Tech Briefs Magazine - September 2021 - PIT-7

Backside
Illumination
Gets Fast
New Sensor Drives
High-Speed Imaging
Improvements
W
ith speeds of over 1million
frames per second,
high-speed imaging
is an invaluable
research tool capable of capturing the
most fleeting events in scientific and
engineering applications. Yet, achieving
this speed comes with strings
attached. Most high-speed cameras
are designed to achieve a maximum
gigapixel/second (Gpx/sec) throughput,
offered in a trade-off of frame
rate to resolution. For example, a 25
Gpx/sec camera reaches 25,700
frames per second (fps) at a resolution
of 1280 ¥ 800 pixels and can
achieve a higher frame rate of 28,500
at a smaller resolution of 1280 ¥ 720.
Both combinations have almost the
same throughput. Very
high frame
rates such as 1-million fps are
accompanied by very small resolutions,
making it more challenging
to see the subject matter.
As frame rates increase, the exposure
time a pixel has to light
decreases. At 25,700 fps, each
frame has a maximum exposure
of 39 microseconds
(µs), and at 1 million fps, the
maximum exposure time is
only 733 nanoseconds (ns).
The short
exposure
times
require high levels of illumination
to compensate for the short
time the pixel receives light. In fact,
Photonics & Imaging Technology, September 2021
Cov
ToC
many high-speed applications are light
starved, meaning that, given the very
short exposure times at high frame
rates, the available illumination won't
deliver enough light to the camera's
imaging sensor to produce an ideal
image and may even be impractical in
certain applications.
High-speed camera operators have
become adept at balancing their need
for speed and resolution with their need
for adequate illumination. They are able
to capture spectacular images that
advance the frontiers of scientific understanding
and engineering analysis, but
the trade -offs
be come more difficult
to manage as
The Phantom
TMX 7510
users push the boundaries of high-speed
imaging.
Recently, a technical breakthrough
was made that eases the speed-resolution-sensitivity
constraint. A new highspeed
image sensor has been developed
that employs backside illumination
(BSI) to increase the pixel surface area
that can capture photons. Because it's
more effective at capturing light, the BSI
sensor is better suited for applications
requiring high frame rates. In this camera,
throughput - max frame rate times
max frame resolution - increased by a
factor of three-times compared to previous
generations of high-speed CMOS
imaging sensors. The new sensor
debuted March 2021 in the
new Phantom TMX cameras,
the fastest of which can shoot
76,000 fps at a full resolution
of 1280 ¥ 800 pixels.
Bringing BSI To HighSpeed
Applications
Until now, the CMOS sensors
used in high-speed cameras
have been based on
frontside illuminated (FSI)
architectures,
in which the
sensor's metal circuitry sitting
above the pixels' photodiodes
is facing the light source. This
metal circuitry prevents some incident
light from reaching the pixels,
7
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Tech Briefs Magazine - September 2021

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