r/OpticalAlignment

When lenses are assembled using the drop-in method, the lens elements are constrained by the cell bore into which they are “dropped”. Fig. 1 gives an example of a drop-in assembly where the first lens in is supported by a seat and the cell wall while subsequent lens elements are separated by spacers.

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For all the lenses in this assembly their diameter is critical. If they are too large, they won’t fit in the cell and if they are too small, they will be randomly decentered. Thought of another way, if the diameter is too small the optical axis of the lens may be parallel to but will not be concentric with the axis of the cell.

If the lenses have wedge, this is also a problem because the optical axis of the lens will not be parallel to the axis of the cell even if the lens is centered. This means that every lens must be inspected for diameter and wedge before kitting will a cell for assembly.

We do drop-in assembly because it is cost effective for commodity type lenses. This means the method of inspection must also be cost effective. The singlet in Fig. 1 is used to illustrate a method of inspection using our Bessel beam centering station in Fig. 2.

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To the left in Fig. 2, the centering detector, a PSM, is centered on the projected Bessel beam before the lens in inserted. Since the lens is the correct diameter and free of wedge the Bessel beam propagates through the lens without any deviation. A Bessel beam is used because it propagates like a single ray in a ray trace. The PSM centroids on the Bessel beam with a sensitivity of < 1 μm.

In the middle view, the lens diameter is too large, and this shifts the optical axis off the projected Bessel beam in the plane of the Figure causing the beam to bend so that it passes through the focal point that is on the optical axis. The PSM which was centered on the Bessel beam will show the deviation in proportion to how far it is beyond the focal plane of the lens. Typically, the PSM is placed 10 times the lens focal length so a 2 μm larger diameter will shift the beam by 10 μm at the PSM (10 times the radius difference of 1 μm).

Because the lens has no wedge in this case, its optical axis is parallel to the incident Bessel beam. This means that if the lens is rotated 180 ° against the edge constraint the optical axis does not change position and the decenter of the Bessel beam at the PSM remains constant.

On the right, the lens has wedge, so the optical axis makes an angle with the incident Bessel beam which in turn means the Bessel beam is deviated going through the tilted lens. Now, however, if the lens is rotated 180 ° the optical axis tilt is in the other direction and the Bessel beam will also be deviated in the opposite direction. This is how you tell wedge from a difference in nominal diameter. Fig. 3 is what you see on the PSM monitor if you insert a lens for inspection.

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The origin in Fig. 3 is the Bessel beam centroid location before inserting the lens on the window in the centering station and locating it against the edge constraints. For a lens that has a diameter that is too large and some wedge you might get a centroid at the position noted as Beam at 0 deg. Once the lens is rotated 180 degrees you get the second centroid. The distance from the origin to the average location of the two centroids is proportional to the error in diameter from nominal. The distance from the average to either centroid is the error due to wedge.

This simple inspection can be done almost as fast as you can set the lens on the window and push a button to log the centroid location.

Everyone seems to agree that the line between the centers of curvature of a single lens element is the optical axis. Because it is the line between two spherical surfaces, it is normal to the surfaces so a light ray along the optical axis is not deviated but exits the lens coaxial with how it enters. Does anyone see a reason not to adopt the same definition for an assembly of lenses such as a camera lens or Cooke triplet? To the best of my knowledge I do not know of anyone who has discussed this question. I know several people who claim there is no optical axis to an assembly of lenses, but I don't believe they were thinking in terms of my definition.

The most basic optical instrument used for alignment and testing of optics is a collimator, an instrument designed to produce a parallel beam of rays, or a plane wavefront, from a point source of light. Turning words into hardware we have something like Fig. 1 where a single mode optical fiber serves as the point source and is placed at back focus of an infinity corrected doublet lens.

Fig. 1 A simple collimator with a point source of illumination.

An illuminated target in the same plane could serve as the source

Collimators are used as a light source for testing camera lenses on a nodal slide optical bench. The collimator simulates a point source, or in astronomical terms, a star, at infinity. For lens testing, the focal length of the collimator is typically 5 times or more the focal length of the lens under test so that the star appears “perfect” to the lens under test. Collimators are also used in MTF measuring instruments to project targets with a structured pattern into the lens under test to measure the lens quality.

