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Lighting in Machine Vision Applications
by Nello Zuech, Contributing Editor - AIA Posted 05/05/2005
In real estate one hears repeatedly that value is associated with location, location, location. In machine vision applications one can say that success is associated with lighting, lighting, lighting. Often creative lighting design can make the difference between success and marginal performance.
When machine vision was in the pioneering stage 25 years ago, integrators had to make do with commercially available lighting arrangements that were designed for applications other than machine vision. More often than not it was the same lighting being used in industrial settings where people were performing the inspection task. While it was often adequate (and just adequate) for humans, the same lighting was ill suited for the camera used or was terribly unstable or inconsistent with high speeds or was nothing more than room light.
Ultimately the growth in the machine vision market has been as much due to the creative lighting arrangements that have emerged and are often application-specific. Lighting designs dedicated to a specific application provided cameras with consistent images and images where concerns are magnified and variables of no concern are normalized into the background.
Over the years, many of these lighting designs targeting applications that are widely proliferated within an industry or across industries became catalog designs. Sold in volumes, the fundamental prices declined as development costs were spread across larger numbers. Along the way, solid-state lighting in the form of light emitting diodes evolved and their price/performance has lead to even more reliable and more consistent performance in machine vision applications.
To catch up with the current lighting activities we solicited answers to the following questions from all those companies selling lighting for machine vision applications. The following responded to our questions:
- Todd Johnson - Senior Engineer, Fiberoptic Systems, Inc.
- Steve Giamundo – Vice President Sales & Marketing, Fiberoptics Technology Inc. (FTI)
- Mike Muehlemann – President, Illumination Technology
- Thomas Fisher – Sales Engineer, Lumitex
- Greg Dwyer – Marketing, NERLITE
- Suleyman Turgut - Sales Manager, Fiber Optics, SCHOTT North America, Inc.
- Matt Pinter - Design Engineer, Spectrum Illumination
- Ivan Maksymyk - Product Manager, StockerYale
- Brett Kingstone - President/CEO, Super Vision International
1. Can you provide a brief general description of your lighting product line targeted specifically at machine vision applications?
[Todd Johnson – Fiberoptic Systems] We offer stock ring lights, gooseneck light guides, quartz halogen illuminators, and standard light guides, which our customers use in their machine vision applications. These are .55, .25, and .66NA borosilicate glass based products.
Most of our products are custom or OEM based, including spectroscopy probes, sensing probes, medical handpieces, and others too numerous to mention. Many of these are fused silica based fiber optics. If it involves fiber, we build it.
[Steve Giamundo – Fiberoptics Technology Inc.] FTI manufactures standard and custom fiber optic lighting products and illumination sources. The fiber optic products can be used in concert with a range of illuminators for front, back, oblique, area or line lighting, as a signal transmitter for absence/presence detection, or as a light conduit for alternative light source transmission (LEDs for example).
[Mike Muehlemann – Illumination Technology] Automatic decision-making has become a major aspect governing our recent product designs. Products that can be pre-programmed and do not require process operators are key to reducing production costs on modern production lines. To meet these requirements our engineers have developed Ethernet compatible interfaces that provide two-way communication for both control and real time performance evaluation. Multi-leveled self diagnostics now provide accurate information regarding all aspects of the lighting systems and sub-systems so that the higher level process controllers can either be ensured that the data being collected is accurate, or that there are system level problems that may have an impact on the reliability of the end inspection. This technology has been highly instrumental in the rapidly emerging compliance sensitive markets including pharmaceutical, medical device, diagnostics and other emerging biotechnology applications.
[Thomas Fisher – Lumitex] Lumitex is a custom manufacturer of fiberoptic backlighting. Typical applications will vary; however, our backlighting technology offers the following advantages:
- No Heat. The heat is filtered at the light source.
- No EMI/RFI
- Uniform & very diffuse.
- Thin profile (typical panel thickness: .068' to .145').
- Remote Light Source for hazardous environments.
- Durable product & low maintenance. Fiberoptic backlight is virtually good forever.
- The only component that would have to be maintained would be the incandescent or halogen bulb.
- Design Flexibility. Simple in design, thin & inert. Responsive design teams.
- Custom built to meet exact backlight dimensions required
- Short lead times (typically 4 weeks, after receipt of order).
- Standard temperature range +85C to –50C.
