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Collimating image from an LCD display

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  1. Sep 10, 2016 #1
    I have an LCD display I'd like to use in a collimated HUD display, however it seems that I can't get the light (image) from it to collimate properly. I'm trying to use a lens with the focal distance of 4.5 cm. When I place the display 4.5 cm away from the lens' center I expected to achieve the collimation however the only thing I can see in the lens is a blurry mess.

    Questions is: Maybe I'm putting the lens in the wrong side of the display - in order to get a collimated image from it do I actually have to pass already collimated light through the display (replace the display backlight with a collimated light source) or is it somehow possible to collimate the image coming out from it (it's shown this way on all schematics of collimated displays/collimators i've been able to find)?

    Optionally, are there other means of collimating an image from an LCD display?
     
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  3. Sep 11, 2016 #2

    CWatters

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    Perhaps I misunderstand but ... If you put an object at the focal point wont the image be at infinity?
     
  4. Sep 11, 2016 #3
    Yes, that's how HUDs and collimator sights work if I understand it correctly.
     
  5. Sep 11, 2016 #4
     
  6. Sep 11, 2016 #5

    mfb

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    How precise are the 4.5 cm of the lens and your 4.5 cm distance? Can you vary the distance a bit and see if it gets better?
     
  7. Sep 11, 2016 #6
    Can you get the screen in sharp focus at one point but the rest is blurry? Is it all blurry all the time? I ask the first question because your lens may have a limited aperture. That will mean that it can only collimate a small portion of the image and the rest will be blurry.
     
  8. Sep 11, 2016 #7

    mfb

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    Chromatic aberration might be another issue: the focal length will be different for different colors.
     
  9. Sep 12, 2016 #8
    I can get a focused image, although it's not the goal I'm aiming for, instead I need to achieve collimated image from the display (focused at infinity).

    I've tried the method I described above (replacing the display backlight with a collimated light source) with good results although the collimation wasn't perfect due to the quality of my light source it still was noticeable. Queston still stands: is it possible to collimate the divering image from the display without replacing the backlight?
    For additional information I'm using a lens from oculus rift dk2 with diametet of 4cm and focal distance of 4.5cm. I also have ordered a 10 diopter 5.5cm diamater lens, perhaps that'll give different results
     
  10. Sep 12, 2016 #9

    Drakkith

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    If you look through the lens at the display and move the lens forward and back, is there any distance at which the display becomes focused?
    When you say you can get a focused image, are you looking at that image directly through the lens, or are you projecting it onto a surface and seeing the image that way?
     
  11. Sep 12, 2016 #10
    Yes, if I move the lens back and forth I get a focused image at some point when looking through the lens
     
  12. Sep 12, 2016 #11

    mfb

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    Then where is the problem? That is the right distance.
     
  13. Sep 12, 2016 #12
    Well is that a collimated image? Because as far as I know, doing that simply results in the light rays to focus at some point on the other side of the lens, but I need an image focused at infinity
     
  14. Sep 12, 2016 #13
    One of the major features of an HUD is that the produced image is collimated and thus always in eye focus independent of the current eye accomodation. That's what I'm aiming for.
     
  15. Sep 12, 2016 #14

    mfb

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    If you look through the lens and see a sharp image with the eyes adapted to infinity (not necessarily, but that is the typical application), your HUD works. That is exactly the use case.
     
  16. Sep 13, 2016 #15
    Okay, much thanks for the info!
     
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