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LED selection

Discussion in 'LEDs and Optoelectronics' started by Maramor, May 5, 2012.

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  1. Maramor


    May 5, 2012
    OK, I want to build a light box to make a projector, I don't really know what I am doing LoL.

    I have seen several things about using a overhead projector and a LCD monitor.
    So my thought is the base of the overhead is just a light box that sends the light through the screen mounted on the bed and mirror's are used to send the picture to the surface (wall).

    So for the light source I was thinking of building a LED array to get about 2000-4000 lumens (I think thats what it should be?). Did a little research on LED's but well I don't understand how to read the information. I was thinking if I make the box itself reflective inside and bounce the light around I would get a uniformed light source through the LCD. If reflective does not work I would try White to get a similar effect.

    Any issues with my thoughts?

    Does anyone have any suggestions on what LED's would be good to array together and get 2000-4000 lumens?

    How do I exactly read the information to figure out how many I need to array and determine power requirements?

    Also is there a easy way to read the lumens output?

    Sorry for all the questions, just hopeful people will be kind and be helpful,

  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    There are several issues.

    Firstly there are the optical issues.

    If you employ a point light source and a condenser lens, you will get the maximum contrast in your projected image.

    With LEDs, it will be hard to get a point source unless you use something like a 10W or 20W LED. If you use more than a single LED then you'll have either a diffuse light source, or something between point and diffuse. A diffuse light source does not require a condenser lens (that's the fresnel lens under the OHP glass).

    The advantage of a diffuse source is that dust and scratches will not show as clearly. However you need to weigh that up against the potentially lower brightness and lower contrast.

    Then there's the LED side of things.

    High power LEDs are typically rated in lumens, so that makes life easier for you. However they're sometimes a little optimistic. The other issue is the width of the cone of light produced. If you only need a 45 degree half angle and the LEDs have a 60 degree half angle then you're wasting maybe half of those lumens.

    The third issue is driving them. It is vitally important for high power LEDs that you drive them from an appropriate supply, and that means a constant current source. It's also vitally important to properly heatsink the LEDs.
  3. Maramor


    May 5, 2012
    So if I bounce the light around in a reflective box I will still need something to help unify the light for the screen (is that what a condenser does)? Is this so you don't get light/dark spots?I am thinking overhead projector like in school and not the small projectors you see more often. My second phase might be to adapt to a more common smaller form.

    Are you saying I can try a regular 10-20W (20W 1200 lumens) light bulb? That might actually work at least for a low cost solution. If I was to use a normal bulb I am betting I will have some heat to remove from the box which maybe fans can help. If I used a normal bulb I could use a normal light fixture like in a lamp and plug into wall. I can't seem to find any information on the heat output of these light bulbs.

    All sounds easy to me, except the whole condenser, fresnel lens, and OHP glass stuff. Gonna start looking up what those actually mean and where I might need them.

    Just had a thought, on the projectors at school they might had a lens on the arm with the mirror....hummmmmm. Maybe I can get a cheap projector arm LoL.
    Last edited: May 6, 2012
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    no, you can't use a lens if you bounce the light around, only with a point source light.

    Yes the condenser tries to ensure that the majority of the llight from the globe is directed at the lens/mirror which projects the image.

    No, it helps ensure that the light goes to the screen and not to general room illumination

    I am thinking overhead projector like in school and not the small projectors you see more often. My second phase might be to adapt to a more common smaller form.

    No, a 10W or 20W LED emitter. Quite a bit more expensive. Or use the sort of bulb they already have.

    The problem with high power bulbs is that they emit a lit of heat and that heat may make your LCD display unhappy.

    Assume that the heat output of a bulb (any type) is their power rating. You won't be more than a couple of % in error.

    Everything is easy except the optics. :D

    Might be better to start with an OHP which will have all of that stuff already in it.

    Well, you really want the entire unit...
  5. Maramor


    May 5, 2012
    Thank you very much for walking me through the whole concept. I think I understand the concepts and now just have to decide how I am going to start. I will see if I can find a old overhead projector or one not fully working and see if I can modify it. This would be a good start and I can see how one is built piece by piece. I have always been interested in how things work :).

    Again thank you.
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