Tests with Lenses

One of the most likely possibilities to create a collimated beam of light would be to use lenses, which will cause the beam of light to be bent back into a central point, and prevent the beam from continuing to spread, and expand. A lens is a piece of glass that has curved sides that can concentrate or disperse light rays. If I therefore find the correct lens I should be able to concentrate the light rays passing trough it to create a relatively collimated beam (same diameter all of the way down).



During these initial tests with lenses I am going to text the effect of enlarger and condenser lenses, that I was able to borrow from John Bunyan (a technician from my university). A condenser lens works by focusing the light that passes through it onto a specific point, commonly seen in microscopes. A condensor enlarger (seen in photographic darkrooms) collimates light, this means that the light is bent so that the rays are all paralell to each other. This is done with large convex lenses which the curved surfaces facing each other. Although I do not have large versions of this I was able to borrow a set of larges that will work to the same effect.



Although it was extremely useful to read about the lenses and the effect that they would have, I decided that t would be better to test them, and see in person the effect that they really have.

I set the tests up like I had with the others, using the same spotlight torch that I used during the initial tests. As before I blew smoke through the light, allowing it to be more visible. I then positioned the lenses in front of the light and recorded the results.









As you can see the lens cause the light to bend, and be flipped, causing the light to invert, similarly to a pinhole camera.




if you take one of these lenses and set up a small bright light source (i suggest a 12v small halogen lamp with a battery or 12v adapter) you should find a distance from the lens from which it projects a columated beam - check with a piece of paper - the lens should project its own shape and size on the paper as you move it away.



Enlarger lenses are different to camera lenses, in that an enlarger lens is designed to transfer a flat image onto a flat image, so it's optimized as a flat-field lens. When using a substandard lens, you'll see light falloff at the edges. In this drawing you can see how the light has to travel a longer distance to the corners of the print. When using a good lens, you may have to burn in the edges to account for this, when using a substandard lens, the impact of the fall off can be too much to adjust for, or may even cause the edges to be unsharp, and reduce contrast.