Thread: Optical flats.

Originally Posted by SC_RUFCTR

Hi All

Educate me about Optical Flats. I've seen them around in labs and tool making areas but I have no real idea what they're used for.
The reason I ask is because they are such a precision item with impressive tight tolerances and characteristics.

Hello,

I'm certainly no expert but in my Uni days we used optically flat glass (optical flats) when using laser for very accurate and sensitive distance measurement. There are many different types of apparatus that are also designed to be optically flat. The glass is typically made from thermally stable material e.g. Pyrex so that diminesional changes due to temperature differences are minimised, similar to the manufacture of telescope lenses and mirrors. Example of use: There are certain circumstances when you may wish to pass a beam of light through a medium such as glass and not have it scattered by surface imperfections. Another reason why the glass needs to be accurate relative to the wavelength of light is because small variations in path travelled by the same beam of light (due to surface imperfections) will cause optical interference which may be undesirable in certain circumstances. The term "optically flat" infers that the surface imperfections are configured to within the order of wavelengths of light. For instance, an optically flat piece of glass may be configured to within a half wavelength. The optical spectrum of light is between 400 - 700 nano metres (1 nanometre = 1000 microns) so take 555nm (middle of the spectrum, colour green) half a wavelength would be approx 280nm or 0.28 microns or just over 1/100 of a thou.

Just a small example for the uses of optically flat glass and what is means.

Cheers,

Simon

Optical flats are deemed to be (in the world of metrology) as the flattest an object can be.

They offer flatness within 25 nm (0.000001").

The are use to check out of flatness condition of gauges and instruments such as gauge block or the anvils of micrometers.

So long as the surface in question is relatively flat, flat ground and lapped and reflective, the light passing through the optical flat placed on top of the item being checked will reflect back. A fringed pattern will appear as shown attached. This is a quick way to see the flatness of the entire surface. Under green or pink monochromatic light, the dark bands are roughly 250 nm apart.

Hi Guys,
Great stuff but I am struggling with some of your numbers
"(1 nanometre = 1000 microns)"? Backwards or did you mean something else?

"roughly 250 nm apart" If the lines are only 0.000250mm apart doesnt that mean that block in the picture is only about 0.000750mm wide? What am I missing?(or is this how it looks under normal light?)

Should "roughly" and "nm" be in the same sentence?


To many 0's

Stuart

I don't think the dimensions of the fringes are significant, just the population. I have a nifty little Mitutoyo mic calibration set here that has an odd assortment of blocks, and an optical flat. It was meant for cal labs to use in certifying micrometers. The flat is used to asses the anvils, and the gauge blocks are used to check the mics at random positions on the screw*

* Most user cal rods that you see measure mics at multiples of the screw pitch. Thus, they are only checking the same radial position on the screw. A set of cal blocks, on the other hand, checks the thimble screw at non-repeating intervals and hence is a much more trustworthy staistical approach to metrology calibration.

If you were measuring for sheep stations you'd of course want the same deviation map as your gauge blocks. My set varies all over the place +/- a millionth or two, but they were last checked 40 years ago. Now they could be anywhere until they are again sent in for cal. which ain't going to happen any time soon. If the optical flats say a particular block is suspect then it'll get red sharpie all over it and I 'll make do without it.

Greg

Aiming to have 1.137 beers before dinner

Originally Posted by GZBMW

Optical flats are deemed to be (in the world of metrology) as the flattest an object can be.

They offer flatness within 25 nm (0.000001").

The are use to check out of flatness condition of gauges and instruments such as gauge block or the anvils of micrometers.

So long as the surface in question is relatively flat, flat ground and lapped and reflective, the light passing through the optical flat placed on top of the item being checked will reflect back. A fringed pattern will appear as shown attached. This is a quick way to see the flatness of the entire surface. Under green or pink monochromatic light, the dark bands are roughly 250 nm apart.

Hi there,

I'm interested in the fringes in the picture. Being used under monochromatic light (light of a single wavelength) I was under the impression they would be interference lines or an interference pattern. If this is the case then wouldn't they be spaced by integers of whole wavelengths?

Or are these lines created by some other optical phenonema?


Hi Stuart,

It is customary to group numbers into groups of thousands, or 3 orders of magnitude. For instance,

SI unit for distance is Metre unit m

so going up, 1 Km = 1000m (nothing new here)

and, going the other way in smaller dimensions....