An autocollimator (AC) is a collimator with a beamsplitter and an eyepiece so you can see where the reflected “star” falls in the eyepiece of the instrument, as in Fig. 2. A common method of packaging an AC is with a precision ground barrel designed to mate with a mount so that the axis of the barrel can be adjusted in 4 degrees of freedom (DOF). The reticle crosshair in the AC is centered on the axis of the barrel. When the AC barrel axis is normal to a plane mirror in front of the AC, the reflected image from the mirror will be centered on the crosshair and will not move when the AC is rotated about its axis.

Fig. 2 Simple autocollimator shown for visual use, or for a point source and digital camera.

For autocollimators that do not have a mechanical reference axis such as a precision barrel, a cube corner reflector is used to center the reference crosshair on the axis of the instrument. The cube corner reflects light back upon itself so an image of the source as seen in the eyepiece is centered on the source. The crosshair in the eyepiece is set to zero on the image of the source.

Notice that an AC is only sensitive to two DOF, the two angles the plane mirror is tipped from being normal to the axis of the AC. Typical barrel type ACs have a full field of view of about +/- 1° but the reticle is labelled to give the angle between the plane mirror and the axis of the AC, or +/- 30 are minutes.

Currently there are electronic ACs on the market with sensitivities of 0.1 µradian or better. This precision can only be obtained if the plane mirror is close to the AC because of environmental influences. A particularly useful AC and mount are made by Nikon, see Fig. 3. The mount is well designed to orient the AC in any possible direction and a single axis filar eyepiece reads the angle in both directions. The instrument is especially helpful for measuring odd prism angles because of the agility of the mount. It can also be focused slightly to keep the crosshair crisp in case the plane surface has a slight curvature.

Fig. 3 Nikon autocollimator fitted with a digital camera (from Nikon online catalog)

Notice that ACs cannot be used without some auxiliary hardware because all they measure is departure from normal. As an example, assume the AC, mounted vertically looking downward, is zeroed out against a plane surface such as the base of the Nikon instrument. Then the AC measures parallelism when sample plane mirrors or windows are set on the base plane surface. They are also useful for measuring errors in prism angles where the faces of the prisms are parallel to each other looking through the prism.

Another instrument in this class is the alignment telescope (AT), an AC with more parts to give it more functionality. We use an AT to determine where an axis is or use it to set up an axis because the AT focuses in any plane between the instrument and infinity. This gives information to determine a line, 4 DOF, in space rather than just two angles.

I have used Fig. VIII from the 1957 Kueffel AT US Patent 2,784,641 in Fig. 4 to show the optically important parts. Starting at the left there is a Galilean 5:1 reverse beam expander and a meniscus element (714) which if decentered, decenters the field of view in the eyepiece, without changing the focus. This is followed by a Newtonian telescope (716) focused on a crosshair reticle that is projected from the AT. Following the reticle is a beamsplitter to bring in a light source, and an erecting eyepiece so the view is right side up when viewing thorough the telescope.

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The objective (716) on the Newtonian telescope is used to focus the AT from very close to the front of the instrument all the way to infinity in an almost perfect straight line. Because the aperture and corresponding focal length are about 5 times smaller than the main objective (702), the distance the Newtonian objective must move to achieve this large focus range is very much reduced from moving the main objective.

As opposed to an AC, an AT determines 4 DOF, and thus an axis. It does so by first focusing on a far target, and then on a close target. When both targets are centered on the crosshair in the AT, the axis of the AT is coaxial with the line between the two targets. Making this adjustment is a little trickier than it first sounds. It is an iterative process, and the adjustments must be made in the right order, or you get farther and farther from alignment.

When focused on the far target, change the angle of the AT tube to bring the far target on the crosshair. When focused on the near target, translate the AT to bring the near target on the crosshair. Even following this order of adjustments, you are usually not rotating about the optimum center, so you end up either over or under-shooting the angle adjustment. With patience good alignment is achieved to the precision of the instrument and your ability to set the target on the crosshair.

Since this is a rather long blog, I will put off a discussion of the last instrument, the autostigmatic microscope until next week. I will also discuss a few mechanical gauges and tools that complement optical alignment tools.

For reference see:

https://www.brunson.us/products/optical-tooling-products/optical-instruments-amp-accessories/alignment-telescopes.html

https://industry.nikon.com/en-us/products/optical-manual-measuring/others/autocollimators-6b-led-6d-led/

https://www.vermontphotonics.com/electronic-autocollimators

https://www.taylor-hobson.com/products/alignment-level/autocollimators/ultra-dual-axis-digital-autocollimator

https://trioptics.com/us/products/optitest-visual-measurement-instruments

If I have overlooked anyone in this list, I apologize.