The Basics of Scanner Field of View and Strategies for Improving It

You’ve decided your process requires the speed and precision of a galvanometer-based laser scanner and you’re faced with determining which field of view your application will require.

Many questions may come to mind, including:

• “What is a field of view?”
• “How will my laser and optical equipment affect my field of view?”
• “Which strategies exist to increase my scanner’s field of view?”

This white paper will identify the specifications that can influence the scanner’s field of view and also highlight hardware and controller-oriented solutions for improving it.

What is a field of view?

A laser scanner’s field of view is the area in the focal plane where the system’s laser beam can be focused by a lens. On lens datasheets, this area will often be represented by a square similar to the one in Image 1. This image, called a theoretical spot diagram, is used to characterize the beam’s focused spot size at any given point within the galvo scanner’s field of view.
Knowing the expected behavior of the spot size helps predict how big or small the laser mark will be at the focal plane and, consequently, helps predict the expected energy density at any given point in the field of view.

Why does the size of my field of view matter?

Any features outside the scanner’s field of view require the addition of linear or rotary stages to reveal the next unmarked substrate section. These additional stages will result in both higher system cost and added complexity from controlling both a scanner and a servo stage subsystem. Additionally, moving from one area to the next, often referred to as “step and scan,” limits the process to the speed of the servo stages. Not only is this strategy time consuming, but also there are almost always imperfections between scanning areas. As seen in Image 2, these imperfections, called stitching errors, are the result of distortions found at the edges of our field of view mismatching from one section to the next. Before adding servo stages to the system, determine if the field of view can be expanded to fit the process through other means.

How do I increase my field of view and how will my process be affected?

Certain setup components can be modified to enhance the scanner’s field of view. The components covered in this article include focal length, laser wavelength and input beam size. Changing each component presents its own unique set of advantages and drawbacks.

One of the simplest ways to improve the field of view is to select a lens with an increased focal length. The illustration in Image 3 demonstrates how a change in focal height can increase the field of view. This increase in distance, however, comes at a price. Changing a scanner’s field of view often incurs a trade-off between spot size and field of view. Fundamental optical principles dictate that spot size is directly proportional to focal distance. Therefore, as the distance from the focal plane increases, unfortunately, the spot size also increases. If the spot size area cannot be sacrificed, other ways to increase the field of view must be identified.

The laser’s wavelength directly influences the spot size and field of view. This can be seen when viewing Images 1 and 4 and considering the differences in the spot size and field of view. Both of these spot diagrams were generated using a 163 mm telecentric f-theta lens. When using a 1030 nm wavelength laser as in Image 4, the available field of view increases as much as ~47 percent, compared to the 515 nm laser used in Image 1. This appears to have achieved the goal. However, upon closer examination, it is apparent that the spot size has increased by as much as four times. Once again, if the process cannot accept this increase in spot size, other means must be considered to increase the field of view while maintaining spot size.

All other components being equal, smaller scan heads with smaller input apertures can allow a more favorable mechanical setup in which the lens can be located closer to the final turning mirror. This arrangement yields a larger field of view. However, the smaller aperture size and smaller input beam diameter will increase our resultant spot size. Alternatively, by increasing the size of the incoming beam, the spot size shrinks within the field of view. However, as the size of the laser beam and input aperture is increased, the size of the mirrors used to reflect the beam to the workpiece must be increased as well. This increase in mirror size adds more inertia to the motor assembly, which can decrease the scanner system’s dynamic performance.

Component- and specification-level changes for enhancing the field of view each come with their own benefits and disadvantages. If neither focal length, wavelength, nor input beam diameter changes will yield an acceptable field of view and spot size, hardware and controller-oriented solutions must be considered.

Scanner Solutions for Increasing Field of View

Single pivot-point scanners provide a solution for operators to increase a scanner’s field of view with a given f-theta lens. These scanners introduce a third mirror to the traditional two-mirror scanner system. This additional mirror prevents the beam from walking on the final mirror, thereby making it possible to locate the final mirror and the focusing optic very close to each other. This positioning enables more efficient use of the f-theta lens.

This closeness provides a larger field of view, smaller maximum spot diameter and less variation in spot size within the field of view. As seen in Image 5, the Aerotech AGV-SPO, which uses this three-mirror arrangement, can increase the field of view of a 355 nm wavelength, 255mm focal length, telecentric lens by as much as 2.5 times.

There are drawbacks to this three-mirror arrangement. The third mirror requires an additional axis of control hardware. Compared to a traditional two-dimensional scanner, the dynamic performance is slightly decreased due to the inertia of the additional mirror.

Another strategy for increasing a scanner’s field of view is incorporating a post-objective scanning setup. Most of the technologies and components identified thus far incorporate a focusing optic in a pre-objective scanning setup (i.e. the scanner mirrors direct our beam into a focusing lens). In post-objective scanners, the beam focusing occurs before the scanning mirrors, which can be used to increase the field of view beyond that of a typical galvanometer scanner.

By focusing the beam before the galvo scanner mirrors, the achievable scan angle is no longer limited to the focusing lens’ input aperture size. Instead, the scanning area’s new limit becomes the mechanical limit of the galvo motors themselves or the exit aperture of the scanner housing itself. The following table shows the achievable spot sizes and corresponding field of view while using the Aerotech AGV3D scanner.

Post-objective scanning also gives the added benefit of focusing the beam at different focal lengths. Substrates are not always flat and, at times, can be quite complex. With post-objective scanning, users gain the ability to tackle complex, three-dimensional surfaces and volumes that could otherwise not be processed with a traditional galvo scanner.

The drawbacks to this arrangement include spot distortion at the edges of the field of view and increased complexity in control architecture. In a pre-objective scanning setup, a telecentric f-theta lens ensures that the laser beam contacts the focal plane at or near orthogonal to the scan head. With post-objective scanners, the beam makes contact with the substrate at a larger angle of incidence, which may cause the spot to distort at the field of view’s edges.

Controller-Oriented Solutions for Expanding Your Field of View

Sometimes the substrate is just too large to fit within any scanner setup or lens field of view. Luckily, there are controller-oriented solutions that enable the synchronization of linear or rotary servo axes with laser scanners. Aerotech’s Infinite Field of View (IFOV) feature accomplishes this automatically in the controller without the need for optimizing on a path-by-path basis.

Synchronizing servo and scanning axes allows users of laser scanners to greatly extend the operating area of their systems. Patterns that are larger than the scanner’s field of view can be processed continuously, which improves processing quality while reducing programming complexity and cycle times. The position command sequence is split between the scanner and the servo stages. The scanner handles the high acceleration, complex motion, while the servo stages keep the material within the working envelope of the scanner.

This technology is ideal for ultra-fast lasers or when small spot sizes are required to keep the laser power density above the material ablation threshold. With these technologies, a short focal length lens can be used to reduce the laser spot size, while still allowing the scanner to access a large work area. As a result, the user is able to select the best optic configuration for the job without compromising the working area.