Fresnel lenses, a bit of problem

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Hello everyone! I was building my projector this eastern but I ran out of time so the project was put on hold. What I remember though is a problem that I had with my fresnel lenses.

I got a cold mirror, IR glass setup with a 7" lilliput. The reflector used is a elliptical cnc lathed designed special for my lamp(150W MH). Due to the elliptical reflector am I not using a precondenser lens. Now on to the problem:

The fresnel lenses are split. And I get a Focus of Xcm and Y cm from the both bits. (measured symmetrically) When I move them towards each other they form a new FL of Z cm, symmetrically on both sides! This makes up for a problem due to the fact that the fl of the "not split" fresnel is too short, and not at all according to the factory spec. I know that some abbrevation of the fl lenght can occur when you split the fresnels but should the fl really be the same? This gets a bit tricky when i should assemble the projector due to that the box gets really small.

I can't say that i'm 100% sure on this but it's what I recall by memory, I hope someone got an idea what I could do!
If I understood you correctly you are trying to extend the focal length of the fresnels?

I know that one member here, in order to use 330mm fresnels with a 17in lcd and a lens with a rear focal of around 450mm I believe bent the front fresnel a slight bit in order to curve the light more to the outside, effectively creating a longer focal. Of course in his design the fresnels were not split.

In your case you could either follow in his footsteps and bend the front fresnel, or because your frensels are split if you only need a bit more length simply move the front fresnel closer to the lens b/c in theory the light traveling to that fresnel would be paralell so distance wouldn't matter. (Obviously there is a limit as to how far you can go, but for a small adjustment it should work.)
not a problem

The combined focal length does not matter. This is where the combined lenses would focus a bundle of parallel rays (ie. from the sun). You never use the fresnels this way in a projector.

Let's look at an example where you have a 220 mm fl condensor fresnel and a 330 mm fl field fresnel. If you put the lamp arc 220 mm from the condensor fresnel, then it will refract the rays to be parallel on the other side. If those parallel rays enter the field fresnel, it will refract the rays to focus on a point 330 mm from the fresnel. Since the rays between the two fresnels are parallel, it does not matter how far apart they are from each other.

If you put the lamp arc only 200 mm from the condensor fresnel, then the rays coming out the other side will be diverging instead of parallel. (If they travel a long distance to the field fresnel, then you would lose some light.) When they go through the field fresnel, they will be focussed at a distance longer than 330 mm. To calculate that distance, you can the use duplet equation (which includes the distance between the fresnels). But you don't need to do that! The duplet equation calculates the focal length on each side of the combined lens system, but that focal length has to be measured from the principal planes of the two lenses. If the lenses are not together, then those two principal planes are far apart. But this is an unnecessary complication.

Calculate the field fresnel to focus point distance as if the two fresnels were together:

1/EFL = 1/220 + 1/330 so EFL = 132 mm combined fl

1/132 = 1/distance A + 1/distance B so distance B = 388 mm

So with the lamp arc 200 mm from the condensor fresnel, the field fresnel will focus the light to a point 388 mm FROM THE FIELD FRESNEL. If the two fresnels are 0 mm apart or 50 mm apart, it will not matter: The light will still be focussed 388 mm from the field fresnel.
hi GG, long time didn´t post near you :D.

I agree with you that doing it to much complicate (including fresnell lens separation effect) maybe is not neccesary.

But if owr friend is meticulous and likes to be exact and precise then;

Ft=(f1*f2) / (f1+f2-D)

Ft=total focal, f1, f2 are the two focals, and D is the separation on the lens.

Then you can work out the ideal placent of the bulb so it is focused on the triplet. (anyway, the two distances GG told you need to be measured from the conbined virtual fresnell, not from the rear non from the field fresnell, but somewhere betten them.
did you move the lamp to fix it?

1)set the triplet where the image is focused (no not care if it isn´t huniform brightness).

2)move the lamp so the light enters the triplet to get huniform brigthness, it shouldn´t affect to the focus in theory.
Those fresnels are fine

Rox is right: Those fresnels will work fine.

With a 10 foot throw distance, your LCD to projection lens distance will be 310 to 360 mm. If you make a split fresnel design, then your 300 mm fl field fresnel goes about 30 mm after the LCD. So the fresnel to projection lens distance will be 280 to 330 mm. A 300 mm fl fresnel is perfect for that distance.

Start with the 220 mm fl condensor fresnel exactly 220 mm from the lamp arc. Once you setup the projector in the room where you will use it, (so you have the correct throw distance and projection lens adjustment), then you can try adjusting the lamp to fresnel distance just a centimeter or two in each direction to get the best image. Even with no adjustment, it will probably work great.
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