to the center of your screen in an unsplit setup. You will not be able to keystone.

There aren't majic numbers but there are some ideals.

The lamp should be close to the FL of the collimator fresnel away from the collimator, and as close to on center as possible. On-center is more important than the distance

The collector fresnel should be as close to the FL of the fresnel away from the projection triplet as possible.

This is not always possible. As such, you treat the collimator and collector as a single lens, and use them to bring the lamp arc to a focus at the entrance to the triplet. By doing so, you will make the best use of the available light. You can adjust the distance from the lamp to the collimator fresenel to do this. Bringing the lamp closer to the collimator allows the triplet to be further away from the collector fresnel, and vise versa.

The fresnels should be perpendicular to the projector's primary axis, (The imaginary line that goes from the lamp to the projection triplet.) This is because the fresnels are most efficient in this position.

The LCD, screen and triplet should be parallel. Ideally, these will also be perpendicular to the projector's principle axis, but this is not necesarily the case. It is more important that these be aligned with each other.

Hi Gurus
It seems the consensus is science is not absolutely the best instructor for the perfect projector.
We cannot simply follow the FL of the lenses and place the components thereat to the best of its functionality.

Why is that? If we compare this to a fixed telescope (of which I believe the principles are the same), the 'only' adjustment the user makes is to the clarity of its subject. Similarly the only adjustment we should make is the distance of the screen away frm the projector (if all else within the insides are alligned and placed perfectly according to science).

Quasi_Mojo, sorry about the parallel and perpendicular misnomers.

Your telescope analogy is flawed.

In the case of a telescope, there is one other lens in the equation: Your eye. Your eye does the final job of actually focusing the image. There are, of course limits to what your eye can compensate for, but since with a telescope, we're (usually) dealing with images of very distant objects, the final adjustment tends to be a very small one, and as such we work very near the FL of the lenses involved.

A better analogy would be a microscope, where we're dealing with objects that are very close, and the adjustments are a signifigant percentage of the focal lengths of the lenses involved.

Still, both of these deal with only 2 elements. The lens and the image. This is the equivalent of just the triplet and LCD, ignoring the matter of the lamp and fresnels.