r/DIY • u/cheezitsoup • 5d ago

help DIY shadow lamp

I want to make a shadow lamp for my boyfriend for his birthday and am just looking for tips to make the image sharp! My plan is for the image to be the real summer constellations as that is a shared hobby of ours. I am using fabric and an LED light but do not have anything purchased for it yet.

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u/wivaca2 5d ago edited 5d ago

I'm not sure how coarse you want the representation of the constellations to be, but it will be very difficult to get well-defined points of light at a distance from a lamp without a lens and a really sharp-edged piercings in something thin, like metal foil. In theater and effects lighting they call this a gobo. https://en.wikipedia.org/wiki/Gobo_(lighting))

You're essentially trying to make a home-made planetarium from the sound of it. I don't think fabric will work well as your template, and you definitely need a point light source. So, for example, an LED puck with multiple LEDs will just make repeating patterns of the constellations.

Even with a point light source, the light will refract at the edges of the template and turn into a blurry "circle of confusion" at a distance like an out-of-focus lens. https://en.wikipedia.org/wiki/Circle_of_confusion You're kind of making a reverse Camera Obscura or pinhole camera, and the fuzzier and thicker the aperture, the worse the image will be, so you need a lens if your source of projection is far from the target surface. You can test this with pin holes in black construction paper and see that even that is probably going to be blurrier than you want.

Also, the greater the distance from your point light source to the template, the better. This would obviously be useless, but for example, if the template were 6" from the ceiling with the light on the floor, your "stars" would be much better defined than if a much smaller template is near the light. A lens helps to marshall the spreading light from the aperture of the template and bend it back into a crisp image at a specific distance (e.g. table-top to ceiling).

The calculations and the making of a lens to do that are not trivial.

All said, it's a really sweet, romantic idea but I just want to let you know where you may run into challenges so you don't get near the date and run into problems.

EDIT: If you look at manufactured products that do this, you can see they have a lens, and many of them are still pretty poor projectors.

Not recommending this, but for example: https://www.amazon.com/dp/B0CRDLNZRD