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Feature Articles

Optics in Machine Vision Applications

by Nello Zuech, Contributing Editor - AIA


Interested in learning more about
Optics & Lighting?
 

Useful tutorial sessions are offered at the
International Robots & Vision Show, September 26th
in Chicago!

Attend the Beginning Lighting & Optics tutorial
session where you’ll gain an excellent introduction
to lighting geometry and the basics of illumination
optics. Unlike traditional lighting and optics courses,
this tutorial is designed to give you a firm
understanding of how image quality is specified
and used to improve overall system performance.

Or you can attend the Advanced Lighting & Optics
tutorial session which is designed for the engineering
professional, you'll learn real-world techniques for
putting together illumination and optical systems that
work. You'll learn how to select proper illumination
wavelength, both visible and non-visible, as well as
how to deal with complex part surface geometries.

 
There are three elements that make up the 'staging' for a machine vision application: lighting, camera and optics. By “staging” I mean the arrangement of these three elements with respect to the object under examination. The specific optics is application-dependent. In addition to the normal parameters to be considered (focal length, field-of-view, depth-of-field, working distance, aperture, etc.), the camera style itself influences the selection of the most appropriate optics: megapixel camera, line scan camera, etc. The nature of the application is also a factor. If one is making measurements or engaged in precision positioning then telecentric lenses often make sense.

The optics creates an image such that there is a correspondence between object points and image points where the image is to be sensed by the sensor, as well as contribute to object enhancement.  Except for the scaling or magnification factor, in an ideal optical system, the image should be as close to a faithful reproduction of the 2D projection of the object. 

Consequently, attention must be paid to distortions and aberrations that could be introduced by the optics.

Many separate devices fall under the term 'optics.' All of them take incoming light and bend or alter it.  A partial list would include lenses, mirrors, beam splitters, prisms, polarizers, color filters, gratings, etc.  Optics have three functions in a machine vision system:

  • Produce a two-dimensional image of the scene at the sensor.  The optics must place this entire image area (called the field-of-view or FOV) in focus on the sensor’s light sensitive area.
  • Eliminate some of the undesired information from the scene image before it arrives at the sensor. Optics can perform some image processing by the addition of various filters. Examples include: using a neutral density filter to eliminate 80% of the light in an arc welding application to prevent sensor burnout, using a filter in front of the sensor which only allows light of a specific color to pass, and using polarizer filters to eliminate image glare (direct rejections from the lights).
  • Optics can be used in lighting to transfer or modify the light before it arrives at the scene in the same manner as optics is used between the scene and sensor; items 1 and 2 above.

To catch up on what is happening in the optics field of importance to machine vision, input for this article was canvassed from all known suppliers of optics for machine vision applications. The following took the time to provide answers to the questions that follow:
Spencer Luster – President, Light Works
Stacie Errera – Marketing, Tamron
Stuart Singer – VP Industrial Optics, Schneider Optics

1. Can you provide a brief general description of your optic product line targeted specifically at machine vision applications?

[Stacie Errera – Tamron] Tamron, a manufacturer of precise and sophisticated optical products for a broad range of industries, has an extensive line up of machine vision lenses based on the 2/3' format (6.5mm, 8mm, 12mm, 16mm, 25mm, 35mm, 50mm). These lenses are designed specifically for machine vision applications.  We began concentrating on this market from the early days of our industrial optics division and have served our customers' needs ever since.

[Stuart Singer – Schneider Optics] Schneider Optics is a subsidiary of the Jos. Schneider Optische Werke GmbH, located in Bad Kreuznach, Germany.  We design and manufacturer high performance OEM industrial lenses (visible thru near IR corrected) in both manual and motorized irises for industrial and machine vision.  These include ultra-high performance lenses for: machine vision, robotics vision, documentation scanning, parcel scanning, industrial inspection, gaging (telecentric), surveillance, flat panel glass Inspection and macro inspection.  Our lenses cover all different sensor format sizes ranging from small C-mount area arrays thru large 12k linear sensors.  We also specialize in custom work from prototypes through large-scale production.  We have two facilities in the US (Hauppauge, New York and Van Nuys, California).

[Spencer Luster – Light Works] Optical products targeted at the machine vision industry:

  • Standard and custom telecentric lenses.
  • Optical view splitters (allowing one camera to acquire views from multiple fields, or the same object from different directions.)
  • Custom imaging and lighting assemblies.

