Checking and calibrating a cheapo microscope.

[DIYSwede]

I bought one of these cheap BG microscopes* a few years back, for checking tool grind finish etcetera,
but have always found the screen lacking any sort of scales a big drawback:



Winging some sort of DIY shadowgraph from one of these has for long been a thought of mine,
actually doing some measurements with it another. Still lacking some darned scale.

Up until now I've been searching for some kind of reference scale to put in for comparison,
I've tried umbrella nylon fabric and other found objects with disappointing results.

When putting a problem aside, I usually (unconsciously?) come up with something after a while.

Like today - finding an old Dell monitor in the electronics dumpster at work,
I remember tearing one of those apart (because reasons...) a few years back,
and between the LCD and the backlighter there was some odd looking opaque and weird foils.
One of these might be worth taking for a spin?

A 5 min monitor instant teardown revealed the needed foil with its rainbow sheen,
and a few scratches with a nail revealed it was ribbed in one direction, and finely so too...
Its purpose is to diffuse the top and bottom cold-cathode tube's light into an even screen light.
In the cine business there's a gel filter called "Brushed silk",
which turns the round output of a luminaire into a wide oval - same stuff but VERY much finer grade in this foil.

I cut off a piece, put it in my wallet and just tried it out when getting back home to my "Closet Workshop":

Minor triumph I'd say: It consists of linear ridges spaced 1 thou apart, as shown:





1 sq mm paper for background, and 40 lines/ mm... equals 0,025 mm c-c, which is about as good a resolution I'll need.

Field of view (@ highest mag) of my particular camera* is 94 thou wide and 71 thou high,
so now I'll get working to score some OH foil for the office laserjet,
wing together some bottom scale grids, overlay circles, rulers and protractors to lay on the screen.
Followup will come when time permits. But MY scales won't be "Inferial" but "glamorous Metric" ones!

Perhaps a DIYed X-Y table beneath with fine adjust differential screws: M4 x 0.7 & M6 x 0.75 (equals 0,05mm pitch)?

*MUSTOOL G 600, 4,3 " LCD @ 3.6MP and 20 bucks.
Yes, the "microscope stand" IS a friggin discarded orange press with a hard drive disk as an adjusting wheel!
Told you I was cheap-to-the-bone!

Just my 2 cents, and YMMV.

Cheers, Johan

PS: Any Uncheap HMT:ers can easily shell out 230 bucks +postage at Edmunds Optics
for a "real" 70 lines/mm, 1" sq diffraction grating if you so choose, it works just the same -
but then you'll at least have a Branded product, instead of my free piece of salvaged plastic!
https://www.edmundoptics.eu/p/70-gro...splitter/6555/

[Toolmaker51]

I too have a seriously underutilized microscope, held on to for same use, with the same shortcomings. In desperation, only did comparative measurements against pin gauges, serious juggling involved.

Bookmarked your post Mr. Johan DIYSwede by title alone, maybe before the page loaded fully.

In all time, had one shot at a genuine toolmaker microscope, only to be outbid by mere cents. Timed auctions were that way, before a buffer to collect bid-swarms.
Anyway, will be following the progress closely. Not dismayed by metric units, that's what calculators are for.

[DIYSwede]

The Cheapo's Guide to calibrating the ditto USB microscope for relative measurements:

Background: USB microscopes are cheap and readily available, and mostly used for merely looking at different magnifications
but could still be used for pretty advanced measurements hitherto reserved for much more expensive units.

Though manufacturers brag about magnifications, Über-resolution etc - very few (if any?) offers any reticles or rulers within the gadget.

The following is merely an introduction into "how-to" use yours for measurements of length and angle, by using
1) A small reference object with known dimensions and properties, and
2) a ruler to measure the object's pic on the screen of your choice.

From there you can deduce the actual magnification for that given setup ( scope height, zoom grade, focus, etc ),
and make DIY printed screen OH film overlay reticules, rulers, protractors or circles of known dimensions,
with which to compare the next unknown object.

