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Figure 1: Intra-scene Switchable Gain Output
Figure 2: A scene with both bright and very dark components,
imaged by a standard IT-CCD (left), a standard EMCCD (center),
and an Interline Transfer EMCCD device (right)
designs that can range to multiple
megapixels in resolution.
Key to the performance of this
technology is an Intra-Scene
Switchable Gain feature, which
avoids overflow in the EMCCD output
register under bright illumination
conditions by selectively multiplying
only those portions of the scene
that require it. This output design
is shown in Figure 1, where charge
from each pixel passes through a
non-destructive sensing node which
can be read by the camera control
electronics to provide an initial
measurement of the signal level for
each pixel. This information is used
to drive a switch in the sensor that
routes charge packets to one of two
outputs based on a camera-selected
threshold.
Pixels with high charge levels
(corresponding to bright parts of the
image) are routed to a standard CCD
output for conversion to voltage,
while pixels with low charge levels
(corresponding to dark parts of the
image) are routed to the EMCCD
output for additional amplification
before conversion to voltage. These
two datasets are then merged to
generate the final image. Since the
charge from pixels with high charge
levels does not enter the EMCCD
register, this output architecture
allows both very low light levels and
bright light levels to be detected
while avoiding the image artifacts
associated with overflow of the
EMCCD output register.
The power of this technology can
be seen in Figure 2, which shows
image captures of a single scene
that includes both a bright light as
well as very dark shadows, where
the darkest portion of the image
is illuminated only by moonlight or
starlight.
A traditional image sensor (the left
image in Figure 2) images the bright
part of the image well, but doesn’t
have the sensitivity to “see” in the
very darkest part of the image. A
traditional EMCCD (center) can be
configured to image in the very
darkest part of the scene, but when
the gain is turned up to enable this
low light imaging, artifacts from the
bright part of the scene destroy the
image integrity. Interline Transfer
EMCCD technology (right) allows
the scene to be imaged continuously
from the brightest to the darkest
part of the image, where “dark”
can extend all the way down to
illumination only by moonlight or by
starlight.
Having been moved forward from
the research labs to use in production
devices, Interline Transfer EMCCD
technology is being used today in
a growing family of products. ON
Semiconductor’s KAE 02150 image
sensor uses Interline Transfer
EMCCD technology to enable low
light image capture at 1080p (1920
x 1080) resolution while operating
at 30 fps, making this device well
suited to security, surveillance, and
situational awareness applications
that require high sensitivity image
capture with video frame rates.
For higher resolution needs, the
8 megapixel (2856 x 2856) KAE
08151 image sensor is designed in
a square aspect ratio with a 22 mm
diagonal, aligning with the native
optical format of many scientific
microscopes and other medical
equipment. By leveraging the
advances available with Interline
Transfer EMCCD technology, these
devices are the first in a new class
of image sensors that achieve high
levels of performance under low
lighting conditions.
New-Tech Magazine Europe l 61