Simple optical lens design help (small projector)

[HexNuclei]

I need to design a mini projector as a part of a project I am working on. The body of the projector is approximately 7 inch tall and 5 inch wide.

I don't know what kind of a lens I need to ensure the projection is sharp and crisp (it is only black and white writing). I know I would need a zoom function on there (to save moving it forward and back) but I not sure what kind of lens(es) I need to make it operational.

I think way it will work is similar to an old overhead projector (without the mirrors) so I think it is:

Light source>projection film>lens>projection screen

The projection screen with be about 2-3 meters away from the projection film. The light source is LED and will be 5 cm away from the projection film, which is 180° dome. The projection film is approximately 5in by 5in.

What kind of a lens do I need, and how many would I need?

I've tried all sorts and keep failing. Only success I have had is holding a magnifying glass 10cm away from the projection film, then it works wonders. But I don't want to do it that way, I want the lens to be no more than a few cm away from the film. Can this be done?

 

[berkeman]

Welcome to the PF.

Can you post a sketch with dimensions shown? The hemisphere film is confusing to me. What are you going to project it on? Another hemisphere, or a flat reflective screen? How much light does the LED source put out? Will the viewing room be dark?

 

[tech99]

I need to design a mini projector as a part of a project I am working on. The body of the projector is approximately 7 inch tall and 5 inch wide.

I hope you can send the details mentioned by berkeman.
I think you are under estimating the problem.
1) You need lots of light due to the distant screen. OHPs use a powerful bulb.
2) Zoom lenses are difficult as they involve lenses being moved by a mechanism. Maybe a big camera zoom lens would do. OHPs do not have zoom lenses as far as I remember.
3) On the lamp side of the slide, you need uniform illumination. This is not easy, but means a condenser lens.
4) 5cm from light source to film seems too little to achieve uniform illumination because your film is so big.
5) If you want the lens very close to the film, obviously it needs to be very big, and must have short focal length. That makes spherical aberration a problem.

 

[HexNuclei]

Welcome to the PF.

Can you post a sketch with dimensions shown? The hemisphere film is confusing to me. What are you going to project it on? Another hemisphere, or a flat reflective screen? How much light does the LED source put out? Will the viewing room be dark?

Thank you :)

Hope it makes more sense.
https://ibb.co/hYV42Q2

Light>2-4cm gap>hemisphere film>smallest gap possible>lens>1.5-2.5m gap>projection screen

The screen is a flat surface, I only need the top 2 or so centimeter radius reflecting sharply, the rest can be blurry I don't mind. The room will be dark. Unsure how much light is emitted in terms of lumens but it is bright enough I think. If I have the same set up as above I can do it with 8 magnifying glasses of varying strengths (ordered loads of different ones from Amazon to see if any work; only work if I put all together) but main issue is the height as they all add a total of 10cm height.

 

[HexNuclei]

I hope you can send the details mentioned by berkeman.
I think you are under estimating the problem.
1) You need lots of light due to the distant screen. OHPs use a powerful bulb.
2) Zoom lenses are difficult as they involve lenses being moved by a mechanism. Maybe a big camera zoom lens would do. OHPs do not have zoom lenses as far as I remember.
3) On the lamp side of the slide, you need uniform illumination. This is not easy, but means a condenser lens.
4) 5cm from light source to film seems too little to achieve uniform illumination because your film is so big.
5) If you want the lens very close to the film, obviously it needs to be very big, and must have short focal length. That makes spherical aberration a problem.

Thanks for your comment.

1- Interesting. Because it works with a magnifying glass, I wouldn't have thought the light is an issue. Could this be the case or am I wrong?

2 - I can probably 3d print a mechanism to help with zoom, I reckon if I could a lens that works, I can design a mechanism to work around it.

3 - is a condenser same as a collimator? If so, would one that allows 180° illumination be of any use as my film is 180° hemisphere

4 - I don't need complete uniformity, just the top part (2cm radius from the top of the dome). The rest can be blurry or as it is.

5 - this I'd need to look into properly. Because I only need the top section clear, I can maybe get a small lens and see if I can make it work. Again, not sure what lense I'd need

 

[HexNuclei]

I tried using a 50mm focal length convex lens today and it worked close to the film. I'm going to try using a shorter focal length to see whether I can close the distance and if that works, then look at building a focus system.

Any advice?

 

[jim hardy]

Since the advent of digital cameras
Junkshops and yard sales have old slide projectors for just a few bucks
From them you can pillage a condensing lens and a projection lens, probably a lamp too.
and an optical system you can study.

if you're lucky you might score an old Kodak Carousel with a zoom lens.

 

[CWatters]

Not sure if it helps but some years ago I purchased a projector that you could put on top of a drawing and have that projected onto a wall or screen. Used it to draw cartoons on the wall of a child's bedroom. Was similar to the artograph projectors on this page.

https://www.cassart.co.uk/painting/accessories_1/lighting

To make the image bigger/smaller the whole thing has to be moved further away and refocussed.

 

[HexNuclei]

Thank you all for your input, it's been really helpful and I think I have finally gotten somewhere.

I've slightly redesigned my projection film. As only the top part of the dome needed clear projection, I've cut a whole at the top of the dome and inserted my slide. This makes it a flat slide that makes it easier to project.

I've tried it with a 50mm focal length lens and it works good. The issue I have now is my film is around 60mm. If I wanted to use a plano convex lens, 50mm focal length, what's the smallest diameter it could be so it projects all my film?

At the moment I've tried a 50mm diameter lens on a 30mm slide and that has failed to project the entire image.

 

[Tom.G]

I've tried it with a 50mm focal length lens and it works good. The issue I have now is my film is around 60mm. If I wanted to use a plano convex lens, 50mm focal length, what's the smallest diameter it could be so it projects all my film?

At the moment I've tried a 50mm diameter lens on a 30mm slide and that has failed to project the entire image.

It is likely the light from the edges of your slide arrive at the lens at too steep an angle. The critical angle (the incident angle from perpendicualr at which there is total reflection) is 41° for common glass. If you can reduce the incident angle to 35° you will lose only 10% to that effect.

With a sketch of your layout with the maximum slide size and the distance to the lens, you should be able to work out the required lens diameter to accept light rays at 35° incident angle.

If you can accept a smaller projected image, move the projection screen closer to your device. This will require a larger distance between the slide and the lens to achieve focus, thereby accepting more of the edge rays.

Also be aware that in a simple optical system, it is normal for illumination to fall off dramatically towards the image edges. A light loss of at least 50% in your configuration would not be unusual.

Cheers,
Tom

 

[dLouie]

Not sure if it helps but some years ago I purchased a projector that you could put on top of a drawing and have that projected onto a wall or screen. Used it to draw cartoons on the wall of a child's bedroom. Was similar to the artograph projectors on this page.

https://www.cassart.co.uk/painting/accessories_1/lighting

To make the image bigger/smaller the whole thing has to be moved further away and refocussed.

That is a good concept for much simpler design.