research activities in 2007 - CSEM

High Dynamic Range Versatile Front-End for Vision Systems
P. Heim, F. Kaess, P.-F. Rüedi
A 128 by 152 pixel array with very high intra-scene dynamic range has been integrated in a 0.18 µm optical process. It features a data
representation which encodes nearly 7 decades of illumination with a 10 bit data word. Furthermore, the data representation used facilitates
subsequent data processing such as contrast computation.
There is an increasing need for optical sensors optimally
suited for systems whose purpose is not to restitute an image
to a final user, but to analyze the content of a visual scene
and make a decision. The main requirements for such a
sensor are a wide intra-scene dynamic range and a data
representation facilitating processing. Standard image sensors
have a too narrow dynamic range to cope with the
tremendous change of illumination occurring in natural visual
scenes. Logarithmic imagers offer a wide dynamic range and
a data representation which easily discard illumination
changes in an image. However, up to now, the logarithmic
compression has been performed in the analog domain,
bringing a high pixel-to-pixel fixed pattern noise, which makes
them unusable for commercial applications.
The visual front-end developed at CSEM circumvents this
issue. It incorporates a high dynamic range pixel array with
logarithmic compression in the digital domain to avoid the
large fixed pattern noise associated with analog compression.
Figure 1 shows a block diagram of a pixel and the logarithmic
time generator. Each pixel integrates the photocurrent
delivered by a photodiode on a capacitor. The resulting
voltage is continuously compared to a reference voltage
(VREF). Once VREF is reached, the content of a 10-bit digital
word distributed to all pixels in parallel is stored in the pixel
memory. This digital word evolves over time to code the
logarithm of the time elapsed since the beginning of the
integration. Once photo-current integration is terminated, the
10-bit words stored in the pixel array are read-out.
Figure 1: Block diagram of the pixel and logarithmic time generator
The data representation delivered by the sensor enables to
easily discard illumination and compute the contrast between
neighbouring pixels.
The circuit encompasses an array of 128 by 152 pixels, with a
pixel pitch of 14 µm and a fill factor of 20% in a 0.18 µm
optical process. Figure 2 shows a microphotograph of the
circuit.
Figure 2: Micrograph of the circuit
The left of Figure 3 shows an image acquired with the sensor.
Notice that the face of the person and the outside background
are simultaneously visible, illustrating the high dynamic range.
The right of Figure 3 shows the contrast representation
obtained by simply computing the difference between
neighboring pixels. Notice the independence on the
illumination level.
Figure 3: High dynamic range visual scene
A system-on-chip [1] incorporating a 320 by 240 (QVGA) pixel
array based on this principle, an icyflex [2] processor, RAM and
communication interfaces is now in the process of being
integrated. It will enable vision applications (image capture
and processing) to be performed on a single chip.
[1] C. Arm, et al., “icycam , a System-On-Chip (SoC) for Vision
Applications”, in this report, page 30
[2] M. Morgan, et al., “icyflex, a Low Power 32-bit Microcontroller
Core”, CSEM Scientific and Technical Report 2006, page 20
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