The Cheap and Dirty Foucault Tester

Mark III

Derived from the highly successful Mark II, the Mark III is much more capable thanks to the addition of measuring scales (for use with a a Couder screen) and various other minor improvements.

Here's the back side (the side toward the mirror):

Here's the front side (the side you sit behind):

Here's the auxilliary laser pointer alignment tool. This makes aligning the mirror a snap.

And finally, the scales. I hacked them up with Photoshop, printed them out and stuck them to the tester. Each rotation of the screw moves the carriage 1/20 of an inch which corresponds to one division of the linear scale in the back. The scale on the knob is divided into 50 increments each of which is just 1/1000 inch. With a little practice I've been able to get repeatable measurements to about .005 in.

Further Improvements

• I added three rubber feet on the bottom to make it sit stably.
• I swapped the LED for one with both red and green emitters. This lets me test in light more closely matched to the eye's most sensitive region. But the green LED is pretty dim so a completely dark room is essential.
• I added a weight (a 1/2 lb salmon ball!) to the carriage to help prevent it from jumping off the nail.

Remaining Problems

• the linear scale is hard to use: the pencil doesn't usually align to a mark when the knob is at zero.
• the carriage screw still wobbles. Maybe a T-nut would help.
• there's too much friction between the lateral adjuster screw and the cubicle name plate. A piece of glass would be better.

See the Mark II page for more details.

In Action