[Greg Dwyer – NERLITE] NER designs and manufactures a broad line of machine vision lighting under the NERLITE brand. Our line includes general purpose LED arrays (spot, rectangular, linear and ring formats), low angle designs for dark-field illumination, dome illuminators and the DOAL, SCDI and CDI designs for some of the more challenging applications. The LED based units have become our mainstay, but we also offer FO models of the DOALs, SCDIs and CDIs. We are also distributing a new line of current regulating light controllers in the Americas.
[Suleyman Turgot - SCHOTT North America, Inc.] Schott Fiber Optics provides both fiber optic and LED based illumination systems. The fiber optics systems include light lines and ring lights as well as the halogen and metal halide based light sources. The LED systems include light heads in a variety of configurations (ring light, light line, spotlights, etc.) with programmable controllers to allow strobing and triggering.
[Matt Pinter – Spectrum Illumination] What sets our company apart from our competitors is our drive at providing the latest technology to the vision inspection industry. Spectrum Illumination was the first to bring HB LED's to the vision industry. We are also the first to provide a complete line of wash down lights and UV LED lights. First with long linear lights, large spotlights and large domes. We have solved many long distance robotic application for the robot manufacturers. We continue to be the first in new LED lights for the vision industry.
[Ivan Maksymyk – StockerYale] StockerYale specializes in lighting for machine vision. We are the only company to offer four types of lighting for machine vision applications:
- Lasiris lasers that generate structured light for 3D applications;
- Fluorescent lighting mostly for microscopy and machine vision;
- Fiber-delivered halogen for microscopy and machine vision;
- High-power LED illuminators for a wide range of inspection applications in machine vision (paint quality inspection, PCB inspection, web and linescan, etc.)
[Brett Kingstone – Super Vision] Fiber optic and LED lighting.
2. What are some challenges that you have offering products to the machine vision industry?
[Steve] Although the standard product offering is significant in scope and depth, we commonly find ourselves manufacturing modified standard or custom components to a specification... and we're challenged to manufacture the design within the time constraints of the customer... which is usually 1-2 weeks. From the customer perspective, any lead time over 2 weeks is painful, over 4 weeks is unreasonable, and over 6 weeks intolerable... basically, customers want what they need, when they need it...FTI’s greatest challenge, and a major point of differentiation, is continually satisfying this requirement while remaining profitable for the long term.
From a technical perspective, the applications continue to grow in complexity, and engineers continue to stretch the bounds of performance. We notice current applications are not as straightforward, nor as forgiving, as they might have been in the past, as technology and customer demands continue to evolve. Where good was once acceptable, great is now the norm. For the future, FTI’s bigger challenge is to maintain its IP and consultative resources for its customers’ benefit.
[Mike] All of our lighting products are targeted specifically at automated inspection applications. Our continued growth is coming from two key areas. First, the classical industrial machine markets continue to expand, although much of the real solid market growth has shifted outside the traditional G-7 industrialized nations since this most recent recession.
In these areas we see continued demand for both Tungsten Halogen/Fiberoptic and LED based lighting systems. In many respects the product offerings from our company and many of our competitors have finally matured. These products are well engineered for these applications, and most of these applications can be adequately solved using off-the-shelf components that are manufactured in sufficient quantities to ensure high quality and low cost of entry. Our particular products are targeted at applications where overall intensity, spectral stability, industrial reliability and full automation capabilities are of primary importance.
Secondly, we see strong growth in the G-7 sectors in the biotechnology markets. These markets provide a wide range of new opportunities for machine vision technologies that have been developed and perfected over more than two decades in these industrial applications. In these markets, many of the same lighting system capabilities remain important and the issues surrounding medical compliance and turnkey automation are even more critical. In these areas our company has developed very simple to use auto calibration products that meet the stringent compliance requirements established by the FDA and similar regulatory agencies.
[Thomas] The fiberoptic backlight might not be bright or uniform enough for some applications.
[Greg] Trying to deliver more for less! We have a talented team of machine vision lighting professionals, arguably the most capable and experienced in the industry, so solving the lighting applications has become less the challenge. Profitably meeting the downward pressure on pricing while staying a US based manufacturer is our biggest challenge.
[Suleyman] There are two main challenges in the machine vision industry. The first is the lack of physical interface standards between light heads and cameras. The second is the wide variety of products that require inspection, i.e. from pharmaceuticals to food products, to automotive products, which requires illumination providers to customize each solution in size and configuration for relatively low volumes.
