Assembly of Optics

Automated Cementing of Lenses

For many lens manufacturers the cementing of two or three optical elements to a doublet or triplet is still a laborious process accomplished manually and prone to operator errors. The classical technology requires an accurate rotation device - whose axis serves as a reference axis - and a sample holder which must be either very accurate (and expensive) or precisely aligned to the reference axis.

The operator needs to accurately align the lens holder until the axis of the bottom lens coincides with the rotation reference axis. Once the bottom lens is aligned and the cement layer dispensed on the joint surface, the operator adjusts -mostly manually- the upper lens until the axis of the upper lens surface coincides with the reference axis. It is obvious that the accurate alignment of the bottom lens in a first step and the alignment of the upper surface in a second step is a time consuming and rather unstable process depending largely on the operator skills. Moreover, both alignments of the bottom and upper lens are carried out relatively to a reference axis and not directly to each other. Consequently, inherent inaccuracies will always go
along with this technology.

The new cementing technology developed by TRIOPTICS is controlled by the software module SmartAlign^®. The basic idea of this concept is to accurately measure and assess the positions of the three centers of curvature of both lenses. The measurement is done relatively to an accurate reference rotation axis in just a few seconds with submicron accuracy. In a next step the upper lens is aligned in such a way that the center of curvature of the upper surface coincides with the optical axis of the bottom lens without the need of any mechanical pre-alignment (see Fig. 8).

In the automated process, the measurement data set containing the position of the three centers of curvatures is transferred to an automatic alignment device equipped with appropriate lens manipulators. In the next few seconds the upper lens is aligned to the bottom lens with an accuracy in a range of 1 μm. The complete process cycle including measurement and alignment lasts less than 10 seconds. If required, an UV-curing device can be integrated into the automated system.

The SmartAlign^® technology is supplied as a turnkey system under the product name OptiCentric^® Cementing Station. Certainly, upgrades of existing OptiCentric^® MOT instruments equipped with rotary air bearing are available.

In conclusion, the SmartAlign^® technology provides a software and technology concept for cementing of lenses with highest accuracy and repeatability and without the need of any mechanical pre-alignment. Further benefits result in the very short cycle time and the stability of 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.