Splitting images

When we needed an inspection system that offered both high magnification and also a large field of view, we found turnkey systems to be insufficient. We solved our problem by developing a custom system that splits one image into two paths (Figure 1), with the first path achieving a much higher magnification than the second. The first path needed a 0.15-mm field of view while the second required a 1-mm field of view.

Figure 1. Higher- and lower-magnification images of the same object yield idfferent informaiton. High magnification (top) provides a smaller field of view, which can make it difficult to keep track of what part of the object is in view. A dual-magnification system provides users with two images at different magnifications of the same area.

Typically, high magnification implies a small field of view, but a small field of view makes it difficult to see which part of a larger object the system is imaging. This problem pops up in a number of applications, including semiconductor and electronics inspection and biological imaging. Our application required that we see a fairly large field of view (for alignment and object location) while still being able to resolve extremely fine detail on exceptionally small components. The detail and field-of-view requirements far exceeded the capabilities of standard off-the-shelf systems.

Several standard approaches can provide different magnifications in a system. Our first thought was to use a zoom lens, but zoom lenses could not provide the necessary resolution at extremely high magnifications. They also take time to zoom from one magnification to another, which reduces a system's efficiency and increases production costs. Time spent adjusting the zoom lens is time that would be better spent inspecting components.

We also considered using fixed magnification lens systems such as microscope objectives. Microscope objectives, however, are designed for specific magnifications and yield only one field of view in standard configurations. (This is why microscopes are generally designed with rotating turrets: to allow for a variety of magnifications.) And as with zoom lenses, fixed-magnification lens systems waste time while changing magnifications. Also, switching from low to high magnifications introduces alignment problems between the objectives.

We continued to look for a more creative solution. We broke the application down into its most basic elements and decided what we really needed was a system that looks at two specific fields of view at the same time. One image needed to have extremely high magnification, resolution, and contrast. The other image needed less magnification.

We knew that many microscope objective designs offer some flexibility in the magnification that they can produce. For example, video couplers take the images transferred by the objectives (which are almost all designed for use in visual-based instrumentation—that is, designed to be viewed with eyepieces) and send them to CCD or film cameras. Because visual-based instruments seldom produce the same image sizes that a standard CCD camera is designed to capture, video couplers for most microscope systems come in different magnifications, designed to best match the image size to a specific camera array size.

Our application had a similar requirement: the same objective yielding different magnifications. Although our application required more range of magnification than is available from microscope video couplers, the concept behind them helped us move toward a solution.

Figure 2. A beam splitter integrated into an inspection system produces two images with different magnifications through a single objective lens.

Figure 3. As the distance between the objective and the tube lens increases, light rays coming from off-axis positions of the object may no longer pass through the aperature of the tube lens. This resuls in lower light levels at the edges of the image.

We calculated the total magnification of a system that uses an infinity-corrected objective by dividing the focal length (FL) of the tube lens by the focal length of the objective:

Varying either of the focal lengths alters the magnification of the system. For our application, we used the same objective lens and two different tube lenses to achieve the magnifications needed. But we also needed a way to view both magnifications at the same time.

We decided to use a beam splitter to split the image after the objective lens (Figure 2). With the splitter, the system sends an image to two tube lenses of different focal lengths, so two different final magnifications of the same scene arrive at two different cameras.

Luckily, the design of infinity-corrected microscope objectives offers a great deal of flexibility. Many optical components—including beam splitters, filters, mirrors, and prisms—can be placed behind such an objective with theoretically little change in the optical performance of the objective.

Adding too many components between the objective and tube lens, however, will lower the overall image quality. Placing components in the system will also extend the distance between the objective and the tube lens. As this distance increases, rays coming from the off-axis positions of the object may no longer pass through the aperture of the tube lens and will not make it to the image plane (Figure 3). This reduction in rays ultimately will lead to lower light levels at the edges of the image and even the inability to view a portion of the object. For our application this did not become an issue.

Our system employs two beam splitters after the objective. One beam splitter integrates in-line illumination into the system, while the other splits the return image and directs it to the two tube lenses. Our system has no moving components for imaging the dual field, which eliminates the issue of alignment. (The objective moves for auto focus, but this is not related to the dual-field issues.) The larger-view system shows a field of view around 1 mm in size, while the second one has a roughly five times higher magnification and correspondingly smaller field of view. Because users view two fields simultaneously, they can measure in both fields at the same time.