Packaging Optical Systems
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Sauter Industrial Design
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By Ken Sauter
Sauter Industrial Design
I have to say up front that I don't design optics, nor am I an optical expert, nor do I know everything there is to know about the mechanical side of optical systems. What I do know, though, is how to mount lenses and optical elements so as to make an optical prescription work as intended. It is not necessary to know all about optics to do a good job of packaging an optical system. It is necessary, however, to understand what is important and must be dealt with. This article is addressed to mechanical designers who are relatively unfamiliar with optics.
Optics are very unforgiving: When packaging optical systems, it is frequently necessary to hold tolerances of 0.0001-0.0002 inch, although tolerances in the 0.001-0.003 inch range are much more common. Lens trains cannot bend or twist in use. Because optical systems require such tight tolerances, machined parts are usually used to mount optical elements. Even with machined parts, though, it is usually not possible to maintain tolerances of 0.0001-0.0002 inch in production at a reasonable cost, so it's necessary to rely on other methods of setting and maintaining those tight tolerances. Molded plastic offers the benefit of consistency from part to part, and can usually hold a tolerance of 0.001-0.002 inch from part to part. With plastics, however, it's usually necessary to do some production tuning to get the plastic to repeatedly hold lenses in the proper location.
Unless the system you are designing is incredibly simple or has low performance, it will be necessary to make precise adjustments in every production system to make the prescription work. Production testing is a regular part of building optical systems. One of the difficulties in adjusting systems is that the adjustment has to be locked in place once it's made. Turning the jam nut or whatever part is used to lock the adjustment usually readjusts the adjustment, so you have to do some back-and-forth to get the adjustment to hold in the proper location. Your production people should be familiar with this phenomenon and know how to deal with it. Designing a way of fixing an adjustment without changing it is the best solution.
Optical elements are usually round with spherical surfaces, but come in all shapes and sizes. You may have to mount prisms, square windows, cylindrical lenses, or pellicles. The important thing to remember is that every optical element must be precisely held in place for the life of the product.
You may have to package two or more prescriptions for a given device. For instance, you may have an objective prescription to create an image, and then have an eyepiece prescription in order to focus the image on the viewer's eye. If you're dealing with light of different wavelengths going to different parts of your system, you may have a beam splitter and a different prescription for each wavelength. Filters and coatings may be used to control which wavelengths of light get to different parts of the system.
Another thing to remember is not to stress optical elements. Unlike many things that get retained or screwed down, optical elements cannot handle much stress without damaging optical performance. Just tightening a lens retainer too much can make optical performance unacceptable. Ideally, you will mount all of your optical elements so they are held solidly in place with no stress. That allows the prescription to perform as intended.
What makes up an optical system?
Every optical system starts with a prescription, which, in turn, begins with optical requirements established by the ultimate customer for the device. It will assume a certain object size, distance from the viewer, resolution of the image, and other optical requirements. It will include clear aperture diameters, lens thicknesses, lens radii, and any unusual optical forms used. The prescription, or the optical designer, should provide you with the mechanical tolerances necessary to make the prescription work.
You will have to sit down with the optical designer who generated the prescription to find out exactly what the prescription requires. Prescriptions seldom make things easy to understand or provide all the information you need, yet you must know all relevant information to design the system properly. One thing that optical designers can forget is the annular ring you will need to mount the lenses. They are concerned about making the optics work, but you have to physically mount it, so you need to tell them how much diameter beyond the clear aperture you need to provide a mounting surface and a place for a retainer to hold the lens.
The systems I have worked with usually have an objective, which is the foremost lens assembly on the system. This is the lens that gathers the light and forms an image inside the system. They also usually have an eyepiece, which looks at the image and allows the user to focus on it. Eyepiece focus adjustments are measured in diopters, which correspond to an axial movement of the eyepiece. The optical designer will determine what those adjustments need to be. The objective also usually has a focus mechanism so that the image can be precisely focused on the reticle or image plane.
An optical system could have any number of other elements, depending on its purpose. Prisms, collimators, filters, windows, image tubes, and light sources all have a purpose in some system. It may be a simple magnifier or a complete observation system with laser range finders and digital imaging, or it may be a huge astronomical telescope. Every system has its own peculiar requirements.
Understanding a Prescription
Since every optical design begins with a prescription, you'll have to learn to understand them, at least enough to package the design properly. A prescription lists a lot of information about an optical design, but the part you really need to understand is the Surface Data Summary, which lists each surface of the prescription in consecutive order, from where the light enters the system to where the image is. This is what you are designing around.