[Matt] Monster Lights are our very popular line of high output lights using LED Light Engines. The new HB LED's or Light Engines are 10 to 50 times brighter than standard LED lights. Our new lights have taken the vision industry by storm. Our lighting competitors have rushed lights to market to compete with our Monster Lights. Spectrum Illumination saw 400% growth in 2003 and 340% growth in 2004 with a majority of this growth in the Monster Lights. Fanuc and ABB robots are large customers that are using the Monster Lights to illuminate long distance, large areas that standard LED lights cannot light. These new Light Engines have lenses that are replaceable to alter the angle of light produced.
There are many different angles from narrow to very wide. Matching the angle with the working distance means more light on the working area and less light is lost or wasted. We have lights on robots that are inspecting engine blocks where the lights are over 20 feet from the parts.
These new Light Engines have allowed us to make very large lights. We have standard linears with light output areas up to 57.5' and dome lights of 29.25'. Large high output lights are now possible with high output Light Engines.
Our LDM Driver is patent pending because of the importance of driving the new HB (High Brightness) LEDs or Light Engines. These new Light Engines require 350ma to 1.5 amps each. Limiting current in the past could be accomplished with resistors on LED's running 20ma or 30ma. Today's higher current LED's/Light Engines cannot maintain a stable light intensity limiting current with a resistor. Spectrum Illumination designed this driver for specifically running high current Light Engines. The LDM constant current regulator based LED driver provides control for LED lights requiring stabilized light output in demanding inspection applications. This patent pending controller utilizes a feedback loop to maintain current within +/-.1%. Standard features include DIN Rail mount, 0 to 3A LED light source output, high speed strobe control, constant output and industrial standard 24VDC input. The LDM driver is designed to run the latest HB (High Brightness), high current LED’s. The LDM constant current driver is used to drive Spectrum Illumination’s flagship line of LED high flux illumination, called Monster Lights®. The LDM Controller provides features unmatched in the vision industry. Spectrum Illumination's high current Monster Lights are the brightest LED lights in the vision industry. The largest LED Light Engine manufacturer has purchased our driver to run their LED light engines on machines in house on vision inspection systems.
Another area in which we have produced new lights that is growing rapidly is UV LED light. We have UV lights for inspection in the vision industry. 395nm is popular and is currently solving many UV excitation inspections. 365nm UV is newer and solving many more applications in the luminance of UV material. Machine vision has used fluorescent black lights in the past but now UV LED's have many advantages. UV LED's can be focused, pulsed or strobed for higher output and lights can be manufactured in many more different styles than a linear or ring like fluorescent. We solve many applications for UV in products like ink, glue and plastics. We are solving applications for UV in the automotive, medical, packaging and pharmaceutical industries.
Wash Down lights or harsh environment lighting is one of our specialty lines. No one makes industrial lights for the wash down industry besides Spectrum Illumination. All our wash down LED lights are FDA compliant. The lights are made from stainless steel 316 and have FDA compliant gaskets and silicon. We have spotlights, ring lights and linear lights in wash down models. We have worked with some large food manufacturers for making a specialty light to solve a common problem with food packages and inspection systems. A couple large food producers asked us to solve a problem with lighting food products. Many types of food products have stretch wrap or clear packaging that is reflective to light. These manufacturers need to inspect food inside the clear packaging for quality. Problem is that the clear packaging is reflective making it unable to view the product. The solution is to have a 'cloudy day' or light tent type of illumination. A dome light or light tent will provide a 'cloudy day' illumination and is used to solve applications in the vision industry where reflection is a problem. A dome light cannot be made or housed in a wash down enclosure. Geometry and lighting angles prevent the light from being used in an enclosure. We have taken the idea of providing a dome light/light tent with even illumination and making a sealed wash down enclosure. This light we call our OXY light is able to provide the wash down industry with a solution for inspecting products that were not possible.
[Ivan] Perhaps one of our major challenges stems from the fact that our customers, while they are often leading experts in their respective fields in machine vision, are typically less well versed in the illumination aspects of their application. People tend to take illumination for granted, but it is actually central to any successful machine vision system. Therefore, we at StockerYale must devote resources to helping our customers understand their illumination needs.
Another challenge that faces us is the wide variety of machine vision applications, which we serve. These applications range from semiconductor inspection to lumber processing. This requires us to have broad knowledge, and to have a fabrication platform which is flexible and can be adapted easily. One OEM order of lasers might have extremely high pointing stability, while the next OEM order might require a high degree of wavelength stability, etc.
