For many machine-vision industrial applications, cost-effective throughput is very important. This means using charge-coupled device (CCD) linescan and area-scan CCD imaging cameras with the right combination of high speed, high resolution, and sensitivity to low-light conditions. Applications like production-line inspections of flat-panel displays (FPDs), printed-circuit boards (PCBs), IC wafers, document scanning, and web inspections require such key parameters (Fig. 1).
Dalsa Corp., which also makes CMOS imaging cameras (see “CCD And CMOS Image Sensor Breakthroughs Promise A ‘Bright’ Future”), addresses these needs with its Piranha enhanced-sensitivity (ES) CCD cameras. These devices deliver cost-effective solutions using time-delay integration (TDI) technology. They also bridge the sensitivity gap between single-line cameras and the Piranha high-sensitivity (HS) family (Fig. 2).
“For certain applications, customers are looking for enhanced sensitivity at lower costs. The ES cameras will meet their expectations while achieving excellent signal-to-noise ratios that cannot be achieved using single-line or dual-scan technology in low-light applications,” said Xing-Fei He, sensor product manager at Dalsa.
HOW TDI WORKS
TDI is a method of line scanning that provides more responsivity than other video scanning methods. It is based on the concept of accumulating multiple exposures of the same (moving) object, effectively increasing the integration time available to collect incident light. The object motion must be synchronized with the exposures to ensure a crisp image. TDI also permits much greater scanning speeds in low light and allows reduced lighting levels (and costs) at conventional speeds.
A total misalignment (either translational or rotational) of one pixel or less across the length of a TDI sensor won’t affect image quality. In most applications, a 96-stage TDI device can comfortably tolerate a 2% to 4% velocity mismatch between the inspection web and imager. This isn’t difficult using a webmounted encoder to supply a sync signal to the camera, even with webs that change speed. Thousands of successful applications use this approach.
TDI generally lacks anti-blooming capability to contain “glints,” or shiny, specular reflections from smooth objects or surfaces. While this ensures 100% fill factor, it can attenuate incoming photons in the deep blue region. To compensate, Dalsa developed enhanced blue response TDI sensors.
“Dalsa is the only company that has produced TDI CCD cameras in high volumes in the last five years, and we have 50% to 60% of market share for linescan CCD cameras,” said Xing-Fei He.
The Piranha ES line consists of two speed grades featuring data rates of 320 and 640 MHz. Its three models (ES-82-04k80/ES-80-04k40/ ES-82-04k40) feature maximum line rates of 110/68.6/55 kHz, resolutions of 4096 by 16/4096 by 32/4096 by 16 pixels, and responsivities of 3940/1970/3940 DN, respectively. (Responsivity is expressed in nJ/cm2 broadband, at 12 bits and 0 dB.) According to Dalsa, this responsitivity is 30 times greater than that possible using conventional single-line scan cameras.
The ES cameras use 7- by 7-µm pixels. They’re capable of bi-directional operation with selectable 32 or 16 stages while preventing overexposure and anti-blooming. They also provide a Camera Link-compatible output. Camera alignment is extremely flexible, and the number of Camera Link taps (up to eight), throughput, and line rates are software controllable. And, the ES cameras measure 150 by 80 by 65 mm and dissipate less than 20 W from a 12- to 15-V supply.