Working Image Projector "Magic Lantern"

On a recent visit to RHS Wisley Gardens in the UK I was surprised to stumble across a magic lantern that looks very much like my model. They used to use a magic lantern to show slides to the students of horticulture who were studying there. I've discovered that my model is about 1/3rd scale!

Magic Lantern is a vintage piece, so I thought I'd see what it looks like with other vintage models. Here it is alongside Cipher Machine and the Typewriter, I think it looks quite nice!

I'd like to give a huge thanks to everyone who has supported Magic Lantern - THANK YOU! If you have any friends or family who you think would like this project, please share it with them, and let's try and get to 10K.

Here are some more pictures of Magic Lantern in action. You can see that even in a lighter room, the image is still quite strong! Enjoy.

To give you an idea of the brightness that can be achieved using the phone's flash LED, here's a quick video of the projector in action in a dark room. I'm not aware of any LEGO lighting element that is this bright, which is why I went for the phone approach.

Because the projection lens is only 1cm in diameter, creating the slides was a bit of a puzzle. The images must be a little bit smaller than the lens, so in this case around 8mm. So how to get tiny images onto transparent plastic LEGO pieces!?

1: Actual slides. Probably the best option. Would have been great but it's hard and expensive to create slide images this small.

2: Shrinky Dinks. Unfortunately these ended up wobbly and more frosted than transparent!

3: Laser printing onto acetate. I thought this would be a great option, but the resolution wasn't good enough. When you think 8mm will be blown up to maybe 1m, even 600dpi is looking too low. The laser printed image was too high contrast and it uses hatching to create shades, which didn't look good.

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4: Inkjet printers have a higher print resolution, so I printed my tiny images at 1200dpi onto acetate. Once cut out these were glued to the transparent flag piece.

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I also experimented with different background shades from white to black. I found that white backgrounds produced a lot of light bleed, which lit up the room and made the image harder to see. The black backgrounds were the best, as they provide a high contrast with the image and kept the room dark.

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I was pleasantly surprised that the minifig magnifying glasses work as well as they do. They're a little rough at the edges but the centres of the lenses are actually pretty good. We use three of them in this model.

On a bright day hold the lens near a wall that is opposite a window. Find the distance from the wall where the outside image comes into sharp focus - that is the focal length of the lens. You can see in the picture below that the image of my street outside (not the window frame!) is sharp at about 22mm.

Two lenses are used in the base of the lantern to collimate the light rays coming from the phone's flash light. This just means concentrating the spread-out light rays so that they form a parallel beam of rays. Two lenses are used because we need quite a strong bending action - as we're so close to the light source the angles are quite sharp. Luckily because they're quite small the lenses have a focal length of just 22mm so two are sufficient.

We need to collimate the light because the light passing through the slide must pass through in parallel beams so that it can be manipulated properly the other side of the slide.

Also the collimator needs to be as close to the phone as possible - that way we capture much more of the light that would otherwise just bounce around inside the box and not contribute to the projection of the image.

A single lens is used as the projection lens. We can't just fire light through a slide and hope it makes an image - it will work to a certain extent but the image will only ever be the size of the lens. The projection lens enables us to project much larger images by deflecting the incoming parallel light rays back into a spread. The slides have to be put in upside-down because the projection lens flips the image top to bottom and left to right.

So now moving the lantern away from the wall makes the image bigger (but dimmer). In a dark room, you can easily get a big bright projection from a couple of meters away, which would be just under a meter in diameter.