CCDs Aid Exploration
Paula M. Powell
Whether imaging the galaxy or intracellular activity, cameras based on CCD sensors continue to capture ever more complex images of the known universe. Still images that are generated with full-frame CCDs can present stunning vistas, but images can become even more intriguing when they capture dynamic motion at speeds beyond the ability of conventional commercial cameras.
As evidence, consider these images of primary neuron cultures from rats, acquired by engineers at Solamere Technology Group (see figure). This time-lapse sequence, which illustrates intradendritic transport of particles labeled with green fluorescent protein, was captured with a confocal microscope and the XR/MEGA-10EX, an intensified CCD camera from Stanford Photonics Inc. of Palo Alto, Calif.
A CCD is simply a collection of pixels that produce an electrical charge proportional to the amount of light received. Many cameras designed to take still photos use a full-frame architecture, where the imaging sensor is an area array that uses CCDs both to collect photo-generated charge and to transport it. To capture extremely fast dynamic motion, many end users still turn to interline CCDs, one of the topics covered in the series of articles that follows.
Although the advantages of high-frame-rate CCD technology were perhaps first realized by the biophotonics community in applications such as single-molecule fluorescence, Ravi Guntupalli of Princeton Instruments, a division of Roper Scientific Inc. of Trenton, N.J., reports that the surveillance market has taken an increasing interest in it. He sees growing excitement about devices with on-chip multiplication that combine high frame rates with low noise and high resolution.
One fairly common surveillance task these days is that of on-road license plate inspection, usually using CCD cameras mounted on some type of over-the-road gantry. Toshi Hori, president of JAI Pulnix in Phoenix, said that in this type of fast-event capture, image clarity and resolution are critical. System and camera must compensate for changing light conditions, variable vehicle speed, positioning of the vehicle within the target zone, color or legibility of the subject plate, light reflections and a host of other variables, he added. Progressive-scan cameras reportedly can capture 30 full-frame images per second easily, which can be sufficient for sequential imaging, even at today’s freeway speeds.
Now consider the dynamic action taking place at rates as high as 120 fps in the low-light scientific imaging arena. Here, there is often interest in short-exposure, fast-frame imaging, especially when samples are light-sensitive. At present, some interesting competition appears to be brewing between electron-multiplying and intensified CCD camera technology in this application area. According to scientists at Andor Technology in Belfast, Northern Ireland, electron-multiplying CCDs can offer higher, broader quantum efficiencies, with back-illuminated formats exceeding 90 percent.
It will be obvious from reading the article by Mike Buchin, president of Stanford Photonics, that the jury is still out on this competition, however. Ultimately, both types of sensors will most likely have some role to play in this market.
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