Reading Optical Flats Part 1

I’m new to optical flat inspection, so bear with me as I walk through some of the things I’m trying to learn. This first of a series of videos presents the basics . . . possibly in excruciating detail . . . but, hopefully, a few points worth presenting to establish a firm basis for things to follow.
I started using optical flats to evaluate progress while generating small lapping plates using plate to plate lapping techniques. The goal was to evaluate flatness on the order of 1/4 wavelength . . . about 6 micro-inches; although, I’m also interested in evaluating old gage blocks and other old metrology equipment . . . a potential subject for some future videos.
While other metrology equipment offers the potential for accurate measurements . . . certainly in the 40 micro-inch (about 1 micron) range and better . . . optical flats offer the potential for measurements down to a few micro-inches with simple equipment. While new optical flats can be quite expensive, some are available used and as new old stock at tolerable prices. Even used monolights can be expensive so we’ll discuss some shop made options in a later video. For those interested in amateur . . . or even more serious . . . metrology work, optical flat inspection is worth consideration.

[@brucewilliams6292]

This is the very best explanation of how optical flats work that I have found. I sincerely appreciate all the work you put into this video series. Merry Christmas and Happy New Year!

[@workshopengineering]

Thanks for the kind words. Hopefully more to follow.

[@hightttech]

Agree 100%. Saving this one for future reference.

[@gjvnq]

I think the units would be a bit more clear if they written as "μin" for millionths of an inch and "μm" for micrometers

[@danproctor15]

Thank you for making a great easy to understand video. Wish I'd seen this earlier.

[@workshopengineering]

Glad it was helpful!

[@xyshawnlau5522]

Can't wait you make more video about optical flat. You're the best.

[@workshopengineering]

Thanks. Working on more as I learn about this subject.

[@CharlesXavierRoy]

Wow, great video !

[@workshopengineering]

Thanks!

[@fenghao9736]

Sir, I like this video pretty much and will wait for your next!

[@workshopengineering]

Thanks. I've released Part 2. It's on my channel.

[@HTMWorkshop]

Nice video!

[@workshopengineering]

Thanks!

[@Plawski-English]

So I understand that to use these you need a special light? I bought one of these really cheap on ebay for a couple bucks years ago (no reserve $1 auction for a new-old-stock item) but don't see any paterns forming when placed on a polished metal surface no matter if viewed under daylight or artificial room light

[@preetamyadav7952]

Nice

[@angelus_solus]

So THIS is what Gingery was talking about.

[@pdrcanada4005]

What lighting are you using to get a yellow glow?

[@Metaldetectiontubeworldwide]

Awesome explanation , well done !
Subed directly !☆
-One question i always asked myself , but never got answered nl.:
-What if we use a monochromatic ultraviolet lightsource , the wave lengte is way shorter.
Meaning we could even get smaller measurement scaler ?
Or am i misding something ?
Grtzz from the Netherlands Johny geerts

[@workshopengineering]

Yes, if the shorter wavelength were still long enough to see (or use an imaging sensor that could). But there are other factors such as view angle (that has significant impact upon fringe spacing calibration . . . to be discussed later) and light bandwidth and part surface finish that affect fringe width. Conventional monolights are probably a reasonable, practical sweet spot for typical measurements without more complex setups.

[@Metaldetectiontubeworldwide]

@@workshopengineering thanks for answering my question ,!
Good video´s a Joy to view

[@sto2779]

Can I use an optical flat to check flatness for a surface plate?

[@workshopengineering]

Generally, No. First, an optical flat should be at least as large as the inspected surface to accurately measure flatness of the entire surface and a surface plate sized optical flat would be very expensive. Second, the surface plate surface finish would need to be very good (preferably on the order of a few microinches) to form clear interference fringes. The surface needs to be quite reflective as well. Granite plates typically don't meet these requirements. While a scraped metal plate probably couldn't meet these requirements, a lapped metal plate could.
While optical flats are best suited to measuring flatness on the order of several tens of microinches and better, even high accuracy surface plates exhibit flatness errors large enough to make fringe interpretation difficult (if fringes formed) and other instruments are better suited to this application.

[@emilioarroyomohamed]

I believe the source of monochromatic light you have is a sodium lamp not a helium lamp.

[@workshopengineering]

The DoALL literature says helium. Others may use sodium. Charts that I have say that helium wavelength is 23.2 microinches while sodium is 23.6 microinches so they're very close; just at the breakpoint between yellow (helium) and orange (sodium).

[@emilioarroyomohamed]

@@workshopengineering Yeah i saw in video number 2. Is kind of surprising for me, because for what i know helium light is not monochromatic, maybe there is some kind of filter to absorb some of the emission lines. I made my own with a sodium vapor lamp i bought on ebay and it works very well even if it needs about 5 min to heat up to working temperature. Thank your for your answer and your video explanations!

[@csonracsonra9962]

People watching this should consider that peices of granite from ancient Egypt have been found to be absolutely optically flat. How is this?