Lights! Camera! Action! Washington DC

Two decades ago, when he founded Mercron, Ken Zeiler learned that his machine vision customers were searching for answers to their illumination needs. Most of the time, they didn't know the questions. Literally and figuratively, it was his job to shine a light on their applications. Today, the job is a little easier because of experience and the existence of knowledge-packed data bases.

"One of the biggest problems is the fact that commercial lamp literature is written for commercial and industrial uses," Zeiler says. "Take those same parameters to machine vision and they don't fit. Industrial light is based on the human eye. With a camera, it won't be as good. These are the kinds of details engineers have to make sure of. The hardest thing for an engineer is to figure out the light needs."

Too many machine vision systems developers don't spend enough time planning their lighting needs, says William Biederman of Vision Light Technologies, the U.S. representative for France-based Phlox. "People who are informed figure out lighting and presentation of the object before picking a machine," he says. "You should spend 80 percent of the time on lighting before you can use your application. I've seen people pick out their camera, lens and software, but hadn't figured out how to light the thing."

Andy Falconer, director of V Cubed in the U.K., agrees. "If you get the illumination right it simplifies it and makes sure you get the right information. Illumination highlights what you want and makes it work for you."

MISTAKES

"Illumination is a crucial aspect of any application. New users need to spend time selecting illumination. It can be the cheapest and simplest, but not always. How many people make penny decisions on dollar machines?" asks Patrick Varley of Volpi.

One way to avoid that error is to pick the right lighting as early as possible, says Dave Muyskens, President of Spectrum Illumination. "If they don't have the room later, it's hard to compensate for poor lighting. The earlier you can get involved in a project, the better. The design stage is better than trying to fit it in later."

Elements to consider: correct type, whether indirect or dark-field illumination or backlight. Most people know the different types but not necessarily how to use them. Lighting companies can tell them what will work best for their project. It's not all cut-and-dried, but until they actually test it they might find out that something may work better for their project.

Zeiler points out a food company that had a system designed for tungsten lighting. It was using too much power, so they switched to metal halite lamps. "They didn't put out light in the colors they needed," he says. "Those things should have been determined before the system design."

In the food industry, for obvious reasons, light sourced with glass has to be shielded from the product. The lamps will overheat. The fluorescent lamp was invented in the 1930s. It was designed to meet specific applications. The main thing for longevity and brightness is that if it gets hotter or cooler the output will go down, Zeiler says. "If you have a plastic box with a lamp inside, air needs to get in there to hold the temperature down. The output will decline up to 50 percent. The lamp driver watches the temperature and holds the light level constant. If the temperature goes up, the lamp driver adds power to maintain brightness. It could go to thermal runaway [the driver adds more power to hold brightness, but the result is less and less light]. In this case, just put a fan inside the box with the lamp. It will raise the ability to get rid of the heat four times."

While the cameras can pick up any color, the light has to have the target color, Zeiler says. We all know almonds are brown, but you're looking for black or yellow (defects) so the light source needs those. "French fries have to be white. If they're too yellow, they're out so the light source has to have features that will reject yellow. You can use tungsten or metal halite. They have the color bands."

Biederman mentions a problem he often runs into: thinking that one single geometry of light will do the application. "You have to think about the size of the object and the speed," he adds. "In bottling applications, it's really difficult to do 1,200 a minute. You need outstanding lighting and control—geometry, the angle the light hits the object—how it gets delivered. A customer will say, 'I've got this object and I need to see 30 different things on it. I need a bunch of lights, sequenced, and I need different wave lengths, color and positioning.' Oftentimes it's not a single-camera presentation. People find problems they didn't think they had. They want to assemble some parts together, then realize that, 'Oh, this part is out of spec.' It all comes back to knowledge."

THE TREND

"Traditionally, halogen coupled with fiber optic was the default lighting," says Varley. "For the longest time, that's all we did. The benefits are very intense light and it's controllable. There are some subtle differences, especially in whites. The limitations are bulb life, which is in the 100s, maybe 1,000 or 2,000 hours. That's a big detriment. If something is fine-tuned, the replacement is not exact, so you need another calibration procedure.

"It does have its uses," he adds. "The intensity is great and it's a cold light source. Heat is not transmitted through the fibers. If you're looking at biological samples, for example, with an LED the heat is under the sample. With this, cold light is under the sample."

Eventually, though, people started to turn to LEDs. It started with small, classic LEDs of 3mm or 5mm. They have longer life and different varieties of wave length. Control is pretty quick. Instead of a ½-inch or ¾-inch fiber bundle going through the machine, you had them in the power cord instead, Varley says. There was no routing of the cable, replacement of the bulbs or outside power supply. You lose light snaking through. With this, you're not losing light. That was the first change from halogen/fiber optic.

"What we're starting to see emerge is a combination," he adds. "Instead of halogen/fiber optic, we're using an LED source with fiber optic delivery. You get the long life of LED and separation of heat from the point of illumination. You get better light. We see that as the current trend and it's going to continue. There's long life and the color availability of LEDs, separation of heat from the object [because of the fiber device] and pretty consistent control and light source."

LED lighting is the fastest growing because there's been such a huge increase in intensity, especially white, says Muyskens. "It has doubled in intensity from a year ago. The home market is trying to replace standard incandescent bulbs, and that's why white is advancing so much. You won't see it that fast with other colors. It's gone from 45 lumens to 90 lumens just that fast, but you can drive it to over 200 lumens per LED.

"Advances in LED [combine with] the disadvantages of halogen or fluorescent or xenon strobes, [which] have such a short lifespan. LEDs have made huge advances; they're getting closer and closer to where they can compete with the brightest. I have some projects switching from xenon strobe to LED because the brightness is getting so close."

Most LED users require a driver, or controller around each light source. LED current controllers typically provide a digitally controlled, repeatable constant current output, along with accurate pulse timing with variable delay and pulse width and selectable intensity. Current and pulse timing are set manually or dynamically from software and are stored in the controller, explains Peter Bhagat of Gardasoft Vision in the U.K.

The pulses are triggered on an opto-isolated digital input, usually from a PLC, frame grabber or camera. The pulse width and the delay before it fires can be controlled. "Often two or more views of an object are taken using different lights at different intensities," Bhagat says. "With an LED lighting current controller it is possible to switch lights on and off and to switch between several preset intensities."