Lighting Makes Its Mark on Vision Systems

You must carefully illuminate the units you want to inspect, and the geometry of those units will dictate your lighting choices.

C.G. Masi, Contributing Editor -- Test & Measurement World, 5/1/1998

When you design a vision system that will inspect PCBs or other assemblies, you’ll probably concentrate on the camera’s specs. Remember, though, that you must carefully illuminate the unit you want to inspect. A camera “sees” only the light the PCB and its components reflect. So, the geometry of the units you want to inspect will usually dictate your lighting choices.

The type of camera you use for inspections will determine how you arrange the light source. Inspection systems typically use two types of cameras—array and linescan. A conventional array camera acquires a rectangular or square picture all at once. A linescan camera has a linear array of charge-coupled device (CCD) sensors. Linescan cameras used for PCB inspection usually have around 6000 pixels.

A lighting system must deliver as much light as evenly as possible to the units in the camera’s field of view. At the same time, the lighting should control specular reflections and shadows. The illustration in Figure 1 shows several lighting choices.

Bright Reflections Cause Problems
Bright specular reflections from shiny and flat surfaces cause saturation and blooming in a CCD camera. Saturation overloads pixels with light. The saturated pixels just can’t hold any more electrons. In some cases, excess charge from saturated pixels spills into adjoining pixels and saturates them, too—an effect called blooming. The bright specular reflection from the top of a flat IC package could saturate pixels and prevent a camera from reading the printing on a package, even if the printing is non-reflective. Although light might not saturate the pixels that “see” the printing, blooming from nearby saturated pixels could affect them.

Some lights create shadows and make it difficult, if not impossible, for a camera to see everything on a PCB. If you use a point-source light (Fig. 1), for example, a shadow falls over part of the resistor at the left of the PCB. The camera has no chance to observe the end of the resistor closest to the tall component. So, most inspection systems don’t use point-source lighting.

Figure 1. You can choose from several types of lighting to illuminate a PCB. Different types of lighting let you examine a PCB for different things. Diffuse lighting, for example, lets you examine a PCB for proper component placement.

In general, a diffuse light source provides the best light for almost any automated board inspection task that uses an array camera. Diffuse sources overcome specular reflection by spreading the illumination over a large area and by illuminating the board from many angles. You get a lot of illumination without any part of the board or assembly appearing particularly bright or remaining hidden in a deep shadow. A shadow cast by one section of the light source gets filled in by light from other sections.

Use an LED Array
Manufacturers offer diffuse light sources made by mounting light emitting diodes (LEDs) in a rectangular array. LEDs are ideal for a diffuse source. Individually, they are not particularly bright, but en masse they produce a lot of light.

A diffuse light source should spread light over an area that closely matches the camera’s field of view. Matching the field minimizes the amount of light wasted and keeps lighting equipment small.

A ring light (Fig. 2) offers another choice. This light surrounds the camera lens and floods the camera’s field of view evenly while avoiding light and dark spots. The ring light uses a remote light source that illuminates the ring through a fiber-optic cable.

Figure 2. A ring light clamps onto a camera’s lens to provide area illumination for an array camera. A remote light source illuminates the ring through a fiber-optic cable. (Courtesy of Fostec.)

You may choose to use a linescan camera rather than an array camera. A line camera operates much like a fax machine that scans a document one line at a time. It acquires multiple line images as a board or assembly moves through its line of view.

The camera feeds the line images to a PC, which then builds up an image from successive lines. A line light (Fig. 3) provides an excellent light source for a line-
scan camera. The light delivers bright illumination along the same line that the camera scans across the board.

Figure 3. A fiber-optic line light concentrates illumination in a bright line for a linescan camera. (Courtesy of Fostec.)

Lenses Concentrate Light
In addition to choosing a light source, you can also choose lenses that intensify and shape the illumination field and make it more diffuse. For example, placing a Fresnel lens in front of an LED array will concentrate the light so that it does not spread out. If the distance between the light source and the board you want to inspect is one-half the focal length of the Fresnel lens, all the light will fall into one-quarter of the source’s area, increasing the brightness by four times.

Fresnel lenses aren’t the only way to concentrate light. You also can use other types of lenses, optical fibers, or reflectors for the task. For example, the ring and line lights rely on optical fibers to transmit light and to properly position and concentrate the light. With a line source, the manufacturer forms the optical fibers into a line that directs the light in a line.

Select a Lamp
In addition to choosing how you want to arrange a light source, you also must determine where the light will come from. Most automated PCB inspection systems use one of four types of illumination: laser diodes, LEDs, strobe lights, and halogen bulbs.

Laser diodes are small, high-intensity lights that manufacturers may mount in arrays. Solid-state laser diodes provide a highly collimated beam of monochromatic light, and they have high reliability and long lives. A controller can pulse them to provide variable bursts of light. Solid-state laser diodes are relatively expensive, but they find applications in 3-D measuring systems, which take advantage of their collimated light beam. You can also spread the laser beam by using lenses.

Light emitting diodes (LEDs) are also small solid-state sources. Manufacturers arrange inexpensive LEDs in arrays to generate the high light levels needed for automated inspection systems. LEDs come in a variety of visible and infrared wavelengths, but the wavelengths from LEDs occur in fairly wide bands. The light from an LED can range from a narrow beam to a broad beam, depending on the type of LED. Like laser diodes, LEDs can provide short bursts of illumination under computer control.

Strobe lamps are familiar devices that provide a short (typically 10 ms), bright burst of light. Unlike lasers and LEDs, strobe lights provide a wide-band continuous light spectrum centered at visible wavelengths. These arc-discharge lamps require a high-voltage pulsed power supply, and the bulbs have lifetimes limited to about 500 hr at 60 flashes per s. But most strobe lights are expensive.

A quartz-halogen bulb provides an intense, white, continuous-on light at a low cost. It also produces a lot of heat. Although you can’t eliminate the heat, you can remove it from the inspection area by using a fiber-optic cable to channel the light several feet from the source to the inspection area.

You can adjust the output of a quartz-halogen bulb by using a variable DC power supply, but the spectrum of light varies with brightness. Quartz-halogen bulbs used for board inspection have a lifetime of about 200 hr. Dim-bulb long-life models can last up to 500 hr.

Look at the Surroundings
Choosing between the various light sources and their configurations is a complex exercise, but you can usually limit yourself to a few choices. Suppose you want a strobed light source to illuminate PCBs. A bright strobe light will make working nearby impossible for humans.
On the other hand, a pulsed infra-red source, such as an array of infrared LEDs, could provide pulsed illumination without affecting people.

Choosing between lighting options is best done in consultation with an experienced inspection-system integrator. You can tell the integrator what type of inspection information you want from the system and what other equipment will need that information. You also should consider human factors that might limit the use of particular types of illumination or light sources.   T&MW
 

FOR FURTHER READING
1. Van Dommelen, Carl, “Choose the Right Lighting for Inspection,” Test & Measurement World, Newton, MA, October 1996, pp. 53–58.
 

ACKNOWLEDGEMENT
The author wishes to thank Steve McCool of Graftek Imaging for contributing to this article.

C.G. Masi works as a freelance technical journalist. He is the former chief editor of Test & Measurement World.