The convention is that light enters from the left and exits at the right, but there are exceptions. You will need a ray trace to make sense of it all. Remember that optical systems can be designed for any optical purpose and can vary wildly from one job to the next. You will have to study each prescription carefully, as well as talk to the optical designer, to understand the purpose of the system.
We will deal here with a relatively simple prescription in order to get a handle on some basic principles. You read the prescription from top to bottom and left to right. The thickness and glass type apply to the area between the surface on the line they are on and the next surface below. You will quickly realize that prescriptions are not intended to be easy for mechanical designers to understand.
If you compare the surface data summary with the ray trace, you can compare the two to understand exactly which radius goes with which lens and on what side. One of the first things you will notice is some dummy surfaces that the optical designer uses that you don't care about. In this case, surfaces 1, 8, 9, 10, 11, 25, 26, 27, and IMA are dummy surfaces. You can't ignore the air spaces connected to the dummy surfaces, because the air spaces are part of the prescription. Compare the prescription to the ray trace to determine which are dummy surfaces and ignore them.
Since the convention is that light enters from the left, a positive radius indicates a surface with the radius center to the right of the surface. A negative radius indicates a surface with the radius center to the left of the surface. A negative thickness indicates a reflection from a mirror. A reflection from a second mirror will change the sign and indicate a positive thickness. A radius of infinity indicates a flat surface.
The first lens in this prescription is circled. It has a center thickness of 25mm, a first convex radius of 144.52mm, and a second convex radius of 208.48mm. Note the sign of each radius. If the signs were reversed, it would be a double concave lens. The distance between the mirrors in this prescription is 65mm. You ignore dummy surfaces 8, 9, 10, and 11, but you must add the air spaces to get the distance between surface 7 and surface 12. Looking at the ray trace clarifies what is happening.
I added dimensions to the ray trace to verify that it was accurate. In this case, some of the dimensions were accurate; others were slightly off. This is the reason that you must use the prescription itself as the defining document of the system. CAD data are usually accurate, but with the unforgiving nature of light, you can't afford to be wrong. You must verify and correct your CAD data so it perfectly corresponds with the prescription.
Issues in designing optical systems
In any optical system, you will have to mount lenses. Usually the simplest and best way to mount lenses is to put them into a closely-toleranced bore on a lens seat and retain them with a threaded retainer. The lens seat must be located so as to put the lens exactly where the prescription requires it to be. Lens locations must also be calculated to create the correct air space between them, since the air space is part of the lens prescription. You are essentially recreating the prescription with physical parts. Spacers are frequently necessary to maintain air spaces between lenses.
You may occasionally have a prescription that requires a lens to be de-centered--i.e., moved sideways--in order to ensure the required optical performance. In that case, you have to allow the lens to de-center without allowing it to tilt. Then you have to allow for potting it in place. Since glass is brittle and subject to cracking and chipping, it is not a good idea to have sharp pieces of metal--such as setscrews--contacting lenses, particularly in systems that undergo repeated shock and vibration.
If a lens gets chipped or cracked and the defect shows up within the clear aperture, it must be replaced. You want to avoid replacing lenses in a built-up system; it's time-consuming and frequently messy. Nor is it a good idea to have lenses mounted against a sharp edge. I have seen at least one instance where simply tightening the retainer literally sheared the lens at the lens seat because of a sharp-edged lens seat. Putting a slight radius on every lens seat makes it less likely to damage the glass, as well as reducing debris from chipped edges.
Keep in mind that wherever you mount the lenses, the image plane has to end up precisely where the prescription requires it to be, usually within a few thousandths of an inch. You'll have to do a lot of top-level layout work to make sure that happens. You don't want to prototype your system and find out the image plane isn't where it's supposed to be. Jury-rigging a supposedly finished design can be embarrassing. Go back to your top-level layout frequently, especially after significant changes, and make certain that the image plane is precisely where it should be. Every minute you spend checking that top-level layout is worth it. You may not have to change much because of it, but those changes will be critical. If you don't check for them and make them, you will have problems.
The optical designer should give you a ray trace of the entire prescription, along with the clear aperture of each optical surface in it. Every lens seat must be larger than the clear aperture of the surface it mounts, and no part of the system can be inside the ray traces made at the clear aperture, or the image will be obstructed. Lens retainers must also stay outside the clear aperture.
