An Updated Version of the Tester
Here's the photos of the tester with the scale I use. Black electrical tape keeps the plastic from glowing from the light. This is a slitless tester, so only a single razor blade is used, which is just securely taped to the vertical support which holds the light.Nothing could be simpler than this! Yet it works very acceptably.
The dark marks are degree markings. I made 180 marks in 2 degree increments around the edge and then I convert degrees to inches: 2 degrees is 2/360 of a revolution, which is 1/20" travel with a 20 thread per inch bolt, which is about .00027 inches. Actually even every 5 degrees is good enough (72 divisions for .0007 inch each- less than 1/1000 inch). The marks on the top of the lid are every 10 degrees.
I used a blue Skippy(?) peanut butter jar top for the dial as it had grooves in the outside edge- every degree, as it turns out!!! I used a permanent marker pen to mark the grooves.
The camera (a fixed lens digital) was too close so the image is a bit fuzzy, I apologize.
A better view, from standing over where the mirror rests. Note I added a dial micrometer. If you add one here as I did, you must reverse the signs of the readings, so be careful (I recommend you NOT copy this part of the design).
Note - you do not NEED this micrometer. Just make a dial marked off in 100's or do what I did above. My tests have shown the micrometer and dial perfectly agree. The micrometer is easier to use, although it somewhat encourages 'cheating' when taking readings. I found that my homemade guage agreed with the micrometer to the limits of ability to test +/- .001, which is better than I can judge shadows, so the limitation is NOT in the tester!
MSC sells dial micrometers .001 accurate with about 1 inch of travel for under $30 I believe.