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Old Jan 5th 2019, 04:32 PM   #1
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Light projecting images

Hi,

I'm currently working on a project the aim of which is to build a small night light that uses LEDs. On top of this is a plastic sheet with a design printed on it that is to allow the images to be displayed on the ceiling - but I cannot get it to work!

I have tried amending the mode of projection (via a dome) from using UV printed PVC sheets that have been vacuum moulded, vinyl, other stickers, laser cutting etc and they all have the same issue, the projection is not clear enough.

If i move the dome further from the light, it becomes more clear but smaller. I can't change the size of the nor can I have the dome too far away from the lights as it looks silly.

I'm thinking maybe if the light source was stronger then the images would be more clear?

Can anyone shed some light (no pun intended) on this matter?

Thanks in advance
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Old Jan 6th 2019, 04:12 AM   #2
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This is all about getting a point source of light ...

If the light emitting area of the LED is , say 2mmx2mm then this is equivalent to many point sources spread over that area , so multiple overlapping images are produced , in other words a blurred image.

You need something like this ...



It's the best the industry can produce for high power (10W) in a small emitter area ... you will need a heat sink ... they are used in the zoom-able touches ..

In fact it will be a lot easier to buy an led torch like this one , must be zoom-able... see eBay



the zoom and lens may help , if not you can unscrew and remove the lens.

Some torches have a parabolic reflector , this type is probably not good for your application

Last edited by oz93666; Jan 6th 2019 at 04:19 AM.
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Old Jan 7th 2019, 05:36 AM   #3
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As Oz says,
the geometry of projection starts with the assumption of a point light source.
You should be able to focus the light from your LED panel through a focus point before it reaches the projection film.
You can then use a second lens, after the projection film, to focus the image onto the wall or ceiling.
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Old Jan 7th 2019, 09:20 AM   #4
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Right, I've taken these suggestions on board and have been experimenting. What I noticed works is if I hold a magnifying glass about 10cm away from the projection film. The film is a 5inch dome, 2.5inches tall. I can only get part of one item on the film to project.

I'm assuming a fresnel lens may help over come his. Or any other lense that's big enough i.e. 5inches?

Also, how do I work out that focal length lens I require

I'd like to add that the magnifying glass technique also worked with the normal led that's already on the lamp (magnifying glass held above the projection film, not below). Am I correct that the light is therefore adequate and what I need is definitely a lens?

Last edited by noney; Jan 7th 2019 at 09:22 AM.
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Old Jan 7th 2019, 09:52 AM   #5
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There are many lenses on eBay used with leds ...



The fatter the lens the shorter the focal length , allowing a compact design. They are used with leds with emitting area 20x20mm ... but you must use as close to a point source as possible...

Fresnel lens probably won't get good results.
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Old Jan 7th 2019, 10:10 AM   #6
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Originally Posted by oz93666 View Post
There are many lenses on eBay used with leds ...



The fatter the lens the shorter the focal length , allowing a compact design. They are used with leds with emitting area 20x20mm ... but you must use as close to a point source as possible...

Fresnel lens probably won't get good results.
I tried using one of those on top of the LED but under the projection film and it seems it just concentrated the beam to a 60 degree angle without having any impact on the actual projection
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Old Jan 8th 2019, 01:02 AM   #7
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Originally Posted by noney View Post
I tried using one of those on top of the LED but under the projection film and it seems it just concentrated the beam to a 60 degree angle without having any impact on the actual projection
I'm assuming your using a point source???

those angles relate to using an led with a very big emitting area ..

if you have a close to point source , and position it at the correct distance from the lens you should have a beam you can work with ...

All you can do is get a powerful point source and play around with different positions of lens and plastic sheet till you get something usable . It will not be perfect since this is one of the short falls of leds . Thye do have a relatively large emitting area for their power.

getting a zoom-able torch will make things very easy

Last edited by oz93666; Jan 8th 2019 at 01:05 AM.
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Old Jan 8th 2019, 04:09 PM   #8
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I tried it with a collimator above the LED and still had to use a magnifying glass and didn't achieve any better results than using the magnifying glass alone (in regards to the quality of the projection)

I also tried a zoomeable torch with a magnifying glass and again I didn't get any better results.

On both occasions I had to hold the magnifying glass a fair distance away from the light.

What did work quite well tho was the use of 8 lenses. A bit much lol but it worked. I had 4 Fresnel lenses and 4 normal magnifying glasses. How can I work out what strength lens I need?
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Old Mar 4th 2019, 11:15 PM   #9
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This discussion really helped ..!!
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