New solutions in precision lens mounting
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Several methods have been developed through the years to mount lenses with the ultimate goal of minimizing their positioning errors with respect to the nominal optical layout. This is a non-trivial task, since it requires either very well controlled manufacturing tolerances or alignment of the optical components into their mounts. This paper reviews the classical lens mounting methods and introduces new solutions to improve the centering accuracy. First, an improved drop-in method called auto-centering is described. This method is based on the use of geometrical relationship between the lens diameter, the lens radius of curvature, and the thread angle of the retaining ring to provide centring error typically less than 0.5 arcmin. In addition, an innovative method that relies on geometric principles to auto-center optomechanical parts to each other is described. The method allows to auto-center an optical group in a main barrel, to perform an axial adjustment of an optical group inside a main barrel, and to perform stacking of multiple barrels within 5 µm of centering error. Finally, to improve the centering accuracy of common method used to center injection molded plastic lenses, a new concept using toroidal interfaces has been developed. This method allows a reduction by at least a factor of two the lens centering error compared to methods based on radial clearance fit for the same manufacturing tolerances.
KeywordsLens mounting Auto-centering Molded lens Alignment Centering Optical mount
In the case of aspheric lens, the manufacturing error of the aspheric surface with respect to the other optical surface must also be considered. In fact, aspheric lens present additional challenges, since the centering and tilt manufacturing error between the two optical surfaces cannot be fully compensated either by the mounting or by alignment .
When the centering requirements are at the tolerance-manufacturing limit for the drop-in method, other strategies to align the lens need to be considered. Most of the time, active alignment techniques such as aligning a lens directly in a barrel, sub-cell assembly, or alignment in 5 degrees of freedom with sophisticated equipment are some of the solutions envisioned. All these different implementations of the active alignment provide high-centring accuracy, but also require expensive equipment and more assembly steps to perform the alignment, resulting in a cost increase.
2 Lens auto-centering
Experimental measurements have demonstrated that the auto-centering method results in centering errors typically lower than 0.5 arcmin for the lens surface in contact with the ring. This new lens mounting method has the advantage of providing very accurate centering while relaxing the manufacturing tolerances on lens wedge, lens diameter, and barrel bore diameter as required for the precision drop-in method. Using different thread shape, the auto-centering method allows centering of convex, planar, concave, and aspheric lenses.
3 Aspheric lens auto-centering
For both aspheric centering methods, it is unfortunately impossible to provide a perfect centering because of the decenter and tilt manufacturing errors between the aspheric surface and the optical surface in contact with the barrel seat.
Centering measurements for different aspheric lenses mounted using the auto-centering have shown that this new lens mounting method for aspheres provides a simple and an accurate mounting method that bridges the advantages of the standard drop-in and the active alignment.
4 Precision optomechanical assembly
4.1 Optical sub-assembly auto-centering
4.2 Translatable optical group
4.3 Barrel stack auto-centering
5 Injection molded plastic lens
In such case, a tapered protrusion on the first lens is inserted in a tapered cavity on the second lens to provide the centering. This requires to have a minimal clearance between the two mounting interfaces, so that the lenses can be assembled without mechanical interference, which would result in an axial positioning error as well as a rocking movement of the lenses. Moreover, the minimal assembly clearance to avoid mechanical interference is increased by the diameters manufacturing tolerances.
For the toroidal interface mounting, the protrusion and the groove can both have a diametrical error with respect to the nominal value, but still provide a perfect entering. In fact, as long as the two diameters are the same, the centering will remain conserved. Thus, there are several diameters of the toroidal interfaces that would theoretically result in a perfect centering. This contrast with classical lens centering based on clearance fit, where the minimum centering error will occur only when the inner diameter is at is maximum value and the outer diameter is at is minimum value. As a result, the maximum centering error for the toroidal mounting method is reduced by a factor of two compared with classical methods based on radial clearance fit, but the statistical centering error improvement is even better.
In precision lens assembly, tilt and distance between the lenses also need to be controlled accurately. Since the toroidal protrusion and the V-groove constrain five degrees of freedom of the lens, manufacturing errors on these surfaces will also affect the tilt and the axial spacing between two lenses. It can be seen from Figs. 19 and 20 that a mismatch of the radii of the toroidal protrusion and of the toroidal groove results rocking movement of the lens. Therefore, there is a tilt imparted to the lens in addition to the centering error described previously. This tilt error can be minimized by the use of a larger V-groove angle. In addition, this tilt error remains moderate and is of the same order as the centering error, unlike traditional centering methods, where a small tilt is typically associated with a larger decentering.
Manufacturing errors on the toroidal section diameter as well as on the V-groove section profile will mostly affect the axial spacing between two lenses. To minimize the axial distance variation between two lenses, a good control of these tolerances is required. Since these flange features are machined into the metal of the mold cavity sets, these manufacturing tolerances are fairly precise.
In addition to these centering, tilt, and axial errors, other contributors on the lens positioning are involved as for any other type of mounting interface. Among these errors, there are manufacturing variations associated with the manufacturing processes such as die machining and molding. For example, centering error between the two optical surfaces of a single lens, lens thickness error, and molding distortions are intrinsic to the manufacturing process and are not compensated by the mounting interfaces.
A comprehensive study performed on interaction between lens and mount has resulted in the development of new solutions to mount lenses accurately. This paper summarizes the solutions developed to mount glass and injection molded plastic lenses to improve the centering accuracy of classical methods without the use of active alignment and tight manufacturing tolerances.
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