PTB >> Imaging Systems: Optical Design for Close Quarters

Despite the fact that imaging systems are critically important for many types of machinery, optical design is often addressed only after the mechanical and other system designs have been completed. Consequently, imaging systems have to be fit into the available volume, which can often be tiny and/or awkwardly shaped.
Because integrating optical systems on paper always seems to be easier than it does in actual applications, many systems are simply put together by trial and error. Although this may work in the lab, it can be disastrous when the system is integrated into a piece of equipment.

This article outlines a series of steps that allow system designers to integrate a workable imaging system into a too-small box without rigorous engineering. The six steps are:

1. Define your mechanical constraints.
2. Define your fundamental parameters.
3. Layout the straight line imaging system.
4. Place the illumination and determine minimum f/#.
5. Compare optical design with mechanical constraints.
6. Bend the system.

Figure 1: The final imaging system must be able to discriminate between acceptable and unacceptalbe characteristics of objects, such as the one shown here.

Consider a typical imaging system that requires integration into a confined space. For example, Figure 1 shows the object that needs to be imaged. The current picture is not sufficient because it doesn’t show enough of the object to register indentation locations.

Our goal is to build a system using off-the-shelf parts (to keep costs down) and have as few bends in the system as possible (for simplicity and to minimize the number of components). The resolution of the final system must be sufficient to discriminate between acceptable and unacceptable characteristics. It must also measure the location and size of indentations in shiny objects that are roughly 20 mm in diameter. Building custom components can be considered after this is complete.

Constraints & Parameters
Designers must ask themselves a number of questions in order to determine the imaging systems’ mechanical constraints and fundamental parameters. How big is the box? What space, exactly, is available for the imaging system?
Several numbers must be determined at this point. For instance, the available track, or the length of the space allotted for the optics. The length of the camera, lens, and cables should be included in this measurement. For most systems, room for illumination will also need to be incorporated. Also, estimate how many bends are needed in the system. An example box is shown in Figure 2.
Figure 2: The example imaging system must fit within this oddly shaped box.

The fundamental parameters (see Figure 3) of any imaging system include:

· Object Field of View
· Working Distance
· Object Resolution
· Sensor Size
· Depth of Field

Figure 3: This diagram illustrates the five fundamental parameters of an imaging system.

When looking at fundamental parameters don’t forget the basics. Be sure to keep in mind that the working distance often depends on mechanical constraints. When selecting the object resolution, ask yourself what size defect the system will be required to measure. Remember that both horizontal and vertical measurements are important for determining sensor size.

For this example, we’re looking at a 20 x 25 mm shiny metal object. We are measuring location and size of indentations. High resolution is necessary to maximize accuracies, ideally 10 line pairs per millimeter (lp/mm). We need only a fairly narrow depth of field of about 5 mm to accommodate the depth of the indentations.

Straight Line & Illumination Layout
With the optical parameters in hand, it is time to start designing the optical system. This article does not cover the basics of choosing components, but it does assume that the design uses off-the-shelf components. First, lay out the system in a straight line and check that the design works. Are the fundamental parameters of the system equal to your needs? Where do you need to place the illumination? Determine the clear apertures. Be ready to repeat this step as bends in the system are introduced. Also, determine how sensitive the system is to adjustment. A system that is sensitive to focus and alignment can add cost and complexity later on.

Figure 4: After determining the fundamental parameters of the system, a working distance, sensor size, and field of view can be determined.

For the example system, we found parts and decided that a camera with a ½-inch-format sensor could be used. The lenses have a focal length of 50 mm, working distance of 250 mm, field of view of 28.5 mm, object resolution of 11 lp/mm, and a depth of field of less than 5 mm (see Figure 4). The fact that it does not fit into the box provided can be ignored for the moment.

Now is the time to determine minimum f/#. The f/# is a measure of the light-gathering ability of an optical system. For a lens, the f/# is the focal length divided by the diameter.

Usually changing the aperture is the simplest way to adjust the f/#, but the aperture also alters the resolution and the amount of light getting through. Changing the aperture can also affect the system’s depth of field. Sometimes, changing the aperture will allow you to switch to a significantly smaller lens.
The f/# is also intertwined with the illumination. If the illumination must be brighter because the aperture has decreased, will the light bulb be driven too hard, and will the resulting lifetime be too short? Meanwhile, what sort of illumination is required: Point-source, diffuse, or ring light, and normal or glancing? Some objects can be lit from behind, providing a bright field. Others may benefit from illumination by line generators or other types of structured light.

For our example, we chose diffuse illumination. To reduce specularity in the image we need to make sure the image of the illuminator appears larger than the object, in other words the illumination must cover the entire object.

Reality Check
Now that we have a working design, it’s time to compare the optical system to the space available for it. Does it fit? (Almost never.)

Before trying to bend the optical path, there are three tactics you can try. First, try reducing the camera size. Either a board level camera or a remote head camera can ease the space constraints. Next, consider whether there is a different combination of focal lengths and working distances that could be used. If the system uses fiber optic cables to deliver the illumination, can they be bent at the connection joint? Or, perhaps, compact-but-typically monochromatic LED illumination would work?

Bending the System
Finally, there is no choice but to bend the system. There are a number of considerations at this point. Where will the bends be located along the optical path? Clear apertures must be defined as well as whether to use prisms and mirrors, beamsplitters, or baffles. The mounting and adjustment of each of these elements should also be considered now in order to avoid expense later.
Locating bends is relatively straightforward. Look at the straight-line design, and locate the bends where there is space for a mirror or prism as well as a need to relocate components, such as a lamp or camera.

The straight-line system can also help define clear apertures. Make sure to calculate entrance and exit apertures if prisms are used to bend the system rather than mirrors. Using elliptical or rectangular clear apertures (rather than circular ones) may save space.

When choosing between mirrors and prisms, consider their strengths and drawbacks. Mirrors offer high reflectivity, wide spectral range, minimal image degradation (if mounted properly), and a low cost versus size ratio. However, mirrors can be difficult to clean and align, and susceptible to mounting tension. Precision mounting is costly.

Prisms, on the other hand, are easy to mount, durable, can be designed for easy alignment, and can isolate the optical system from environment. However, they also cost more (for their size) than mirrors, and are heavy. Other drawbacks include that the image is degraded by the glass’s thickness, and that the prism faces reflect light. Reflected light can be removed using baffling.

Figure 5: The final system fits within the given box while utilizing a minimum number of bends.

Similar considerations accompany the different types of beamsplitters: cube, mirror, or pellicle. In our example, we chose a beamsplitter to direct the illumination onto the object while allowing the image to pass through (see Figure 5).

As illustrated in Figure 5, light from the diffuse axial illuminator (shown in yellow) is reflected by the beamsplitter, illuminating the object. Reflected light carrying the image (shown in blue) passes through the beamsplitter. The image light bounces off a front-surface mirror and arrives at the camera. The entire system fits within the box, employing a minimum number of bends. In the end, there is usually adequate room in an overall design for the optical system – even if it does not seem like it at the beginning of the process.

This article was written by John Stack, President, of Edmund Industrial Optics located at 101 East Gloucester Pike, Barrington, NJ 08007. For more information call (800)363-1992. Visit Edmund Industrial Optics at www.edmundoptics.com.