Friday, June 16, 2017

Measuring pinholes with the Teslong USB microscope.

June 2019. When my university retiree account disappeared with a new change in policy, the pictures I uploaded to this blog while logged into that account disappeared. I'm working on fixing that but it's going to be at least a summer long project. 

If you follow me on Facebook, you may recall that I posted about a new favorite toy, the Teslong USB microscope. It's about the size of a pencil with a camera at the end that's held on a nicely adjustable stand.  There's a focuser on the other end so you can adjust it to various distances, and it has LED lights on the camera end to illuminate the subject. It's marketed as a microscope with up to 200 times magnification. (Just so you're not wondering about it, that's a .2mm pinhole at the right of the screen.)

In the comments to that Facebook post, Earl Johnson asked if you could measure pinholes with it. Well, since the magnification is a function of it's distance from the subject, which the device can't report to the computer, you can't tell directly what the size of anything is.

I usually measure pinholes with a scanner. Since the scanner knows exactly where the pinhole is and is scanning at a precisely known resolution, you can just read the actual size right off the screen.  The highest I can scan is 4800 dpi, which makes this .3mm pinhole about 60 pixels across. A bit of a problem is that the scanner software doesn't deal well with bright highlights and the pinhole is made from brass, so it tries to fix that and makes a way too high contrast image and it's often difficult to see just where the edge of the hole is. This example is better than most. Anyway, it works but it could be better.

I just happen to need a .2mm and a .15mm pinhole this week, so I got my brass, needles and emery paper out and tried to see what they looked like on the Teslong and see if I could measure them.

First I had to standardize some things. I wanted as much magnification as I could get, so the first thing I did was see how close I could get to something and still get it in focus. When you turn the focusing knob, you can feel stops at either end. So I put it at what I thought was its near point (lenses, ya know - by the way I just learned that term from Wikipedia) and then slowly pulled it away from the table until it was in focus.  That turns out to be about a quarter inch. It's pretty easy to get it set up to it's minimum focus repeatably.

200x magnification means movement gets magnified 200x too, so pointing is a little tricky. That swirly mark that's on the screen in the picture above was made when my drilling needle, which I was using to hold down the brass against the table, swept across the screen. Pretty exciting. By carefully moving it by nudging the base around you can get enough control moving it to find what you're looking for. The field is much less than that illuminated area, but it's possible to find the approximate center of the spot of light. You don't need the pinhole to be in the center, you just need it on the screen.

The camera uses the shareware video capture program VLC to display the image in a 1280x720 pixel window. VLC wants to capture video, so the easiest way to get a still image is to do a screen capture.

What I need now is a known object to compare my pinholes to.

I happen to have several sizes of Gilder EMS pinholes. They come in vials of 100, but Earl Johnson, I think, still resells small quantities for about $1 USD apiece.  I'll let him give you contact info in the comments if he wants to. Anyway, my stash includes some .2mm apertures - just what I'm trying to measure.

They look pretty good under magnification.

Selecting just the pinhole, it measures 100 pixels across (how convenient), so now I can measure with it and it's a little better than twice as sharp as the scanner  (recall that my scanner had 60 pixels for a .3mm hole).

I happened to have a pinhole that I recently measured with the scanner as .2mm, so I put that under the scope. I took the new image and pasted it next to the image of the Gilder aperture. I guess my measure with the scanner was pretty accurate. They're exactly the same size.  But, what you can see now is that around the edge of my homemade pinhole is a bit of fuzziness, probably dust from sanding off the burr on the exit side of the needle hole.

I bet some canned compressed air would get rid of it, but I don't have any of that. Blowing with my lips didn't do anything. So I stuck the needle tip in as gently as I could and spun it a little. That pretty much did the trick cleaning out the schmutz without enlarging the pinhole. Not bad. I think I'll use this one.

Now I've got to try to drill a .15mm.

I used my standard method for .15mm pinholes, drilling right against a hard table top, this time with the needle held in the eraser of a pencil.  Much to my surprise, it measured 75 pixels - exactly .15mm - and very clean with no fuzz. Two for two. I guess I'm starting to get the hang of this.

Both pinholes are now mounted on cameras so we'll see how they do when I get some film through them.

The Teslong seems like a pretty good deal for $41.

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