Lenses and optics—an integrator’s perspective

David L. Dechow

 A very wide variety of lenses specifically targeted for industrial machine vision applications are readily available in today’s marketplace. In general, lenses can be loosely divided into three technical categories—entocentric, telecentric, and specialty lenses—and there are many variations in the components available within each general category.

Lenses with perspective

In machine vision applications, the most common and widely used of these lens categories is the “entocentric” lens (the term “endocentric” is also used in the industry interchangeably). In optical terms, this category refers to lenses that have their entrance or exit pupils inside the lenses. More practically, these lenses are differentiated in that they have an “imaging angle” and produce a scene that contains “perspective”, that is objects nearer the lens appear larger than those far away. The most ubiquitous entocentric lens is in the human eye.

In machine vision, these lenses are generically called “fixed focal length” lenses, which has nothing to do with whether or not they are “focusable”. At a given focal length—a term that technically defines where the rays in an optical system converge but in usage relates to the lens’s magnification and imaging angle—the lens will create a specific field of view at a specific working distance on a specific imaging sensor.

In the marketplace, entocentric lenses are available in a multitude of focal lengths and sizes and vary widely in quality and capability to reproduce an image—an important factor with increasing camera resolutions.

Gauging lenses

Telecentric lenses differ significantly from entocentric lenses in that they do not have any (or at least have a very small amount) of perspective. Effectively, this means that objects within the depth of field of the lens (the focus range) do not change size at all with respect to the distance of the object from the lens. These lenses sometimes are called “gauging” lenses because part position (perpendicular to the optics) does not impact the reproduced size of the part in the resulting image, and therefore greater precision can be obtained in some applications. Telecentric lenses also can be used with collimated (parallel ray) light sources to help improve the illumination for an object, particularly when backlit. Telecentric lenses generally have a fixed working distance that must be maintained in the imaging, and often this distance is quite small compared with the longer and completely flexible working range of an entocentric lens.

Specialty

Myriad specialty lenses are available for machine vision. Pericentric/hypercentric lenses can image around the outside perpendicular walls of an object. Bore or pinhole lenses can image the inside walls of a hole. Lenses with mirror assemblies to see around or provide multiple views of objects are available. One must be aware of the lens marketplace and be ready to research and apply a specialty lens when it will benefit a particular application.

Specifying lenses and optics

As one might imagine, lens specification must take into account the totality of the requirements for a specific application. This includes things like object sizes and required spatial resolution, necessary illumination techniques that might affect the required imaging angle(s), the type of appropriate component (entocentric/telecentric/specialty lens), the physical limitations of the installation, and more. In discussing lens specification, let us presume these prerequisite details are already known. That said, there are some fundamental things to consider when specifying a lens including the following.

Lens formats

Lenses are design-limited to work with a maximum sensor size, and lens format designations are related to the size of the maximum image circle produced by the lens. The lens format specification must match (or be larger than) the format specification of the sensor that will be used for the imaging.

Optical Resolution

This metric helps ensure that the lens can provide differentiation of small features stated in “line-pairs/mm” (lp/mm). While a lens spec sheet might designate a specific lp/mm capability for the lens, a much more thorough (but complex) metric/specification related to optical resolution is the measure of the Modulation Transfer Function (MTF) of the lens relative to a specific sensor.

Magnification

To calculate or specify a specific lens, one can start with the required magnification. Magnification (MAG or Primary MAG-PMAG) is simply the amount the desired field of view must be magnified to get the view onto the selected sensor (e.g., if the view is 100 mm horizontal and the sensor is 10 mm horizontal the required magnification is 0.1). From magnification, one can determine a required entocentric lens focal length for a given working distance to achieve a specific FOV (sometimes engineers skip the MAG and go directly to examining focal lengths, or perhaps start with working distance and magnification in the calculations). For a telecentric lens, the lens is only specified by its magnification, so the selection is more direct, though again keep in mind the fixed working distance that also will be specified for a telecentric lens.

Important criteria

Once the lens is specified technically as described above, the selection and specification of the component manufacturer is the next step.

Lenses are reasonably straightforward components. As such, the barrier to entry in the marketplace is low and we see many new lens manufacturers emerging. The bottom line is that lenses from both new and the mature manufacturers in the machine vision market generally are very good devices.

However, one still must be well aware of quality—in particular, light gathering capability and resolution. There is no standard uniformly followed in specifications published by manufacturers about these metrics. It is important to do the best due diligence when selecting a manufacturer, including actually testing the lenses. Practically, there ultimately will be lenses from several manufacturers that perform in a very similar way, and at that point the consideration of commercial details might come into play. To be sure though, there is no way to combine low price with quality, and claims of cheap but high quality lenses should be closely examined.

Lens specification really is more difficult also when we start to look at very large format lenses (greater than 1.1 or 4/3 format sensors requiring other than “C-Mount” lenses). These optical systems are more complex (and costly) and often require more research and analysis as part of the specification process.  It might be practical to engage the assistance of the lens manufacturer or distributor in specifying the correct components.

Building the best system for the task

As we can see, it is important to be trained in the use and specification of lenses and optics to adequately and competently choose the best one for the task. Poor optical design and specification can result in machine vision systems that are not as reliable or robust as they actually could be.

Key errors include:

• Only using fixed focal length lenses and not considering the entire optical and illumination system in the specification.
• Using the cheapest lens available to save a few dollars.
• Failing to analyze the overall needs of the application and mis-specifying imaging components as a result.

Here are a few tips to help avoid these errors and to consider when specifying lenses and optics for your next build:

• Have all the prerequisite details about the needs of the system before specifying imaging systems.
• Seek out expert assistance from an integrator or distributor for help in specifying if unsure.
• Learn the technical details of optical specification and be able to creatively use the broad range of optical components to ensure the most successful imaging results.