2. What are some challenges that you have offering products to the machine vision industry?

[Stuart] First and foremost is to fully understand the requirements from the perspective of a potential customer.  98% of the time non-optical engineers are looking for optical support in the machine vision world.  Therefore it is very challenging to understand what people are trying to say and then create a specification, which will eventually allow us to determine if we can support or offer an optical solution.

Secondly, I find it challenging to keep-up with the camera and sensor manufacturers.  Sensor formats as well as individual pixel sizes are changing on a monthly basis.  Clearly, there needs to be much better communication between optical companies and camera/sensor companies in order to meet the needs of the end-users.

Lastly, it is very challenging to keep up with machine vision trends – such as 'Flat Panel Glass Inspection' systems.  Lenses always seem to be an after thought from most end-users.  Cameras/sensors and peripheral electronics seem to be chosen well in advance of a lens.  This often leads to providing a non-optimal optical solution (off-the-shelf) or trying to explain to the end user that a custom lens (define a performance spec., optical design, mechanical design, prototype fabrication) can take 6 months or more.  End-users typically do not have an appreciation for this.

[Spencer]  As a small company our advertising reach is so far limited and again, as a small company, being able to have a broad range of demonstration products to put into the hands of customers or product reps.

[Stacie] We continually challenge ourselves to provide the best optics possible for this industry. This means constant investment in R&D to be sure we meet the high quality standards expected and demanded in this industry:
1)  Keeping the distortion low: our 25mm is so low it is close to '0'
2)  High Resolution: our lenses are known for their superb image quality
3)  Ultimate Clarity: the images derived from our lenses are so clear that they are very useful for inspection applications
4)  Small Size: the maximum diameter of our lenses is kept consistent so that even if the customer has to change the lens to a different focal length, the basic outfit will remain almost identical.

3. What are some challenging machine vision applications your optics has addressed?

[Spencer]

  • Very long distance telecentric imaging.  We’ve built lenses for measurement of very hot materials with a stand-off distances of 66' and hot steel at 102'.
  • Very large field-of-view (up to 200mm) telecentric lenses for various applications.
  • Telecentric lenses for use in the Infrared.
  • Very large optical field splitters (field separation of approximately four feet) for preserving resolution of surface inspection.
  • 5-Way view optical field splitters for fastener inspection as well as for inspection of airbag initiators (full coverage inspection of welds).
  • Specialized near IR telecentric imaging systems for use in the bioassay industry.
  • Long standoff telecentric lens and optical splitter combination used for UV inspection of cooling pipes in a nuclear reactor head.  This assembly had to be very compact, while also providing two different magnification views of the same area from the same direction.
  • Telecentric lenses for use with large format cameras, either megapixel area scan cameras or long linear arrays.  Light Works has and does build lenses for detectors 40mm or longer.
  • Large field-of-view (FOV) telecentric lens for high-resolution color imaging onto three separate linear arrays. We built the lens as well as the split view optics in image space.

[Stacie] Machine vision has a unique set of problems and Tamron has worked hard to keep up with changing technology and eliminate issues that have caused problems for users in the past:

High Resolution: In order to meet the growing need for a diverse selection of optics to fit distinct imaging requirements, Tamron now has three lines of high resolution lenses to choose from for Factory Automation and more: Super High Resolution (25mm), 1.3-Megapixel (16mm, 25mm and 50mm), and Standard High Resolution (6.5mm, 8mm, 12mm, 16mm, 25mm, 35mm, 50mm, 75mm).

Close-up capability: The Megapixel lenses feature a high close-up capability, yet there is no sacrifice in optical resolution, brightness or contrast, providing excellent details of complex items even at these close distances. With an MOD of just 0.15m on the 16mm and 25mm and only 0.20m on the 50mm without the use of extension rings, these lenses are well suited for machine vision applications where close inspection of smaller objects is needed.

Consistent Size: Tamron manufactures a number of different lenses but all diameters are consistent in size so that customers are able to utilize the same filters, and space within the machinery is a constant.

Durability: They hold up even under heavy usage

Vibration: Early in the release of our machine vision products, we applied lock screws to keep the lens settings in position especially for use under severe conditions where machine vibration can shift the settings if not locked.

[Stuart] Pixel sizes are constantly getting smaller and the overall sensor sizes are getting larger.  This clearly is a challenge in designing new lenses.  As the pixels get smaller resolution goes up and so the need for more light (photons) grows ever so important.  As a result of this lenses need to be faster (lower f-numbers) to provide more light.  As the sensor formats grow (i.e. 12K) new lens designs are becoming more challenging to produce in order to meet the demands of large image formats.  Also, the advent of micro lenslet arrays covering the sensors are defining a new breed of lens designs. 

