What is Active Alignment?

What is Active Alignment

As modern devices gradually become smaller and more complicated, the need for fine precision down to the micron is becoming increasingly necessary in assembly environments. To achieve this, many manufacturers are utilizing machine vision and high-precision lenses, but they need to be certain that these lenses are properly centered, otherwise they can lead to inaccuracies.

In their construction, lens manufacturers utilize a technique called active alignment to ensure that this is the case, providing completely centered micro-optics.

This technique encapsulates all elements of the adhesion of the lens to its metal housing, including the adhesives used, the workstations where this is performed, and the measurements that were taken.

All of these intricate processes have been carefully developed to ensure maximum accuracy while also easing elements like length curing times for adhesive and the locking of components while such curing occurs.

In this article, we’ll explain how active alignment works, to provide you with a better understanding of lens construction and accuracy. As with many techniques, there are several variations on this strategy, including mechanical alignment.

How Does Mechanical Alignment Work?

Mechanical alignment is the simplest means for active alignment, as it utilizes physical elements to insert the lens into its metal housing. As the lens cell and the lens itself are produced with relatively tight diameter tolerances, this means that the gap between the circumference of the lens and the metal body is meant to be minimal.

Several surfaces must be properly aligned to accomplish this, including aligning the two lens surfaces to the optical axis, the optical axis to the outer diameter of the lens, and the mechanical housings inner-diameter tolerance.

Quality Magazine has an excellent explanation for how mechanical alignment works, offering insight into the multi-step process. In assembly, the gap between the body and the lens is temporarily filled with three shims of equal thickness that ensure the lens has been placed at the precise center of the housing. Once it is in place, an adhesive is applied to lock it in place, cementing the lens at the center.

After the adhesive has taken a firm hold onto both the lens and the body, these shims are removed and the lens moves on to the next stages of production.

Similarly, this process can also be completed with retaining rings, however, both of these strategies are used with the awareness that there are some tolerances in regards to the lens’s centering.

As these centration requirements become tighter, they can require finely-tuned mechanics and optics that are increasingly expensive, and the process becomes more difficult. Inserting lenses and placing shims can become trickier, and this is especially true in assemblies that utilize multiple lenses.

While this can increase costs, this level of accuracy and precision provides for high-grade vision lenses that are properly suited for industrial uses like manufacturing, assembly, and even life sciences.

Optical Alignment

Similar to mechanical alignment, this differing technique is completed when the technician uses the optical image from the lens to finalize its position. Here the alignment is leveraging how the lens actually performs relative to its assembly, forming a process that compensates for component fabrication errors. This utilizes a number of precision tools such as a precision spindle and an autocollimator to provide accurate focal points for centering around.

The requirements for centration relative to a lens’s outer diameter and its optical axis are diminished, although both components still need to be manufactured relative to one another. Here we are leveraging the skill and prowess of trained engineers to properly align lenses in an ideal manner by utilizing high-end workstations that we’ll detail later.

Which Is Better?

Neither of these methods is known to completely compensate for an optical element’s misalignments. For mechanical alignment, the image displacement caused by misaligned surfaces is a fixed, unchangeable value. The active optical technique enables a composite alignment that can offset some of these inherent misalignments but can also add more time and expenses to the manufacturing process.

What Adhesives Are Used in Active Alignment?

In the construction of lenses, liquid adhesives are inserted in multiple places to secure lens components within a metal housing. Here the adhesive must be designed to cure quickly to reduce assembly times, and will likely be exposed to high-intensity UV light and heat in order to achieve this.

This process even works correctly in shadowed areas of the lens housing through dual-curing, as the adhesive connects itself to all sides in two different cycles, ensuring precision and accuracy.

As explained by DELO Industrial Adhesives in Germany, these adhesives are paste solids that use elastic forces to keep themselves in proper shape. This keeps the adhesive stable so that it can be properly inserted without the adhesive itself causing displacement among the lens.

Additionally, this substance also utilizes surface tension to keep itself evenly applied, allowing it to disperse equally when the glass is affixed to the lens.

Of course, the dispensing of this adhesive is also a highly precise action as well, in which the substance fills a limited space but also provides enough material for surfaces to affix onto. This is why a paste is used, as the adhesive must physically stand above the lens as the housing is pushed onto it, providing a continuous seal.

During manufacturing the lens will also be moved upwards and downwards, during which the adhesive will need to remain attached to the lens. As a paste, this adhesive can be sucked back into the joint to improve its strength and reliability, and even if this area is shadowed it can be cured through heat.

The result is machine lenses that are properly assembled, with even adhesions that apply stress laterally and do not cause unnecessary stress on specific joins. Of course, this adhesive must be applied carefully and properly for maximum effect and requires careful attention. Still, with fast curing and flexibility, this adhesive simplifies many parts of active alignment and lens assembly.

How Are Active Alignment Lenses Made?

In order to accomplish the needed level of accuracy in machine vision, many companies have developed mechanical workstations that help enable this level of precision. These moving installations are stabilized and flexible for a multitude of needs, and all provide the proper tools for centering lenses within their houses.

One of these systems is developed by Integrated Mechanization Solutions, IMS, in the Netherlands. They explain that their workstations are constructed to be modular, meaning they can be seamlessly integrated into existing manufacturing assembly lines, minimizing impacts to cycle time and maximizing ease of operation.

These are also configurable based on scale, meaning they can be designed to align small microlens arrays, mid-sized cameras, or large automotive headlights.

These workstations are built to help with the curing processes as well, which are an important part of constructing vision lenses. Here, the workstation is constructed so that it may continue to work during the bonding process by moving all necessary components in parallel to maintain centering.

This is immensely valuable for high volume production, in which this positional freeze can also be dispatched from the workstation, so the components are still in place even as a manufacturer moves on to the next piece of equipment.

All of these elements work together to ease the manufacturing process by maintaining alignment through mechanical and optical solutions. There are many global companies producing solutions like these that are capable of meeting the needs of vision makers who are set on ensuring the highest quality for their customers.

Conclusion

As we’ve gone over, active alignment is a carefully calculated process in which lens components are properly placed amongst one another and then adhesives are applied to maintain this centering. This process utilizes neatly designed apparatuses, finely-tuned adhesives, and intricate measurements to ensure a high yield of lens products.

These processes are essential as optics manufacturing continues to become increasingly demanding. The fidelity of cameras on mobile devices is ever-increasing, and as technologies like virtual and augmented reality continue to proliferate, these lenses must be as accurate as possible to facilitate seamless interaction with the software.

Lens makers continue to ensure the high quality of their products through these methods so that they can be properly used for machine vision applications, as well as integral life sciences.

Sources:

Photonics Media - Active Alignment Techniques Improve Lens Centering | Photonics.com

PhotonicsViews - Why, When and How to Apply Active Alignment (pg 63-65) | Wileyindustrynews.com

1Library - Active Alignment for Cameras in Mobile Devices and Automotive Applications | 1library.net

Effects of Camera Alignment Errors on Sterioscopic Depth Estimates

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