Another challenge has to do with the fact that photonics components (such as laser diodes, LED chips, fiber couplers, etc.) tend to vary from unit to unit in regard to power output, efficiency and other variables. On the other hand, we at StockerYale must guarantee to our machine vision customers a high degree of repeatability. Therefore, we have been obliged to develop fabrication methods, which compensate for the natural variability of the primary resources that we use.
[Todd] Many new products are a challenge. The biggest challenges include new applications and new materials, as well environmental and packaging constraints. It is also challenging educating users that in general, lighting is just as important as image acquisition. Traditionally design efforts have been heavily placed on the image acquisition portion of a given system. With experience and the recent advent of lighting software, engineers are learning to balance their design efforts equally between imaging and lighting.
3. What are some challenging machine vision applications your lighting has addressed?
[Mike] In an ever more competitive landscape, our company focuses almost exclusively on the challenging end of the spectrum. Our products have been applied to detect micron size defects on 2 meter wide webs traveling at 60 miles per hour, to separate and inspect complex multi-layer assemblies for micron sized defects at high speed and high magnification, to inspect and quantitatively analyze complex cellular samples with high degrees of spectral reproducibility, and just about everything in between.
[Thomas] Looking at alternative light LED sources.
[Greg] Here are some applications we’ve recently solved: Detecting shallow dents on shiny coin blanks; detecting scratches on white plastic that were invisible to the human eye; inspecting fluorescent ink print on a highly specular surface; illuminating multi-color semiconductor chips using multiple light wavelengths; illuminating very large fields of view from distances up to 30 feet.
[Suleyman] Glass bottle inspection for cracks. Semiconductor wafer inspection for macro film defects. High Speed Check/Mail scanning and sorting. Fine Pin Grid Array (FPGA) Inspection for packaged chips. Web inspection.
[Matt] LED lighting for high speed imaging with image capture in the low microseconds. Very high output, pulsing the LED's in the hundreds of amps. Robot applications where the lights are 20 to 30 feet above the part. UV LED's to focus the light and provide a higher output for fast moving medical inspections. Wash down lights for harsh environments that meet FDA compliancy.
[Ivan] High-speed precision linescan illumination is one of the most challenging machine vision applications that we address. The illumination for linescan must be extremely bright and extremely uniform. We address the high-speed linescan market with our COBRA LED-based linescan illuminators as well as our fiber-optic halogen line arrays. COBRA LED linescan illuminators offer superior uniformity, a high degree of unit-to-unit repeatability, the advantages of monochrome illumination and potentially lower cost of ownership. On the other hand, the halogen solution can offer a somewhat lower initial price.
Another challenging application that we address involves structured light provided by the Lasiris laser product line. This product line consists of line generators, dot matrix generators, grid generators, etc. We focus on producing an extremely uniform line, as required in the precision 3D imaging applications that we address. Line uniformity is particularly critical, for example, in the inspection of semiconductors.
[Brett] Underwater lighting and explosion proof lighting for fuel storage and petrochemical processing operations.
[Todd] Since fiber optics are ubiquitous, we serve many machine vision markets. Especially exciting is the use of fibers in medicine, as in oxygen sensors, brain probes, muscle probes, dental curing, and phototherapy.
[Steve] Because it is a high skill, hand-made product, cost and lead-time has relegated Fiber optic lighting to be the choice of last resort... therefore, with the exception of stereo and video microscopy, most fiber optic lighting applications are not straightforward... the most challenging applications involve lighting spheres and irregular shiny surfaces for defect evaluation, lighting deep cavities to confirm material integrity and formation, glass inspection of all types, and high speed web production (intensity and uniformity is a challenge). The most interesting applications use color filtration to create contrast, rather than directionality.
4. Are there some specific lighting properties or features that a buyer of lighting for a machine vision application must be attentive to?
[Thomas] Dimensions of the backlight are self-explanatory; however, the light output and uniformity specifications of each application should be the focal point of discussions.
[Greg] Yes, there are many! First, buyers should try to address the lighting early on when building a new system. Reflected light is what is being measured in machine vision, so buyers must determine proper geometry of light delivery or the camera won't be 'seeing' what is expected. A light’s wavelength or color can be used to either create or diminish contrast in an image, so understanding how to apply color is important. There are choices in lighting optics such as diffusers and focusing elements that can make a huge difference in results. And the light sources themselves, such as LEDs or halogens, xenons or fluorescents, can make a big difference in both initial implementation cost and also maintenance costs later on. An example is the fluorescent or incandescent source that seems like such a cost saver when planning the system, then later on, when you have to routinely shut down production for bulb replacements, you spend much more than what long-life solid state LED lighting would have cost originally.
