Machine-vision developers are often faced with the inspection of a variety of different products that reflect, refract, or diffract light in various ways. Accordingly, they confront lighting suppliers with the task of providing optimal lighting solutions to meet their system needs. For many applications, developers furnish lighting suppliers with sample parts and expect them to develop off-the-shelf or custom lighting solutions to properly illuminate the regions of interest.
"Often," says Jeff Tanimori, vice president of engineering at CCS America (Waltham, MA), "proper illumination methods can reduce the cost of the type of imager needed, limit the amount of custom image-processing code required, and lead to a solution that can be rapidly deployed." Examples of commonly inspected products are aluminum cans, compact disks (CDs), and glass vials (see images below).
LEFT. Proper illumination allows machine-vision developers to rapidly structure and deploy vision systems. For example, laser markings on beverage cans are often hidden when illuminated with improper lighting. By using diffused dome-type lighting, the etchings become more apparent.
CENTER. Compact disks are shiny, flat objects that cannot be illuminated with simple front lighting. Using diffused flat ringlighting, markings at the center of the disk can be more readily seen.
RIGHT. Spectral, diffuse, and diffracted light from improperly illuminated pharmaceutical vials can hide cracks and chips in the containers. When illuminated with diffused low-angle ringlights, the chip in the side of the vial can be detected.
"Each of these products exhibit different imaging characteristics and must therefore be illuminated differently," says Tanimori. In a typical aluminum-can inspection system, for example, high-speed printing is used to mark each can with letters and numerals that indicate the expiration date of the contents and the place of manufacture.
"In most cases, illuminating this type of object with a light-emitting-diode (LED) ringlight produces a "hot spot" in the acquired image where the light is directly reflected into the camera's field of view," he says. To reduce this spectral reflection in such applications, CCS America recommends using an LED diffused dome light. "Because light is provided from every direction by this device," states Tanimori, "fairly uniform light is provided across the base of the can, eliminating hot spots."
"For CD, DVD (digital video disk), and wafer-type inspections," says Tanimori, "direct LED illumination will also produce hot spots." To overcome this problem, ringlighting is generally used. Although this produces light that is evenly reflected from the CD, it may still be difficult to visualize fine scratches and the data at the center of the CD. Diffused flat ringlights provide an excellent solution.
In the pharmaceutical industry, glass vials often need to be inspected for the presence or absence of chips or cracks. Whereas flat LED ringlights can illuminate the aperture of such containers with proper lighting, the shape of the vials can sometimes produce a prism effect where light is reflected and refracted within the vial, making imaging difficult. "Here," says Tanimori, "the use of diffused low-angle ringlights can reduce hot spots and allow the vision system to better visualize any chips or defects that might occur."
—ANDREW WILSON
