What's in an optical bench?

[Grim Tuesday]

I am told that the professional repair people have optical benches that allow them to align lenses, check focus, collimate infinity focus and surely manyother things that us mere mortals can surely only dream about. I'd kinda like to go those things too. How do I build an optical bench for myself?

As an aside to that I've heard of people (mark hansen, in particular) testing lens quality on this optical bench. How the heck do they do that?

 

[Nodda Duma]

An optical bench is simply a specialized bench — legs and a tabletop — that dampens vibrations from the surrounding environment so that precise optical measurements and instruments can be used. The benches are designed to have extremely low resonant frequencies... typically around 1 Hz as opposed to 100-500 Hz for regular tables or benches. They do this primarily through mass...a typical 4’ x 6’ bench can weigh around 1000 lbs.

They are typically extremely heavy, costly, and better ones require an air supply as they are self-leveling. Older benches were made of granite (6” -12” thick depending on the need). Newer benches are honeycombed steel with 1/4” -20 tapped holes on 1” grid or M6 tapped holes on 25mm grid.

As for Mark Hansen measuring optical quality on an optical bench... as mentioned it’s basically a fancy specialized table so he is measuring optical quality using instruments that he has placed on his bench.

Google optical bench thorlabs and you’ll see examples.

Regards,
Jason

 

[AgX]

Grim, you likely mean a collimator.
It projects an image to be seen by the camera as coming from infinity. Basically one can replicate such with home means.

Collimators were/are typical in camera/lens repair shops, not multi-purpose optical benches.

 

[Nodda Duma]

AgX yes I was thinking he might mean an auto-collimator (proper term used for the instrument you describe) or, less likely, an interferometer... or, even less likely, an MTF bench.

 

[choiliefan]

I believe one of Thomas Tomosy's camera repair books shows how to build and use a simple auto-collimator.

 

[Luckless]

If you are looking to build a work bench to dabble in optical systems with, then do remember that massive multi-ton units that need three-phase power hookups and a specially engineered building and foundation would be awesome to have...

But would be many orders of magnitude of "complete and total overkill" for real dabbling.

Fancy benches just make things easier, faster, and more reliable to work with, but a good sturdy table out of an old kitchen and some sandbags can still provide a more than adequate work surface for some surprisingly precise optical work.

More more you "cheap out" on a bench, the more time you spend readjusting and double checking measurements, plus waiting for vibrations to settle down.

 

[AgX]

A plain and simple optical bench from the school's physics room, or even a plastic one with plastic lenses is a good learning tool though.

 

[Sirius Glass]

A good optical bench needs to be absolutely level and flat. That alone can get expensive; think a steel table. The rest of the equipment can be pricey but maybe you can get it used. Then the key is getting is set up properly and you learning 1) to check the set up, 2) be able to align the equipment on your own, and 3) use it to check out your equipment. Personally I would rather pay someone else to do the optical work and spend my money on lenses, film, ...

 

[AgX]

Sirius, what you decribe is a optical table.
A optical bench basically is a rod on feet. On this rod by means of sliding holders optical elements can be mounted. As being just one rod, basically only one optical axis can be installed.

Over here a view camera thus is called optical bench camera.

 

[Sirius Glass]

Sirius, what you decribe is a optical table.
A optical bench basically is a rod on feet. On this rod by means of sliding holders optical elements can be mounted. As being just one rod, basically only one optical axis can be installed.

Over here a view camera thus is called optical bench camera.

I am just so embarrassed. That is all I ever used. I am going to sit in the corner for a while and hang my head.

 

[AgX]

Sirius, of course I checked the terminology before as applied in the USA and so far I seemed to be right.

 

[Nodda Duma]

AgX, it’s not universal and not standardized. I would call what you describe as an optical rail...and it was terminology I picked up early in my career from others in my industry.

An optical bench is also considered the chassis of an integrated optical system...which is well outside the context of a photographic camera.

Perhaps “bench” is a catch-all...essentially any opto-mechanical stable structure for placing of optical components.

There is also an optical breadboard.

In any case, it’s not clear how any of those would aid alignment in and of themselves. As Luckless indicates above, all you really need for anything less than interferometry is a good solid workbench to place the alignment equipment (autocollimators, lasers, whatever) on top of. Even then you can mount your stuff onto dampened posts and get rid of 90% of vibrational issues.

There’s many different ways to check optical alignment and focus, so it’s hard to say what specific method the people mentioned in the OP use.

-Jason

 

[MattKing]

Every time I see a reference to "interferometry" or an "interferometer" I am immediately taken back to learning about Michelson Morley and my youth....
What can I say - I was a nerd!

 

[Mr Bill]

I am told that the professional repair people have optical benches that allow them to align lenses, check focus, collimate infinity focus and surely manyother things that us mere mortals can surely only dream about. I'd kinda like to go those things too. How do I build an optical bench for myself?

As an aside to that I've heard of people (mark hansen, in particular) testing lens quality on this optical bench. How the heck do they do that?

If you'd like some general background on things - various tests that might be done on cameras - you might have a look for the book, "Camera Technology: the Dark Side of the Lens" by Norman Goldberg. I'd suggest to check libraries as the style of the book is not for everyone. (Google Books has a preview.)

To just check focus, etc., an auto-collimator is hard to beat. It's possible to rig one up (commercially they are very expensive). Essentially it is like a small slide projector with a way to view a "return image." The collimator part has a small lamp illuminating a reticle; a lens projects an image of this "at infinity." This means that the "bundles of light rays" being projected are parallel. (Also, if you were to use a camera to look into the front of the collimator then focus on the the image of the reticle, it would appear to be at infinity focus.) Now for the "auto" part: if the instrument is equipped with a "beam splitter" at a 45 degree angle, it is possible to use an eyepiece to inspect a "return image" off to the side of the instrument. (This image should be at the same "virtual position" as the projected reticle.) To check calibration of the instrument you simply place a mirror in front of it, then look into the eyepiece; the projected reticle should be in focus relative to a reference reticle under the eyepiece. (If it is NOT in focus, this means that the projection lens is not making parallel rays - it is not properly focused at infinity.)

Ok, now that this is done, you wonder, how do I use this on a camera? Ok, you open your camera back and place a "front-surface mirror" at the film plane. (It must be silvered on the front as the reflective surface has to be at the film plane.) Now, if the camera is placed in front of the auto-collimator, angled just right, and with the shutter open, you can inspect the image returned by the camera. It will be in focus only if the camera is also focused at infinity. So you can dial the camera's focus adjustment to get a sharp image (via the auto-collimator eyepiece) and see if it matches the camera's infinity setting. If not, the camera's lens mount is not exactly right. The beauty of this system is that the instrument is very easily checked, via a mirror, and that it can be very sensitive - the camera lens handles the image twice (once in, and once out) meaning that the effect of a focus error is doubled.

Or, you could simply place a small ground-glass at the camera's image plane and examine the image with a very strong magnifier. You would need a test image at a very great distance to essentially be at infinity.

If you are using an auto-collimator designed for camera repair you don't need an elaborate vibration-reducing and leveled surface - your kitchen table is fine. You have to make fine adjustments in the camera's angle until the image lines up right, but this capability is built into the bench-top unit. Again, these units have been very expensive new, and today are pretty scarce.