1m = 1000 millimetres (milli means 1/1000)

1mm = 1000 micrometres (micro means 1/1000000)

1 micrometre = 1000 nanometres (nano means 1/1000000000)

1 nanometres = 1000 picometres (pico means 1/1000000000000)

insert another measurement here in addition to the standard numbering system:

The angstrom. 1 angsrom = 10 nanometres. Angstroms are generally used in crystalography and other sciences that deal with dimensions of approx the size of atomic spacings since the average atomic spacing is approx 1 Angstrom or multiples of.

1 picometres = 1000 femtometres

1 attometre = 1000 femtometres

So on and so on.....

Hope this helps!

Cheers,

Simon

Hi Simon,
I was with you till I got to here
"1 picometres = 1000 femtometres

1 attometre = 1000 femtometres"
A quick google got me to "1 femtometer = 1000 attometer"
Not likely to be a problem for me for awhile yet

Thanks

Stuart

Originally Posted by Stustoys

Hi Simon,
I was with you till I got to here
"1 picometres = 1000 femtometres

1 attometre = 1000 femtometres"
A quick google got me to "1 femtometer = 1000 attometer"
Not likely to be a problem for me for awhile yet

Thanks

Stuart

Don't you hate it when your shed warms up by 2 degrees and everything is many femtometres to big. Geez even spell check doesn't know what that is. Good work Simon, you have opened my eyes to the truly small......

Ewan

Originally Posted by Stustoys

Hi Simon,
I was with you till I got to here
"1 picometres = 1000 femtometres

1 attometre = 1000 femtometres"
A quick google got me to "1 femtometer = 1000 attometer"
Not likely to be a problem for me for awhile yet

Thanks

Stuart

Oops. Sorry. I did it from memory but i must admit i may be a tad rusty!

Did a precision grinding diploma back in the eighties The first assignment was to grind an optical flat by hand Cannot say how long that took other than wore out a pair of runners stepping in a circle going round a grinding plate going down through the different grades of emerys to the oxides .Could probably still do the stroke in my sleep today.

Hi Simon,
I do it all the time.
Just making sure I wasn't missing something(which I also do all the time)

Sounds interesting fubar, I've only read about making lenses like that. I guess making a flat is pretty much the same?

Stuart

Originally Posted by simonl

Oops. Sorry. I did it from memory but i must admit i may be a tad rusty!

I'll have forgotten by tomorrow morning, but i guess i have never come across it before. At least next time i do i'll go "hey where have i heard that before?"

Thanks again for the fascinating incite

Ewan

Originally Posted by Stustoys

Sounds interesting fubar, I've only read about making lenses like that. I guess making a flat is pretty much the same?

Stuart

I too thought it was a similar process and would be very keen to hear more about the process. I've heard it's actually not all that difficult (though I guess that's a relative term), but does take a lot of patience.

Pete

Originally Posted by Pete F

I too thought it was a similar process and would be very keen to hear more about the process. I've heard it's actually not all that difficult (though I guess that's a relative term), but does take a lot of patience.

Pete

I'd be interested to hear about grinding a flat too. I know that grinding a parabolic mirror is a relatively simple (but long) process of random movements of one flat over another in a W movement using varying grades of grinding paste. Turning the pieces bit by bit as you go, and the randomness of the process tends to turn the top into a concave and the bottom convex. The longer and more random the process, the more accurate you get. Accuracies in the order of a 1/4 wavelength of light (150nm) are easily achievable by the backyard punter who wants to grind his/her own mirror for a telescope. In theory, a good telescope mirror should be at least a 1/4 of the diameter in thickness. So, an 8 inch mirror should be at least 2 inches thick.

More useless crap from ME!

Simon

Simon

An Attometer is only 10-18 while a


Yoctometer (ym) is 10-24 (0.000000000000000000000001) m


At the other end of the scale we have a Yottameter (Ym) and it is 1024 (1,000,000,000,000,000,000,000,000) m


The estimated size of the universe is 930 Ym (93,000,000,000 light years)


Can’t claim any real knowledge, very interesting website which shows it all graphically is HTwins.net - The Scale of the Universe


Cheers,
RossA