Does anyone know if there is a glass or material that I can put on my studio light table that would collimate the light after passing through the frosted Perspex (acrylic) top? I know by definition light needs to pass through a lens to do that, but I thought there might be a kind of glass that would help
*The light table is a Monfroto light table I use for commercial work and is lit from below with a Profoto D-1. It’s always set up, so it’s my starting point.
The closest thing to what you wish for is a fresnel lens, they are available in several sizes and are made from clear plastic material. Cover your backlight with a piece of cardboard with a hole, mount the lens a few inches (depending on the fresnel lens’ focal length) above the hole and there you go.
You can also put one of your lenses on top of the lighttable…
Or use the sun as a backlight instead (careful with that one)
Anyway, will it do you any good? I’m not sure. Depends on what you aim to get in the first place,
wanting collimated light looks like a means to an end rather than a requirement.
Collimation confusion?
In terms of geometrical optics, collimated light rays are all parallel to one another. One way to approximate a collimated beam of light is to place a point-like source of light at the focal point of a converging lens. I don’t see any way to do so from an extended, diffuse light source. Please explain further what problem you’re trying to solve and perhaps someone can provide you with more help.
Well, nothing too serious or too confusing; just seeing if I could get a more defined grain than what I am getting with a diffused light without building some Rube Goldberg machine. I like the fresnel idea - seems easy enough to try. To be fair, my current setup is pretty sweet and does a great job - just tweaking.
Maybe this on top of my light table and a couple of inches under the neg?
Here’s a better link There seems to be more than a few varieties. Picking the right one is where I could use some help. Unfortunately, physics was not a requirement in art school 
I like the fresnel idea - seems like that might give a more distinct edge to my grain which is my goal.
Thanks!
Well… I spoke with an engineer at a company that sells fresnel lenses. Most of the conversation was way over my head but they recommended this lens 2" below the negative for best results. Pretty sure I might just be chasing my tail on this, but why not try?
Try to get an old condenser enlarger head or try something like this:
The condenser head vs soft head is exactly what I was thinking about. I used to work in a darkroom that had both heads available. Prints from condenser head always felt sharper and more contrasty, but man they were a lot harder to spot tone because they showed every flaw in a neg.
Distance from the light source seems like an easy enough variable to test too.
*whoops meant to say “cold light” head
The Fresnel lens in your link is only 2 inches in diameter with a focal length of 1.2 inches. This sounds an unlikely choice to me for any reasonably-sized negative or slide. If you do try it, please do let me know how it works out.
–Pat
Thanks - I’ll double-check that before I order. Seems like it would be wise to cover at least a 6X7 neg
I’m not optimistic that ANY lens placed between your light table surface and the negative you are photographing will improve your digital copy in any way. Your starting point is not at all like the condenser enlargers of long ago. Your light table plus your camera lens and sensor are irreversibly like a light-diffusion enlarger. This is not to say that you should not try it. Give it a try if you will. It’s just that I do not expect it to be helpful.