[Suleyman] Lighting properties are never certain. A customer should have an inventory of a variety of lighting solutions so they can quickly identify the best solutions, otherwise a sample should be sent to the lighting provider so an adequate solution can be determined.
[Matt] This all depends on the inspection needed. Every part is different and every application has its own unique properties.
[Ivan] Contrast, contrast, contrast. At StockerYale, we see a great many machine vision applications each year. One of the major themes is contrast. For example, many times, at the outset, one might think that white light is required by an application (simply because white halogen or white fluorescent has been used traditionally). In reality, the abandoning of white and the adopting of monochrome lighting (often red or blue) enhances contrast significantly.
[Brett] Yes, applications that consume low or no power at the fixture end.
[Todd] All buyers should know the wavelengths, working distance, field of view, and acceptance angle of their system. Also, illumination level and uniformity are controlling criteria in many applications.
[Steve] I believe a general specification set is pertinent across lighting technologies... uniformity is probably most important, then output shape (or structure), intensity, package size, operating life, and light characteristic (wavelength/color temp)... these features are combined in a multitude of ways to create a lighting preference for each application. The buyer should strive to describe each as succinctly as possible. The lighting supplier can be a significant help defining these characteristics, as long as the application can be openly discussed.
[Mike] Lighting is simply about contrast enhancement, which is really nothing more than effectively increasing signal to noise in 2-D data signal. Of course this is a very application dependent task, however, it is very basic in its approach. If you forget about fluorescence for a moment (which you probably shouldn't!) there are only two techniques that you can use to accomplish this task - structure and spectrum. Structure is a geometrical property, which defines from what angle(s) photons impinge upon a target and what angle(s) are gathered by a detector (camera). Spectrum is a physical property, which defines the distribution of wavelengths (color) for a group of photons impinging upon a target. Both of these can be very effective at enhancing contrast. In practice, about 80% of applications are solved with the proper design of geometrical structure, and only about 20% are enhanced with spectral manipulation.
5. What is it that you require from a prospective buyer of lighting for a machine vision application to assure you will deliver the proper lighting?
[Greg] We ask a lot of questions: What specific features or defects are you trying to capture in the image? How large is the field of view? What camera and lens are you using? What is in the background? What restrictions are there on light placement? Is the object in motion? What is the ambient environment? And probably the most important questions: Can you send us a sample of the object you want to capture images of so that we can test it for the best lighting solution here in our lab? Can we send you a light to test in your production machine? There's no getting around the value of testing the actual application before deciding on the best lighting!
[Suleyman] Understanding of the intensity and uniformity specifications.
[Matt] We ask many questions about every application. We ask for pictures and drawings along with samples of good and bad product. We ask if there was any testing done to lower the risk for finding the best light. We ask what type of defects, is bright field or dark field illumination needed? We ask many different questions about the application. Years of experience in the different industries have given us knowledge for lighting different parts. For instance in the plastics industry, we ask about flash and carbon specs. We will talk to the stamping industry about carbon content, oil and reflectivity on stampings. A stamped part is different every time due to variables like metal composition, metal thickness or carbon percentage. Is the part plated? If the part is plated there are many times where there is inconsistency in plating of a part. Does the part have specified tolerances in color, size, shape or material?
We also ask if the camera is monochrome or color. Are there any restrictions on mounting lights? What type of lens? What is the working distance? What is the image acquisition time? In general we ask many questions so we can become familiar with the application asking intelligent questions and giving expert advice.
[Ivan] We find that we can deliver the proper lighting if the customer has had a chance to experiment with the same type of illumination. We work mostly in high-end applications. A prospective buyer can help us deliver the proper lighting by relating to us his or her experience with a similar lower-performance product. This comparison provides a point of reference that allows us to determine the required product from our product line.
[Brett] Detailed description of the process/application.
[Todd] We need confidence the customer understands his system, and we require a good understanding of what he/she wants. Sometimes this is not possible, so we work with the customer to guarantee feasibility while minimizing cost to achieve this understanding. This may take the form of a site visit, loaning of samples, and/or fabrication of a feasibility model.
[Steve] We'd like to know as much as possible about the application and the objectives. Then we'll recommend the proper components. If we're manufacturing to a specification, we'l
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