In any system with an eyepiece, it will be necessary to allow for focusing to correct for different degrees of nearsightedness or farsightedness in the user. You may also have to focus the objective to adjust the image plane's location. The optical designer will have to tell you if the focusing lenses can rotate or if they must move without rotating. It makes a difference in how you design the focus mechanism.
In all except rare cases, you will have to seal the system. Foreign matter inside the system--whether it be gas, liquid, or solid--will obstruct the image. O-rings will usually do the job, but sometimes you'll have to create some odd seals. For military equipment, this is a critical requirement, and you may also have an immersion or altitude requirement to deal with. Keep in mind that external pressure isn't the only possible problem. Internal pressure at altitude can also cause problems. Be aware of what will happen to your system if internal pressure gets significant. Also be aware of the chemical environment. If your customer uses a cleaning chemical that degrades O-ring material, you'll have a problem.
If sealing is important, it will be necessary to purge the system before finally sealing it. You will need a purge port somewhere on your system that allows you to pull a vacuum or pump air or nitrogen through the system. This requires a pathway for the gas going past lenses and spacers and whatever other elements might be blocking air passage through the system. You may have to add grooves and holes to spacers, grooves through lens seats, or other features to allow gas to flow past optical elements. Once the system is purged, you will need to install a plug in the purge port to maintain the seal. A seal screw with an attached O-ring usually works well for this.
Ideally, you will assemble the lenses into your design, securely retain them, seal the system, and it will work as advertised. That seldom happens the first time you do it. Since you'll have to align your system somehow, some possible ways to do it are listed below. In all cases, once your adjustment is made, it can't move any more for the life of the system, except for maintenance or repairs.
Double eccentric. A double eccentric allows you to de-center an image within a predefined adjustment circle. The difficulty with a double eccentric is that tolerances on the parts can make it impossible to get the image on dead center. In that case, you may have some systems that can't be aligned. Double eccentrics are used in production systems with good success, though.
Risley prisms. These are two wedge-shaped pieces of glass that are rotated independently in order to steer the image. It is possible to get the image on dead center, as well as steer it within a given angle defined by the wedge.
X-Y stage. This is a little more difficult to design than a double eccentric or Risley prism, but you can get some precise adjustments with it. You'll have some amount of cantilevered weight that may cause you difficulty. Remember, glass is heavy. Purchased x-y stages tend to be very large and impractical for use in a production system.
Dynamic adjustments to the system
You may have a requirement for certain elements in your system to be rotated, moved, or flipped after the system is built. Again, you must remember that any movement outside the prescription parameters will degrade the optical performance of the system. Movement of any element must be precisely controlled. You will frequently need a spring to eliminate backlash, which must always be considered when moving optical components.
Differential threads. Differential threads are created by putting one screw inside another. When one screw is turned, the part moves a distance defined by the larger thread pitch minus the smaller thread pitch. For instance, a 1/32-inch pitch thread turned against a 1/48-inch pitch thread will move the element 1/32 (0.03125) - 1/48 (.028333) = 0.0104 inch per turn. You can create almost any amount of movement you want with differential threads, and it is generally fairly simple to lock the element in place. This is cheap and very effective for small movements.
Four-bar linkages. Four-bar linkages are quite useful and show up everywhere. The difficulty in an optical system is that the movement must be precise. Cylindrical fit tolerances at the linkage pivots usually aren't tight enough to maintain prescription spacing. You may use a linkage to move or rotate an element to different locations. The linkage must be designed so that when the moving element reaches a particular location, it is precisely where it should be. You may have to use a spring or some over-center lock to hold it in a particular position. Remember not to stress the lenses.
Pivots. For rotating elements, pivots must be mounted on conical bearings or balls to make the movement precise. Tightening a plate against a shaft in a V-groove also works. Use the fundamental shape and design of the pivot, rather than part tolerances, to keep the element in place. Tight tolerances are expensive.
Focus mechanisms. Threaded lens barrels with a jam nut usually suffice for a focus mechanism. You can also use slots and pins, levers, cams, or whatever does the job for you.
About the author
Ken Sauter is president of Sauter Industrial Design (www.sauterindustrialdesign.com), Garland, Texas. Ken's 35 years of experience in design work includes projects ranging from a large-scale microwave oven for a hospital to an interior for a custom airplane; a flight simulator; plating equipment; sheet metal NEMA boxes; truck bodies; and night vision equipment for the military. Ken also has experience with sheet metal, weldments, molded parts, machined parts, and many other methods of manufacturing that are used to produce his designs.
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