4. Are there some specific optic properties or features that one buying optics for a machine vision application must be attentive to?

[Stacie]
1) Purchasers should be sure the resolution of the lens matches their requirements
2) They should make sure the image delivered will be clear enough to inspect the subject
3) Should be sure that distortion is well controlled in order to accomplish the required job

[Stuart] Sure, as mentioned above in # 3 – sensor format (overall sensor size), individual pixel size needs to be considered at the start of a machine vision system.  Also, your system Working Distance and Field-of-View all share in the optical solution.  I often find that vision engineers don’t take into account the environment the system is to be used in.  For example, when your systems will constantly be exposed to vibration – you must consider the mechanical design of a lens equally as important as to the basic optical properties.

[Spencer] Properties or features when buying machine vision optics. I consider this highly application dependent, but in general:

Lenses: Match the lens type with the application.  For example, while a major portion of Light Works’ business is building and selling telecentric lenses, we end up persuading about 20% of our potential customers that they really don’t need telecentric lenses.  The idea that telecentric lenses are THE lens type to use for machine vision is just false.  In most cases you need telecentric lenses only if you must maintain constant magnification over a certain object depth, or you must maintain constant perspective.

Another example is trying to use cheap or improper lenses for a given application. For example, if you really need flat field, high resolution imaging (especially color) over a relatively large area and with low distortion, almost no C-mount lenses will perform well.  This type of application shows up in the printing industry all the time. Photographic enlarger lenses are usually the best match for this application, but many people in machine vision don’t know anything about them.

Ruggedness and lack of adjustability:  The first property needs no explanation. The second one goes hand in hand with the adage 'Anything that can be adjusted will be adjusted.'  Many times someone will 'helpfully' adjust the focus on a lens, thus ruining the inspection. So, its good to look at fixed focus optics, or at least lenses that have locking focus adjustment.

Use first surface mirrors when a mirror is needed for imaging. Imaging rays of light passing through the glass substrate of second surface mirrors will pick up aberrations.  BUT, make sure the mirror (and most other optics for that matter) have durable protective coatings.  The most common is silicon dioxide, SiO2.  Many times a harried operator will 'clean' a lens or other optic by wiping it with a shop rag or a shirtsleeve.

When using interference-type spectral filters, be wary of using them outside the angular range for which they are designed.  The spectral pass or reflectance properties of these filters are in a part a function of the light incidence angle.  So, if you have a large angular field-of-view, the center of the field-of-view might be 'seeing' green filtering, for example, but the outer portion of the field-of-view might be 'seeing' blue or yellow! (This would be an extreme case, but even small variations could be disastrous for certain applications requiring accurate color discrimination.)

5. What is it that you require from a prospective buyer of optics for a machine vision application to assure you will deliver the proper optics?

[Stuart] Some of this was covered above – however let me recap in simple bullet format:

  • Sensor format/size
  • Camera mount
  • Pixel size
  • Object resolution or object field-of-view
  • Working distance – or total system black box dimensions
  • Lighting conditions   (such a spectrum – i.e. visible, near IR, monochromatic, etc….)
  • Quantity of lenses needed
  • When will the first lens first need to be delivered (project time frame)?
  • Evaluation units needed?
  • Lens performance (MTF) and geometrical distortion values
  • Possible visit from me to the customer’s facility?
  • Budget – price range for the lens(s)
  • Manual or electronic control of focus and iris settings?
  • System environmental factors
  • Optical filters
  • Etc… 

[Spencer] Requirements from a prospective optics buyer. We almost always ask as much as possible about the application, not just what field-of-view is required and what kind and size of camera.  The more information we have (lighting, type of part, purpose of inspection, etc.) the better able we are to help our customers.  This is how we can often tell people, 'You don’t really need a telecentric lens' for example, or offer alternate solutions. Our business is not just optics, but helping make our customers’ applications go right.  But, as a minimum for lens sales we ask:

  • What camera type and size? (area/line scan, detector size, monochrome/color, lens mount)
  • What field-of-view size?
  • What standoff/working distance?
  • What kind of lighting is to be used?
  • What depth of field (for telecentrics) is needed?
  • What, if any, distortion limits should the lens have?

For optical splitter sales we also need to know what size each sub-field-of-view must be, as well as how much room is available in the inspection station, among other things.

