A need for speed

Many inspection systems work well with standard TV-type cameras that operate at 30 frames/s. But as production-line speeds increase, engineers must look to digital cameras that offer higher image rates, often between 60 and 120 frames/s. A change from an analog camera to a high-speed digital camera, however, involves more than substituting one camera for another.

Engineers have often mated analog-output cameras with inexpensive PC-based frame grabbers that digitize video signals. High-speed digital cameras, though, rely on standard digital interfaces such as USB, FireWire, or Gigabit Ethernet that PC vendors supply or that designers can add to a PC at low cost. (The Automated Imaging Association's proposed "GigE Vision" standard will operate at about 800 Mbps, which will accommodate a 2-Mpixel camera at 30 frames/s or a 1-Mpixel camera at 60 frames/s.)

The GE Gigabit Ethernet series of cameras can operate as fast as 200 frames/s with a resolution of 659x493 pixels. Such cameras find use on high-speed production lines and in motion-analysis equipment. Courtesy of Prosilica.

These digital-communication standards impose new limits on the distances between cameras and host computers, an aspect of system integration that some engineers may overlook at first. "Gigabit Ethernet lets you put a camera about 100 m from a host PC," said Ken McDonald, senior applications engineer at JAI Pulnix. "But Camera Link gives you a 10-m distance between a camera and a PC, USB limits that distance to 15 ft, and FireWire sets a limit of about 10 ft."

The use of high-speed cameras also affects lighting and lens choices. "When a camera operates at 30 frames/s, you have a 33-ms exposure time," McDonald said. "But at 100 frames/s you have only a 10-ms exposure time, so you need more than a threefold increase in light intensity to get the same number of photons to the high-speed camera's sensor."

If lighting changes and lens adjustments still don't provide enough light, Furse said you can consider changing the gain of their digital cameras. But a gain increase also may increase noise in a video image. Thus, if you plan to change gain settings, you should specify low-noise cameras—those with a high signal-to-noise ratio.