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High-speed image capture for motion analysis is used in
a wide variety of applications, including vehicle-impact testing (VIT);
biomechanical research; range, aerospace, and ballistics (RAB); and
particle-image velocimetry. Various technologies have been developed for
high-speed imaging over the years, including high-speed analog and
digital video imagers using CCD senors, but until recently the
high-speed film camera has been the workhorse in industrial and military
applications in which both high frame rates and excellent image quality
are critical. Image quality is key in these applications, in which
producing beautiful full-color slow-motion images of vehicles crashing
or of bombs exploding is almost equal in importance to the business of
motion analysis.
The early days
The high-speed CCD cameras of the 1990s offered great advantages over
film in terms of production cost and ease of use. And, whereas
high-speed film technology required chemical processing of the film and
digital scanning of images for computer-based motion analysis, most
high-speed CCD cameras could download images directly to computers for
analysis immediately after a test. Performance of high-speed CCD imagers
of this time was 640 × 480 pixels or below at 1000 frames per second,
with higher frame rates available at lower resolutions. But there was a
growing demand for higher resolutions, better blooming suppression, and
faster shutter times that high-speed CCD cameras could not easily meet.
By 1997, Eastman Kodak Motion Analysis Systems Division (now Redlake, of
the Roper Industries Imaging group) had developed a 1024 × 1024-pixel,
1000-frames/s CCD camera as part of a DARPA (the Defense Advanced
Research Projects Agency; Arlington, VA) project for a holographic
datastorage system. The camera had 64 channels operating at 30 MHz. As a
result of the number and complexity of the output channels and the power
requirements of the sensor and supporting electronics, the camera
mechanical design was divided between a camera head that housed optics,
sensor, and channel preamplifiers, and a half-rack-mounted chassis (to
which the camera head was tethered via a short cable) for the
analog-to-digital converters, power supplies, frame memory, and
image-processing engine. The camera produced beautifully crisp and clean
images at 1024 × 1024 pixels and 1000 frames/s, but the size and cost of
the camera made it impractical for use in any of the traditional
high-speed camera markets. Only a few of these cameras were produced.
The move to CMOS
High-speed camera design has now shifted to an almost exclusive use
of CMOS sensors. Looking back just a few years ago, it was unclear to
most that CMOS sensors were capable of the image quality needed for all
but low-performance, low-cost, high-volume applications such as
camera-on-achip designs for consumer products (see "Comment," p. 57).
So why would high-speed-camera manufacturers shift from CCD to CMOS
technology to get better image quality and resolution? The answer is
that CMOS sensors represent a breakthrough technology for high-speed
imaging because they address most of the high-speed design requirements
better than CCDs.
Typical performance of high-speed CCD cameras of a few years ago was 512
× 384 at 1000 frames/s. Current requirements are for resolutions of up
to 1504 × 1128 at 1000 frames/s and 100,000 frames/s at reduced
resolutions. The less complex output channels of CMOS imagers allow more
channels than were practical with CCDs; CMOS sensors for high-speed
applications may have from 10 to 32 or more output channels. In
addition, for CMOS cameras, pixels can be read out of the output
channels every clock-pulse, while CCDs typically require two to four
clocks per pixel. One can see that by simply doubling the number of
output channels and clocking pixels at the same rate, the pixel rate
increases by a factor of four.
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FIGURE 1. A high-speed CMOS camera operating at
2000 frames/s captures an image of a dart bursting a water-filled
balloon.
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For high-speed imaging, very short exposure times are
required to minimize image blur (see Fig. 1). Full-frame CCDs require a
mechanical or electrical (usually liquid-crystal) shutter, both of which
are limited in speed. Interline CCDs and frame-transfer CCDs use
"electronic shutters," but they can suffer from image smear and are
generally effective only to a minimum exposure of 20 µs. In contrast,
CMOS sensors have precise electronic shuttering in the 2- to 10-µs range
required for ballistics and some laboratory applications.
High-speed
cameras often must operate in situations with very high-contrast
lighting. For example, large banks of high-intensity lighting are used
in VIT (see Fig. 2). Light bouncing off reflective surfaces cannot be
controlled during crash testing. In RAB applications, ordnance
detonations or rocket plumes are often sources of extreme contrast. But
CCD sensors are prone to blooming (the overflow of electrons from one
pixel to the next or to other adjacent structures). There is a tradeoff
in CCDs between sensitivity and blooming suppression—but high-speed
cameras require both sensitivity and blooming suppression. In contrast,
CMOS sensors are not prone to blooming; while saturation effects can
occur, they do not obliterate images the way blooming in CCD sensors
can.
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FIGURE 2. An automobile is imaged at 1000 frames/s as it undergoes
a 40-mph side-impact test. The lack of blooming in the CMOS camera used
in this application allows imaging under very nonuniform lighting
conditions, while the camera's high light sensitivity allows
automotive test engineers to scale back on expensive test
illumination. |
Form factor
is another variable in the migration of high-speed cameras to CMOS
technology. In order to fit cameras to mounts formerly used for film
cameras and lower-performing high-speed CCD cameras, the
newer-generation higher-performance cameras generally need to conform to
the size and weight of the older products. As was seen with the CCD
camera developed for the DARPA holographic data-storage system, it would
be very difficult to design a high-performance high-speed CCD camera
that would conform to the form-factor requirements of the traditional
high-speed market.
Flexible windowing
Another important requirement for high-speed cameras is the ability
to produce higher frame rates at reduced resolutions. The high-speed CCD
cameras of the 1990s generally offered two to four or five resolutions.
In many designs, the horizontal resolution was static and only vertical
resolution could be changed. In other designs, both horizontal and
vertical resolutions could be changed, but the optical center of the
readout would migrate to one corner of the image. Challenges in
designing CCD cameras with flexible "windowing" were apparent. In
contrast, CMOS technologies allow extremely flexible readout of the
sensor. This flexibility is realized in current high-speed CMOS cameras
that may have thousands of selectable resolutions, helping the user to
maximize performance.
Light sensitivity is a vital issue in high-speed imaging. Exposure times
of 5 to 10 µs may be necessary to limit motion blur of a fast-moving
projectile, especially for RAB applications, which often rely on
available light outdoors. Lighting used for VIT applications is very
expensive, so light sensitivity becomes a financial issue for the
automotive market. Finally, light can become a heat issue for laboratory
applications. Early CMOS cameras based on designs with 0.5-µm minimum
feature sizes had inherent sensitivity problems. Advanced CMOS-sensor
designs that use fewer components per pixel and take advantage of
0.35-µm CMOS technology provide better light sensitivity than the
interline CCDs used in most of the older high-speed cameras.
The image quality of these new CMOS high-speed cameras, critically
important for much of this market (and somewhat of a question as far as
CMOSsensor technology is concerned) has been very well received. Image
quality of CMOS-based cameras is finding acceptance today even in the
stillcamera market, as evidenced by the use of sensors based on 0.35-µm
CMOS technology used by Canon in some of its digital single-lens-reflex
cameras. |