You'll need:
USB microscope, stand with height adjustment and table perpendicular to the microscope's optical axis.
Some form of lighting for the object. A ruler for checking screen size, object relative length on screen.
Optional is USB cable and laptop w a larger screen than the microscope's teeny-weenie one.
Then: The crux is getting a reference object, and as I've suggested in the post above (as this IS a forum for Home-made tools)
you might get away with objects with easily determined physical properties like the 25 c-c micrometer prism film mentioned above.
This foil has the decent property of being flat, and will thus stay in focus in the very limited depth of field available.
Gauge pins, Jo blocks etc have some height to them making focusing harder... Then there's edge diffraction.

Another good thing is the many (40 of 'em) lines covering, say a 1 mm gap to be measured, thus minimizing reading error.

So - I established the frame size of MY 'scope at max magnification,
objective distance and focus as being: 2,35 mm wide and 1,77 mm high,
and the frame size on MY laptop measured 209 x 157 mm: giving a scale of 1 : 88,8.
From there I can (in some drawing program of choice) just cobble together a ruler for direct measurements,
(or a reticle/ grid of desired resolution, a hairline cross for using a protractor or whatever crosses your mind)
print it out (mirrored image - you want the print towards the screen for less parallax) on OH film on any 1300 dpi office printer.

This is just some theoretical basics - haven't gotten around actually doing anything yet!

Just a short add-on. More info will follow in due time.

Johan

[Toolmaker51]

All that is perfectly logical. Every comparator made has to overcome these factors. It's all about scale against anticipated part size vs depth of field. Using pins as described required elevating the part, ie juggling.
Largest I've seen was a 60" (?) screen, the housing could have been 12' long. Many are 12" and 20"; those on certain tool grinders are just 4" or 6", supplied by Stocker & Yale.

[DIYer]

Thanks DIYSwede! We've added your Microscope Calibration Method to our Measuring and Marking category,
as well as to your builder page: DIYSwede's Homemade Tools. Your receipt:

Microscope Calibration Method by DIYSwede

tags: measurement

[tonyfoale]

I bought one of these cheap BG microscopes* a few years back, for checking tool grind finish etcetera,
but have always found the screen lacking any sort of scales a big drawback:

................................

*MUSTOOL G 600, 4,3 " LCD @ 3.6MP and 20 bucks.

The cheapest that I could just find was > 90 euros.
I have just the microscope part which you need to use with a laptop and they are around the 20 euro mark. Are you forgetting that 20 bucks was what you told your wife it cost and not what you actually paid?

https://www.amazon.es/s?k=MUSTOOL+G+600&__mk_es_ES=%C3%85M%C3%85%C5%BD%C3%95%C3%91&ref=nb_sb_noss

[DIYSwede]

Are you forgetting that 20 bucks was what you told your wife it cost and not what you actually paid?

Nope - it was on BG at some kind of 30-ish % introductory discount back then.
Today's prices are quite a bit steeper as container shipping costs from China has skyrocketed in the last 6 months,
and their prices also seem to have gone up quite a bit too since then (AFAIK).

I don't have to blow hot air into my dearest's ears, as she sees me being cheap-to-the-bone thru all my daily "honey-dos" too...

BTW: The foil I've scored is from OLD monitors, with CCFT backlighters (which went outta fashion some 10 years ago),
I cannot say what contemporary LED-fed monitors use inside.
Also: My foil HAS only 20 lines/ mm - but when lying with its ridges upwards,
and lit from above each ridge gives 2 light maxima, thus 40 lines/ mm resolution...

BONUS: For HMT:ers checking their microscopes (incl SEMs) with higher magnification,
an ordinary CD has a pitch of 1,6 µm whereas a DVD has a pitch of 0.74 µm



Even simpler and less cumbersome is just zooming in your cell phone display (which resolution could be found out),
as this is a pic of my worn iPhone SE's 640x1136 px display (showing an overexposed white pic):



Note: Vertical resolution IS better than horizontal

Cheers
Johan

[rebuilder1954]

A quick search revealed this diffraction grating from Amazon at a more attractive price. Probably not the equivalent of Edmunds product, but maybe more suited to this task.

https://www.amazon.com/Lines-Linear-...27815255&psc=1