Machine Vision Lighting Expands to Meet Demand

Manufacturers and emerging lighting tools hope to bridge gap between light source and application.

An old saw in the vision industry goes something along the lines of, 'if bad optics can distort features beyond usefulness, then bad lighting can make those features completely disappear.' In the last 24 months, vision integrators and -- at the suggestion of manufacturers -- even end-users are starting to listen to what experts have been saying for years. Namely, that the most sophisticated automated inspection system in the world cannot work without the proper lighting. In response, the numbers of machine-vision light sources - along with small start up companies who hope to capitalize on the market  - are expanding at an unprecedented rate. 

'When machine vision came out you probably had three different vision tools: edge detection, correlation and intensity. Now you have hundreds of algorithms because each is application-specific. Lighting is the same way. Once upon a time you had spot, ring and back lights. Now there's all kinds of variations on sizes, wavelengths and techniques & so people can get off-the-shelf lighting for a specific application,' explains Allen Burns at RVSI, Weare, NH.

Integrators and end-users now have a significantly greater number of lighting solutions, particularly among LED-based solutions, as a result of the growing number of machine vision applications. These lights were original developed for specific applications including BGA inspection (upper left), pharmaceutical label verification (upper right) and wafer sorting/mapping light (lower picture).

A market that started around the microscope has become machine vision specific thanks to a realization that each vision applications, and in many cases, each part requires a specific lighting solution in order to optimize system efficiency. 'Older companies are starting to diversify beyond ring lights,' Burns added. 'Customers are looking for very specific products, such as a light for looking at the bottom of cans or ball grid arrays (See figure), fiducials, or connectors. It seems to be getting very specific.'

LEDs gain ground
Vision consultant Perry West at Automated Vision Systems Inc. (Campbell, CA) agrees that the vision industry is seeing an explosion of new lighting products, but qualifies his experience in a serious enhancement of LED sources and the specific applications that these devices tend to target. 

'We're certainly seeing a broader range of offerings,' West said. 'We're seeing the most entries in LED lighting, while the competing technology of fiber optic lighting remains fairly static in terms of the players. New comers are going into LED lighting as we saw at the Vision Show, but I don't think there's enough applications to support all these companies. I'd look for some shake outs of the players in this area.'

According to West, most of the application-specific lights are of the LED genre: monochromatic, long-life systems for semiconductor inspection. 'The main driving factors for LEDs are the flexibility of their configuration, low [cost] barriers to entry and long life times. When you can build the array [of LEDs] into any shape or size you want, you can direct the light very easily& These systems are making significant inroads against fiber optics, primarily in electronics and semiconductors where monochromatic light is perfectly satisfactory and long life times are desirable,' he said. 'Fiber is still good when you want a broad band light source.'

Structured light has also seen a steady growth. Typically, structured lighting or laser light generators provide excellent narrow-band illumination sources where horizontal or vertical variations in the laser line as it scans across an object provide a source of data to generate high resolution 3D measurements, including those hard to reach places such as the depth of a drilled hole or sharply angled objects. According to Quebec-Canada-based Lasiris, provider of structured light solutions including laser line generators, their business niche has seen more than 50 percent growth per year since they offered their first industrial vision lighting system in 1991. 

While some integrators and manufacturers realize this expanding light library that is now available to the end user, the major companies have yet to change their cataloging techniques to reflect an easy 'choose by application' scheme. The result is that while vision board and system manufacturers get the occasional educated end-user who specifically knows he needs a system that works in hue-saturation-intensity for vector correlation, lighting companies still have to field the 'here's my part, how do I light it question.'

Answering the light question
Since the late 80s, lighting experts have been searching for a way to move lighting out of the occult and into a reproducible science that is accessible to all. Michael Muehlemann, president of Illumination Technologies Inc. (East Syracuse, NY), a lighting and optics consultant for machine vision, uses a flow chart to illustrate the complicated thought processes that go into a lighting choice.

Like Muehlemann, lighting experts always start with the object under test, its characteristics and the criteria that need inspection for a successful test. Also like Muehlemann, most flow charts will repeatedly return to the 'lab' for expert analysis of the spectral characteristics, contrast and other elements that help vision engineers determine the best relationship between the light source and the part. 

Perhaps the best way to emphasize the cause-and-effect relationship between an object under test and the light source is to realize that the object itself acts as an optical filter. For instance, the part may reflect some wavelengths while absorbing others, while changing the optical path. Experts in the field have been trying for many years to quantify and standardize the relationship between different objects and effective lighting solutions; in essence putting the brains of a vision engineer into a useable, automated system. 

Many of these systems are still around today. The Lighting Science Database first developed by Kevin Harding and associates at the Industrial Technology Institute (Ann Arbor, MI) defines seven decision points that help guide a user through the lighting decision process. This work led to the All Purpose Lighting Investigation System (ALIS 500, also known as micro ALIS), a lighting testbed that offers several of the industry's standard lighting configurations. The ALIS 500, which offers several different moveable light sources for easy line testing using almost any vision engine and image acquisition system, is still available today from Dolan-Jenner (Lawrence, MA). 

According to Harding, 'There are countless ways we can filter, direct, or otherwise control the lighting that will either make or break our machine vision application. Clearly, the part itself serves a role in changing the lighting. The next step& is to put [these tools] together as a tool kit that does not require set up or optical expertise to operate. This last step was the philosophy behind the design of [ALIS].'

 More recently, Bruce Batchelor of Cardiff University has released the lighting advisor. Unlike the lighting science database, the advisor assumes that the user's level of understanding will already allow him or her to decide whether they need 'crossed linear polarizers' or 'tunable monochromatic lighting.' Batchelor is currently working on a remotely operated evaluation system that would allow a vision engineer to trouble shoot the hardware set-up as well as develop specific algorithms without being on site. 

Admirable as these efforts may seem, critics - albeit very polite critics - abound. 'There's still a lot of work to be done on these systems,' said West. 'A lot of very knowledgeable people have bent the pick on this one, but there's room for a lot more work and many breakthroughs. Even if you have a program that is good enough, will the users be knowledgeable enough to provide the specific characteristics for the program? For instance, for a question about a filter, you need the spectral characteristics of what you're trying to look at, and people don't always have the means to determine those characteristics.'

'Lighting is still an art form. It's passed the witchcraft phase. In the early 80s , you did it once, but you couldn't always do it again. Now you can repeat, but it's still an art form,' he said.

No substitute for experience
Despite the lighting challenges, machine vision  -- even with its technical requirements and challenges - is one of the most powerful tools available to manufacturers for quality and process control.  The vision industry continues to build on its lighting expertise through the accumulated years of experience of its application engineers.

Although new applications require specific solutions and careful engineering, for many industries and applications, one can find turnkey application specific machine vision systems in which the lighting already has been optimized for the application.  In these systems, the lighting design is essentially transparent to the user.

Today there are many resources available to assist the end user in the selection and optimization of the lighting arrangement.  In addition to the suppliers of the basic machine vision hardware, who frequently employ application engineers to assist users in 'staging' (lighting, optics, camera) issues, several lighting and optics suppliers now offer an application engineering design and build service for staging. There are also many system integrators with some expertise in lighting design. For more information on lighting, contact the companies discussed in this article or contact the Automated Imaing Association for a list of experienced consultants, suppliers and designers.

About the Author:  Prior to accepting his position as managing editor of Wireless Design Online, Winn Hardin spent several years covering the vision industry for a variety of leading trade publications.