Thread: Ray's laser interferometer. Measuring angles

It's all done with mirrors.... I'll see if I can find a good diagram.

Ray

See if this explains the method.. lots of mirrors...
laserAngles-1.jpglaserAngles-2.jpglaserAngles-3.jpg

The setup is the same as flatness measurement, when we did the surface plate calibration, it is the same as using differential levels.

In 10x mode the angular resolution is 0.1 arc seconds or 0.5 millionths per inch.. the range is limited to +-10 degrees.

When you are measuring for squareness, that is, the angle you want to measure is 90 degrees you use the optical square.

the actual meaurement is the difference in path length caused by the corner reflectors being at an angle, so you are measuring the angle.
Here's a better diagram showing that.
laserAngles-4.jpg

Ray

all this stuff about dark and light begs this question... which is faster...dark or light...

It's worth putting that into context of mm/meter, when you are talking about levels.

0.1 arc seconds is 0.00048 mm/meter 5 millionths in 10 inches
1.0 arc seconds is 0.00485 mm/meter

5.0 arc seconds is 0.02424 mm/meter ... that's about where the most sensitive machinist vial levels start

20.0 arc seconds is 0.09696 mm/meter ... ( about 1 thou in 10 inches )

Ray

Another way of looking at these ridiculously small angles..
10 microns/meter is 2 arc seconds.. average dust particle is 2-10 microns,
So if you had a single dust particle under one end of a meter long straight edge that would be an angle of around 2 arc seconds..

Originally Posted by eskimo

all this stuff about dark and light begs this question... which is faster...dark or light...

Douglass Adams answered that one, nothing travels faster than bad news...

Originally Posted by RayG

It's worth putting that into context of mm/meter, when you are talking about levels.

0.1 arc seconds is 0.00048 mm/meter 5 millionths in 10 inches
1.0 arc seconds is 0.00485 mm/meter

5.0 arc seconds is 0.02424 mm/meter ... that's about where the most sensitive machinist vial levels start

20.0 arc seconds is 0.09696 mm/meter ... ( about 1 thou in 10 inches )

Ray

can we just stick to thou..or microns....i have enough trouble with these without bringing another measurement into it

how do you know all this stuff Ray

Can the interferometer determine the direction of angle, or does it only know an angular displacement has taken place, but not the direction..

I also see my above explanation of the workings was incorrect... The interferometer device does not send two light beams back to the detector, but it is the interferometer itself that does the recombining of the split light beam and sends that signal to the detector, and the detector reads it...

This new explanation explains to me how the angular measurement works as changing the distance between the interferometer and the reflector does nothing as far as changing the light/dark bits..... Only angular changes will change the light/dark bits...

If we wish to also discuss level sensitivities... A Talyvel 1, I think measures to 0.2 arc seconds, the Talyvel 2 and above does 0.1 arc seconds and better... I believe for surface plate checking you need 0.1 arc seconds or better....

Originally Posted by eskimo

how do you know all this stuff Ray

I saw Hitchhikers Guide to the Galaxy on TV

Originally Posted by .RC.

Can the interferometer determine the direction of angle, or does it only know an angular displacement has taken place, but not the direction...

You get the direction as well, depending on which beam is longer or shorter the +-sign on the display changes. ( there is a reverse switch on the front panel if you want it the other way round )

So, is the distance between the retroreflectors printed on it? or is there some other way to work that out?(or I guess they could all come from the factory set to a standard number). With out knowing that to some stupidly small number you have a problem right?

Stuart

Originally Posted by RayG

It's all done with mirrors....

Ray

Smoke and mirrors...apparently. Light and dark, as clear as day and night.

Originally Posted by .RC.

If we wish to also discuss level sensitivities... A Talyvel 1, I think measures to 0.1 arc seconds, the Talyvel 2 and above does 0.1 arc seconds and better... I believe for surface plate checking you need 0.1 arc seconds or better....

GQ had a Taylor Hobson, don't remember the model... might be still kicking around somewhere?

Originally Posted by RayG

You get the direction as well, depending on which beam is longer or shorter the +-sign on the display changes. ( there is a reverse switch on the front panel if you want it the other way round )

Ahh OK... I just found Agilent/HP must have all the manuals online and they mention a doppler effect... This must be how they measure beam length to determine direction of angle...

I wonder how many millions of dollars they poured into research way back when to create the laser interferometer..

Originally Posted by RayG

GQ had a Taylor Hobson, don't remember the model... might be still kicking around somewhere?

It is a Talyvel 1... I had to edit my above post, the Talyvel 1 was 0.2 arc seconds, not 0.1 as I originally mentioned... AFAIK Greg still has his..

Originally Posted by Stustoys

So, is the distance between the retroreflectors printed on it? or is there some other way to work that out?(or I guess they could all come from the factory set to a standard number). With out knowing that to some stupidly small number you have a problem right?

Stuart

The distance between cube corners is standardized at 2.0625 inches.. known to within 1 part in 10,000 ( that's their quoted tolerance ) so +- 2 tenths, and yes, that affects the ultimate accuracy.

So if you couldn't know the absolute angle to better than 1 in 10,000.. worst case would be when you are measuring close to the 10 degree limit of it's range, 1 in 10000 would be 0.001 degrees ( ~3 arc seconds ) but the error scales with the measurement, so smaller angles would have greater absolute accuracy? ( I think that's right.. )

Ray