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KnowHow

Assembly of Optics

 

Relevant for: OptiCentric®

Automated Cementing of Lenses

For many lens manufacturers, the cementing of two or three optical elements is a expensive process that must be carried out manually and is thus prone to error. In the conventional fabrication process, two different cementing procedures have been used. First, all the centers of curvature can be aligned on a reference axis. Complex, costly positioning elements and/or sample holders are needed for this. In the other process, the upper lens is adjusted on a reference axis defined by the edge and one center of curvature of the lower lens. In this case, the alignment accuracy is limited by the fabrication tolerances of edge processing of the lower lens. Both processes are carried out manually, so the alignment accuracy also depends on the skills of the operator.

The technology developed by TRIOPTICS overcomes the disadvantages of both processes. The basic idea is that the three centers of curvature of both lenses are measured exactly and adjusted onto a line. Exact prepositioning of the lower lens is thus no longer necessary.

The complete process is shown in Figure 11. In the first step, the centers of curvature are found for the three lens surfaces. Measurement takes place within a few seconds and with a precision in the submicrometer range relative to the exact axis of rotation. Then the measurement results, including the three centers of curvature, are converted into parameters for exact adjustment and transmitted to a piezo actuator.
Quickly, the upper lens is aligned to the lower lens with a precision better 1 μm. The entire process takes no longer than ten seconds. If necessary, a unit for automatic UV hardening is also integrated into the system.

In summary, the SmartAlign® technology is a software and technology concept for the cementing of lenses. It permits the highest possible degree of accuracy and repeatability without requiring mechanical adjustment. In addition, this cementing process is very stable and fast.


centre of curvature
Fig. 11: The SmartAlign® technology is used for the automated adjustment of achromatic lenses in the cementing process

 

 

 

 

 

Automated Bonding of Lenses into a Mount

Bonding of lenses into a cell became the technology of choice for applications requiring very precise lens alignment and where severe vibration or temperature extremes are encountered such as military systems and scientific applications.
When aligning a lens element in a cell it is necessary to adjust the optical axis of the lens to coincide with the datum axis of the lens cell. The best alignment performance is provided by the optical inspection in reflection mode. An accurate reference rotary axis e.g. provided by an air bearing is also required.

The datum axis of the cell is either measured with an accurate mechanical gauge or a non-contact optical sensor. An alternative is the use of precise hydrostatic cell holders ensuring repeatable clamping of the lens cell. Certainly the mechanical axis of the hydrostatic holder must be initially aligned until the datum axis of the holder runs true to the reference axis of the air bearing.

Finally the bonding cement is applied and allowed to set or is cured using UV-light sources.

The bonding of lenses into a cell is a laborious process requiring experience and remarkable operator skills.

For this complex part of the optics assembly process TRIOPTICS developed an automated work station known as OptiCentric® Bonding Station. The features of this work station can greatly simplify the optical assembly, reduce the risk of alignment and centering errors and increase significantly the efficiency and stability of the process.

The heart of the system is a complex and stable software integrating and automating the process of inspection, alignment, cement dispensing and curing.
A schematic representation of the basic process is presented in Fig. 9. The work piece, a cell with the lens element set inside, is clamped using either an accurate hydrostatic holder or a more simple mechanical chuck.

When using accurate (hydrostatic) holders the accuracy of the alignment of the cell to the reference axis depends on the accuracy of the holder itself. The disadvantage of this
method is that the precision holder can be employed for one cell diameter. A cell holder accurate to 1-3 μm is expensive enough, the equipment for a large range of diameters becomes really unaffordable.

The SmartAlign® technology integrated in the software of the work station allows the use of inexpensive mechanical chucks. The datum axis of the cell is determined by measuring the cell with a precise non-contact optical sensor or a mechanical gauge. The datum axis of the cell is compared with the system overall reference axis (the axis of the rotary air bearing). The measured centering errors of
the lens element can now be recalculated taking the datum axis of the cell as reference. The new data set is fed to an automated alignment device performing the accurate alignment of the lens elements. The complete process is shown in Fig. 9:

A) The work piece is rotated around the reference axis of the air bearing. A set of measurement data is taken. The position of thecenter of curvature of the both surfaces is assessed.

B) The dispenser is precisely positioned by means of a stepper motor stage in the space between the cell and lens element. While the sample is rotated, the dispenser needle produces a precise joint around the lens. The dispensing time is coordinated with the lens rotation.

C) and D) The data set obtained during the measurement is transferred to a piezoelectric alignment device. Using an appropriate manipulator, the device aligns the lens element
to the reference axis or the datum axis of the cell.

The process is completed by the automated curing of the cement using a UV-light source integrated into the work station.

Lathe Centering

A common method of producing a precisely centered lens assembly or object ive lens is based on the production of precise sub-assemblies, which are later combined into a barrel of close mechanical tolerance.

The advantage of this procedure is that the mechanical manufacturing and maintaining of the tolerances is better controlled than it is for the optical counterpart. The sub-assemblies are manufactured by lathe centering, where the optic is mounted more or less carelessly into an oversized lens cell. The cell is then adjusted in a special alignment chuck so that the optical axis coincides with the rotary axis of the lathe. Afterwards the cell is machined to the final size including the reference surfaces well aligned to the optical axis. This method is quite expensive, however it provides the highest quality and is usually applied for precision optics.

The first step of this process is to bond each individual optical element into an oversized lens cell with stress-free cement. In a second step the assembly is mounted on the chuck of the lathe and aligned with respect to the rotation axis of the turning machine. For this process two autocollimation telescopes with head lenses focused to the centers of curvature of both lens sur faces are used. As a compromise, a single autocollimator may be used that measures both surfaces sequentially in MultiLens® mode. In the third step, afterlens alignment, the edge of the cell and the flange surface are machined to their final dimensions of close tolerance. The lens cells produced in this way can now be inserted into a barrel with exact internal fit to manufacture an objective lens.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

automated bonding
Fig. 9: Automated bonding of lenses into a mount

Relevant for: OptiCentric®
 

 
Read more about MTF measurement, camera module testing and autocollimators.
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