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Optics and projection

  1. Apr 15, 2008 #1
    Hi all

    Not sure if I posted in the right spot but my question is in regards to optics and projection. To make a little less confusing I will try to explain my dilemma.

    I have a triplet lens out of an old crt projector where the rear focal point is about 10mm form the rear lens surface. So to use for an lcd projector i would have to use an LCD the same size or smaller than the lens diameter.

    My questions are.

    If I want to use a larger lcd panel what type of lens would I put between it and the existing triplet to essentially shrink the image size so that it would act like a smaller lcd at the focal point?

    And would the triplet correct the chromatic aberration from the new lens inserted?

    Thanks in advance

  2. jcsd
  3. Apr 16, 2008 #2

    Andy Resnick

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    You want to convert a CRT projector to an LCD projector?

    Question- How do you know the rear focal length is 10 mm?

    You are confusing focal lengths with field of view, I think. I'd have to look into the details of a CRT projector, but I suspect that it, just like an LCD projector, are projecting the object (something very small, smaller than the lens diameterl) onto a surface some distance away. If you took a large LCD panel and simply placed it at the rear focal point of the lens, you would project a small portion of the LCD to a plane infinitely far away. It would also be very dim, since the brightness is conserved.

    It would be easier to help if you could provide a spec or name on the lens, and give a better idea of what you want to do. That it is a triplet is useful information, could be a simple version of a Cooke triplet. (wikipedia has an article about that). Note that chromatic aberration is entirely different from field flatness, and that inserting additional lenses willy-nilly can cause all kinds of changes.
  4. Apr 17, 2008 #3

    Here are some pics of the lens that I have and the machine that it came out of. The model number of the lens is a TKG139964, The projector is a panasonic PT-102n. The reason I think I knew the focal length was because i put a single light source behind the lens and it projected an light image on the wall about 3m away I moved the light back and forth until it was the most focused and took a measurement.

    The picture of the projector shows the size of the crt tube in relation to the lens.

    The lens consist of a what i believe to be a Biconvex lens in the front, a Plano-convex lens in the rear and I am not sure what the lens is in the center. But it is a great deal smaller than the other two.

    Attached Files:

  5. Apr 17, 2008 #4
    The basics are take a bright light (6500k colour temp) and a stripped down lcd, two Fresnel lens and one triplet. Arrange in order as shown in attached picture.

    1. Light Source
    2. Heat protection
    3a,b. Fresnel Lens
    4. Stripped Lens
    5. Triplet lens

    Please refer to lumenlabDOTcom and go to there forums sections to see other example of these home built projectors.

    I know that with this lens the only lcd I can use has to be smaller that the lens diameter this severely limits my choice (3.5" to 5"). It is for that reason that I asked if there was anyway to add a lens that had a field of view that was larger to accommodate a larger lcd (say 10.4") and then "shrink" its image to fit the field of view of the large triplet.

    I hope this help you to understand what I am trying to do. I had a search for any information on the lens but it was to no avail.

    My ultimate goal would be to change one of the lens in the triplet to create a larger field of view. Perhaps my question should have been which lens in the triplet do I change to change it's focal point and field of view?



    Attached Files:

  6. Apr 17, 2008 #5

    Andy Resnick

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    Ok, I understand better what you are trying to do- unfortunately, that's a little out of my expertise- but I'll give it the ol' college try. I'm not sure why items 3 and 4 are arranged as they are in your diagram. Wait... is 4 a 'stripped LCD'? If so...

    3a serves to simply illuminate the LCD panel evenly (for a suitable definition of 'evenly'), and 3b does the demagnification. So what you need to play with are the focal lengths of 3a and 3b- the focal length of 3a needs to be just large enough to capture sufficient light from the source to illuminate the whole LCD (put the source at the back focal plane of 3a) and the focal length of 3b needs to be about equal to 3a, creating a new 'source' located at the entrance pupil to your triplet lens. Now, the source (the backlit LCD) will be about the same size as the original source, which is good.

    Now, adjust the spacing between the triplet and 3b to generate an image of the LCD on a distant screen. Expect the image to be aberrated, chromatically and distorted.

    More than that: specific distances, adjustable magnifications, aberration correction, ratio of focal lengths of 3a,b to the triplet are beyond my basic ability. That's why optical designers get paid the big bucks.

    Good luck- let me know how it goes.
  7. Apr 17, 2008 #6

    Andy Resnick

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    A few more thoughts:

    From your schematic, you are wasting easily 90% of the light from the source. Either enclosing the source in a reflector (like a flashlight) or minimizing the focal length of the first Fresnel lens will help. Obviously, you are constrained by the size of the LCD.

    Second, if you can adjust the triplet-screen distance, that will probably help any residual distortion. The schematic reminds me of a "4-f" system...
  8. Apr 17, 2008 #7
    Hi thanks for your help

    I do belive there were a few details missing from the diagram like you sugested a reflector for the light source and also I now understand the confusion of part four I appologise it was a stripped lcd.

    I have another triplet lens but it is only 60mm in diameter and has a focal length of 340mm. this lens was specifically designed for this project. I then found that panasonic projector at a second hand store for $5 Aud. ( Almost to good to be true $5 for three 150mm triplets lens)

    I still have to build my optical bench to fit all the peices together but that will be the easy bit as I have acess to a cnc plasma cutter so It will be designed on the pc first. I will make two different triplet holder and test each one.

    Thanks for the help. I will post a message when I build and test is


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