A simple home-made laser microscope

This microscope is based on the excellent plans at http://www.colorado.edu/physics/phys5430/phys5430_sp01/PDF files/Projecting Microscope.pdf. Please go there to read up on the theory, etc. of the device. The rest of this page assumes that you have read and understand this paper...

Here are some tips that make life easier if you don't have an optical bench / you are throwing it together on a shoestring budget:

A rudimentary optical bench can be made with a common table and a bunch of cassette tape cases, CD jewel cases, small pieces of Plexiglass, and steel washers to hold the various lenses, etc at various heights from the table top.
Blocks of foam with round holes cut in them make great lens holders. To cut the holes, pinch a rectangular block of foam between a piece of wood and the (sharpened) end of a piece of pipe. Then rotate the pipe back and forth (about its long axis) to cut a round, pipe-sized hole through the foam. The hole diameter should be slightly smaller than the lens diameter so the lens it held snugly in the hole. Larger holes for bigger lenses are easily cut by tracing the lens onto the foam with a mark pen and then cutting the hole with a serrated steak knife.
Hold the object you are magnifying with the microscope in a paper clamp, which has been balanced on its end and is sitting on top of a plastic CD jewel case or a cassette tape case. You can then easily adjust the position of the object / adjust the focus by sliding the plastic case around.
Focusing becomes much easier if you look at the image on the screen/wall through a pair of binoculars. If the binoculars can't focus that close, place a convex lens (60 mm focal length works good) into one of the eye cups of the binoculars. It can be easily held in place with some masking tape. This will allow the binoculars to focus on closer objects.
Using a strong beam expander is nice because it gives you plenty of room to place the object you are magnifying. Unfortunately, it also makes the image on the screen significantly dimmer. Be aware of this tradeoff.
Try to use a laser that is fairly monochromatic. (Not all lasers are - I have seen cheap HeNe lasers that have simultaneously produced strong lines from deep red all the way through green.) Monochromatic lasers are nice because they make color-corrected lenses (doublets) unnecessary. An easy way to see if your laser is monochromatic or not is to use a compact disk as a diffraction grating. Shine the laser at the CD at an angle, adjust the CD until the first order diffracted beam (not the zero-order reflection) is projected against a white wall or white piece of paper that's 5-10 feet away, and then walk up to the wall or paper and see if there are any dimmer (and differently colored) spots to either side of the bright main spot. If there are, this means that your laser is not monochromatic. You can still use it if it is, but the images won't be as clear if you don't use color-corrected lenses.

Here is my procedure for assembling the unit / optical alignment. Note that this works best in a partially darkened room.

1. Put the laser on a few CD cases to get it an inch or two off the table top. This will allow larger lenses to be centered on the beam without cutting holes in your table. Turn on the laser and point it at the wall / screen so that the beam is more or less perpendicular to the wall / screen.

2. Adjust the laser so it's perpendicular to the screen: mount a mirror on the screen and adjust the screen or the laser until the reflected beam is going right back down the laser tube. It is also recommended to get the laser beam more or less parallel with your table top: measure the height of the beam right as it exits the laser and then measure the height of the beam 4 or 5 feet further down the beam. The heights should be more or less the same.

3. Mount a white piece of paper right in front of the laser, with a hole in the paper that's just big enough to let the beam through. This card will let you see the reflected beam as it comes back towards the laser, making the rest of the optical alignment MUCH easier.

4. Draw a big, black cross hairs ("+") on a sheet of white paper and tape it to the screen so that the cross hairs is centered on the laser beam. The cross hairs now mark the "true, correctly centered beam position", and they will be used for the rest of the alignment procedure to verify that each lens is centered on the beam. (Note that the paper is also nice because you can draw on it later, which makes measuring the sizes of things much easier when you are using the microscope.)

5. Place lens Lx1 (the thick, short focal length beam expander lens) right in front of the laser. (Note that the distance between the laser and Lx1 is irrelevant.) Center Lx1 on the beam by moving Lx1 up and down and side to side until the beam spot is centered on the cross hairs. Then make Lx1 perpendicular to the beam by adjusting the tilt and angle of Lx1 until its two back-reflections (both back towards the laser) are on top of each other and both going straight back down the laser tube again. Then check to make sure the beam is still centered on the cross hairs, moving the lens up-down and left-right if it isn't.

6. Place lens Lx2 (the thin, long focal length beam expander lens) right in front of Lx1. Then move it away from Lx1 (the spot on the screen will shrink) until the spot on the screen is exactly the same size as the beam is just as it exits Lx2. Verify this by holding a paper in the beam close to where it exits Lx2 and drawing lines on either side of the spot on the paper. Then hold this paper up right in front of the screen and verify that the beam spot is still exactly the same size. Once the sizes match, follow the beam from the screen to Lx2 with the paper and verify that the beam size stays the same - all the way from the screen to Lx2. If the beam shrinks to a spot and then gets big again, Lx2 is too far away from Lx1 - move it a bunch closer to Lx1 and then re-adjust the size again. Making the beam perfectly parallel like this is important because we can then move the object being magnified back and forth to get the best focus without changing the size of the object's image on the screen. Now center Lx2 on the beam and get it perpendicular to the beam as done in step 5.

7. Install L1, with the flat side towards the laser. Make it at about three focal lengths away from Lx2, so that there is room to slide the object back and forth to focus it. Note that the object comes into sharp focus when it's about two focal lengths away from L1 (add L1 and L2's focal lengths), so be sure to leave plenty of room between L1 and Lx2. Center L1 on the beam and get it perpendicular to the beam as explained previously.

8. Install L2, with the flat side towards the laser. Start with it just in front of L1 and then move it away from L1 until the spot on the screen gets small and then gets big again. Keep moving it away until the spot is as big as you want it to be. Center L2 on the beam and get it perpendicular as discussed previously.

9. If you want more magnification, put a negative (concave) lens on the exit side of L2. The distance between the concave lens and L2 does not matter. Center it on the beam and get it perpendicular as discussed previously.

If you wish to use the laser microscope to measure the sizes of small objects, then the microscope can be calibrated and used as follows:

Create a "calibration card" as follows: obtain some fine pieces of wire, measure their diameters with a micrometer, and then tape them over holes that have been punched in a thin piece of cardboard, as shown in the picture at the top of this page. Write the wire's sizes on the piece of cardboard so you don't forget them. If small diameter wires are not handy, then human, dog, horse, etc. hairs can be substituted.
Put the calibration card in the microscope and bring one of the wires into sharp focus. It is important that the wire be well-focused or it will be very difficult to make accurate measurements on the screen. Diffraction patterns on the screen make accurate measurements impossible, because diffraction can make large wires look small and small wires look large! Use a pair of binoculars to get the focus sharp, as discussed at the top of this page.
Measure the size of the wire's image with a ruler that is marked in millimeters. Write down this image size and the wire's actual size on a piece of paper. Repeat steps #2 and #3 with each of the different sized wires on the calibration card.
For each of the wires, divide the wire's image size by the wire's actual size. This will give you a multiplier which can be used to convert other object's image sizes to actual sizes. Hopefully, all of the multipliers will similar. If they are not, then errors are being made somewhere.
To measure the size of an unknown object, place it in the microscope, bring it into sharp focus, and then measure its image size (its size on the screen). Then use the multipliers obtained in step #4 to convert image size to real size.