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diffraction patterns in DMD (imaging application)

ewc
Prodigy 40 points
Other Parts Discussed in Thread: DLP7000

Hello there,

I am directing a laser beam onto my DLP7000, and trying to project an image of the DMD onto a screen/sample. The beam goes directly to the DMD without any expansion, and I'm getting diffraction patterns as described by others in various posts. I understand that if using the DMD for imaging, this shouldn't be a problem as a projection lens can collect the diffraction orders and form an image.

However, I am a little confused if I want to image from infinity. I looked at an earlier reply that says:

you may want to use a lens that makes all the orders parallel (i.e. collimated space) then use a final lens to reimage them from "infinity".  This is equivalent to cutting a lens in half and separating the two halves by whatever distance you need. 

Referring to an illustration I found:

Does this mean then that:

1. adding a single lens 1f from the diffraction plane above will collimate the orders

2. then placing a second lens afterwards forms an image of the DMD 1f away from this lens?

So my setup would then use 3 lens total?

Sorry for the confusion regarding basic optics, thank you again!

  • 0
    TI__Guru 54585 points

    Hello EWC,

    Welcome to the DLP section of the TI-E2E community.

    The "cutting the lens in half" is a bit of an oversimplification but works like this.  Imagine a bi-convex lens with no magnification so that the image and the object are the same distance from the lens.  Inside the lens the rays will all be paralled to the optical axis.

    Now imagine replacing the bi-convex lens (equal curvature on both sides with a convex-plano followed by a plano-convex).  Now you can (in theory) separate the two halves by any arbitrary distance.  Now looking at just the plano-convex lens on the image side.  It looks like a lens that takes parallel rays (as though coming from infinity) and forms an image.  This is what your infinity focus system would look like.

    What is it you are attempting to accomplish?

    Fizix 

  • 0 in reply to Fizix
    Prodigy 40 points
    Hi Fizix,

    Thank you for the answer. I'm trying to gain some flexibility in the distance from the DMD to the screen. I understand your answer, and it is also illustrated here:
    i.stack.imgur.com/hNaJP.png

    However I'm trying to wrap my head around the fact that we have to deal with diffraction patterns produced by the DMD. Do I then just treat the DMD as an ordinary object to be imaged, and exactly the same principles apply? If I do that, I will get a diffraction pattern at the back focal plane of the plano-convex lens, and further away I get a magnified image of the DMD?
  • 0 in reply to ewc
    TI__Guru 54585 points

    EWC,
    If you wish to also magnify (or de-magnify) the image of the DMD you will have to adjust this slightly by inserting an element that does the magnification/demagnification in the train before or after the columated space section of the optical train.
    You are correct that you should set up your system as an ordinary object to be imaged. You should be able to set it up with broadband illumination, focus the system as you desire and then replace your illumination with your coherent illumination and it should still be in focus.

    Fizix