How to reduce light diffusion around focused image?

Question

I'm doing an experiment where I am shining an infrared LED through a single element lens (plano-convex lens) onto a surface which is about 3 inches away. I've placed a little square in front of the LED and I'm projecting the square image to the 3 inch surface. The square is nicely focused onto the surface however there are faint blobs of diffuse light emminating from the the squre edges. Can anyone please inform me as to the cause of this and if there is a solution with a single lens. Thank You.

Answer 1

I assume the infrared LED is not so infra that you can't see it. If it's not monochromatic you may be getting chromatic aberration. Or you may be having edge effects from the square or irregular translucence (maybe paper fibers?) at the surface.

Answer 2

1) One normally does not make a projection system that way. Google the optics of a slide projector. If you want a square, use a square aperture later in the train.

2) If it were a laser you would expand and collimate the beam, focus it through an Airy circle-diameter exactly round pinhole to spatially remove everything but clean TEM_00, then recollimate on the other side with an identical reversed lens.

3) It's called "diffraction." You can apodize the scatter by microroughening the edges of the square.

Would meniscus lenses give you fewer aberrations?

Answer 3

Proton led in Lyons Ga says:
Chromatic Abberation is probably the reason. Try to block the surrounding light using an opaque plastic. How about a laser that displays the light through the lens?

- Paul Catignani

Answer 4

its caused by chromatic abberation. you can correct this using a light stop - use black tape or card around the externat 1/3 of the lens - this will correct it the only other way to stop this is by using an achromatic doublet

Answer 5

yeah, dont use a plano-convex lense. when the light goes through the lense it not only goes straight throgh but it also refracts. that's why convex/concave lenses are used in glasses.