[Stacie]
1)  First, we need information of CCD format size,
2)  Second, we need information about the working distance
3)  Third, we need to know about the subject size (horizontal, vertical)

With such information, we are able to calculate the right focal length to accomplish the job and at the same time, we are able to tell what size extension tubes may be required.

6. Are there specific application issues that must be understood insofar as selecting the correct optics for the application is concerned?

[Spencer] Specific application issues.  I think a combination of my previous answers addresses this.

[Stacie] It all depends on what the user wishes to accomplish:
1)  If the goal is a simple inspection such as counting, then the only issue is the focal length
2)  If the goal requires a certain level of resolution, then the lens selection might be limited with the software capability, i.e. resolution, color clarity, spectrum, etc.

[Stuart] I believe this has been addressed above in some of the other answers.

7. How important are optical properties such as resolution and the various distortions in machine vision applications?

[Stacie] Ditto -  Sometimes the software cannot calculate and read the details if the lens does not have enough resolution. The same can be said in regards to distortion.

[Stuart] They are greatly important.  If the machine vision system designer does not properly take into account the necessary system resolution and allowable geometric distortions – then the full system performance will never be reached.  The object side resolution (Pixel Sampled Size) is of paramount importance.  If the pixel sampled size in object space (this relates to system magnification) is not considered, then – you may never resolve the details/information that you are trying to capture/image.  This also parallels with Geometric Distortion in such a way that you might not be able to measure or locate a defect accurately enough to validate your machine vision system.

[Spencer] As expected, again this is highly application dependent. For checking the presence or absence of a gross feature, for example, then almost any old optical set-up will do. For critical gauging (particularly sub-pixel), however, excellent imaging quality and appropriate lighting are essential.  I will say one thing about distortion:  many if not most MV software has some distortion correction function, usually a look up table generated during calibration.  For small distortion values (say 1% to 2% full field), this type of correction is usually fine.  But, if the distortion values are high (say approaching 5%), or if sub-pixel accuracy is really needed, then distortion correction via software may not be sufficient.  There IS loss of information when doing such a correction! – if nothing else by digitization error.  So, for critical gauging applications, low distortion optics is usually required.

8. Are there some emerging technology changes in the field of optics that will positively impact your machine vision product line? What will those impacts be?

[Stuart] New fabrication technology such as “magnetic based technology” for the grinding and polishing of the glass elements.  Recent developments in new optical fabrication equipment will produce:

  • More accurate optical surfaces
  • Greater repeatability form lens to lens
  • Employment of complex surface shapes (i.e., aspheric, etc…)
  • Reduced delivery times

[Spencer] At the moment, I don’t see anything on the near horizon that will impact Light Works optical products. I do think there are a couple of technological developments that we may be seeing more of in machine vision: 1) Broader use of diffractive or hybrid refractive/diffractive lenses and other imaging optics. These can be made lighter and sometimes smaller than conventional optics. Right now, for most MV uses, the cost factor is just too high. 2) Wire grid polarizers, especially holographic type.  This will not get 99% of the hearts in the MV industry pumping faster.  But, for the small fraction of optics geeks working in the infrared, this is big news!

[Stacie]
1)  Today, customers are asking for more customized products to meet their system needs
2)  Also, the Megapixel cameras that are becoming popular may result in a change in product and/or specifications

9. Any advice you can give to someone investigating optics for a machine vision application?

[Spencer] Contact Light Works!  I know that sounds like a blatant sales pitch, but I seriously mean it.  If I were a non-optics person working in MV, I’d want to talk to Light Works.  We help a lot of people with free advice.  Some of them become are customers, others don’t.  Regardless, we try to steer people in the right direction.

Take a look at the “tutorials” sections of the catalogues of companies like Melles Griot, Edmund Optics, and Oriel.  There’s very good stuff there in a compact form.

Don’t skimp on the optical front-end of imaging and lighting.  Take the time to get it right, and spend a few extra dollars if warranted.  An unsolved $100.00 problem at the front-end can easily become a $10,000.00 problem at the back-end!

[Stacie] Since the application is becoming more complicated and challenging, customers should really look for a reliable product that can be use with confidence for a long time. Tamron provides products with high durability and high resolution developed through years of accumulated optical knowledge and prowess.

[Stuart] Speak with and solicit advice from a known optical company.  Leave the optical work to the optical engineers.   Again, most machine vision systems engineers are not classically trained optical engineers.  The system engineer needs to know his or hers limitations when it comes to optics/lenses - leave this part to the optical engineer.  

 

 

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