[Metrology] How Thick Is A Sliver Of Light

Glenn Brooks

Glenn Brooks

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so, not sure how to properly phrase this question... Here goes:

When using the eyeball method to determine flatness of a workpiece, using a backlit straightedge, what is the 'thickness' of the smallest sliver of light one can see between the work surface and the straight edge?


Don from Suburban Tools mentioned in a video that the thinnest sliver of light he can see with the naked eye appears blue in color. And If I remember correctly, he reported this sliver of light was around .0002". Others have said the minimum thickness is around .002 - which seems rather a large gap...

Can anyone clarify??

Thanks much!
Glenn B.
 
I used to have to grind chisels to fit the curved sides of cutting dies. In my experience it was around .001 - .002.

Of course, Don Bailey is very knowledgeable. Maybe he has better eyes than me.
 
I've watched that video and didn't think to question it.
Good work.
I'm actively watching this thread, and am wondering if there it somehow ties in with metrology optical lens.

Daryl
MN
 
I'm wondering if a profilometer type device would measure it. They measure surface roughness and, I believe, flatness. The only issue may be the scrape from the contact type stylus. There sre non contact ones also.
 
Now you've done it!
No one is supposed to ask impossible questions before breakfast.

"In our experience, most tightly-toleranced ground or lapped metal parts become mirror-like at wavelengths longer than 5μm."
http://www.aspe.net/publications/Annual_2001/PDF/PAPERS/SFMETRO/1154.PDF
What is a "μm"?

I wonder if the thickness, or at least the ability to identify the thickness might have something to do with finish. The profilometer that TommyD references moves us that direction. Don Baileys stuff at Suburban is nicely ground. Perhaps it's why he gets tighter readings.

Daryl
MN
 
I would need to probably give it more thought but I suspect that the limit is similar to what limits magnification in optical microscopes (hence the need for electron microscopes at large magnifications).

This is referred to as the rayleigh criterion which pretty much means you are limited by the wavelength of the light you are using, which in the visible spectrum is from 300-700nm (thats 0.3 microns or about 0.000007 inches).

Of course this excludes the eyes ability to see such a thin slit of light.
 
Glenn Brooks said:
When using the eyeball method to determine flatness of a workpiece, using a backlit straightedge, what is the 'thickness' of the smallest sliver of light one can see between the work surface and the straight edge?
Click to expand...
I should think that would depend on many variables such as the quality, intensity and direction of the light, the quality of the machinist's eysight, and the color and finish of the materials.
 
Take a dial, vernier, or electronic caliper and open it to .002". Hold it up so you are looking through the gap at fluorescent lights. Now tilt the calipers gradually to narrow the effective gap, little by little. At some point you will see green light with diffraction patterns in it. Try it, it's pretty cool!
 
Bob Korves said:
Take a dial, vernier, or electronic caliper and open it to .002". Hold it up so you are looking through the gap at fluorescent lights. Now tilt the calipers gradually to narrow the effective gap, little by little. At some point you will see green light with diffraction patterns in it. Try it, it's pretty cool!
Click to expand...
Try this test before and after happy hour and see if there are unknown variables in the findingso_O.
 
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