CSEM Scientific and Technical Report 2008

Towards Miniaturized Time-of-Flight Cameras
C. Gimkiewicz, S. Schneiter, R. Cook, M. Columbus, D. Beyeler, Y. Li Schrag, H.-R. Graf, Y. Zha, B. Schaffer, S. Neukom, C. Urban,
N. Blanc
Time-of-flight (TOF) cameras extend classical imaging towards the third dimension. They measure the distance for each image pixel and therefore
acquire true three dimensional images. Required is an active illumination for which the travel time from the camera to the object and back, the timeof-flight,
is measured. New developments like on-chip demodulation drivers in the image sensor, a dedicated digital controller and a high efficiency
VCSEL illumination reduce the size and the power consumption of the TOF camera down to values comparable to conventional (2D) cameras.
Including distance information into an image is advantageous
for many applications since it allows to simply and precisely
locates objects in a scene from a three dimensional image. An
elegant way for such distance measurements is the
measurement of the time-of-flight (TOF), the time that is
required for light to travel from an emitter to an object and
back to the camera. A TOF camera thus requires an active
illumination and a sensor capable of demodulating the
incoming light. In practical implementations the TOF is
measured by the phase shift between the emitted
continuously modulated light intensity and the detected signal.
With modulation frequencies of 20 MHz a distance accuracy
of approx. 1 cm can be achieved. Illumination is typically in the
near infrared and appears invisible to the human eye.
An important feature of the TOF technology, contrary to
classical 3D imaging methods like, for instance, triangulation,
is the potential for miniaturization without restricting the
performance. In the context of the European project ARTTS a
miniaturized and low-power TOF camera has been
developed [1] .
Figure 1: Picture of the miniaturized TOF camera
The distance resolution, i.e. the standard deviation of the
measured distance, is mainly determined by the shot noise [2] .
The shot noise increases with decreasing signal amplitude
and decreasing modulation frequency. In case of a low power
design, the optical power available in the field of view is
reduced as well. In order to compensate for the larger noise
figures, an increase in the modulation frequency is a necessity.
The new TOF camera is powered solely over USB; i.e. the
power consumption including also the active illumination is
below 2.5 W, which is only 25% of the power consumption of
currently available TOF cameras. The higher modulation
frequency required a novel illumination module, and a fast
demodulating image sensor with a low power consumption. A
further step in size and power reduction is the integration of
multiple functional blocks in a dedicated digital controller ASIC.
Due to the use of vertical cavity surface emitting lasers
(VCSELs) the active illumination achieves modulation
frequencies of up to 80 MHz [ 3] . The image sensor provides
integrated demodulation gate drivers capable of demodulation
frequencies of up to 80 MHz. 3D images are delivered at
frame rates of up to 30 fps in QCIF resolution (176x144 pixels).
The newly developed controller ASIC contains analog to
digital converters, image memory, digital signal processing
functionality and a programmable sensor timing block. A
generic digital interface is used to connect to a high speed
USB controller. The controller helps to reduce the chip count
drastically and thus increases the compactness and power
efficiency. The camera has a size of 4x4x4 cm3 only.
Figure 2: 3D image, the distance is given by the brightness level
Figure 2 shows a 3D image, where the distance is color coded.
The system is still under development, but first distance
measurements have proven that the expectations into the
miniaturized system have been fulfilled: The distance standard
deviation ranges from seven millimeters at one meter down to
more than three millimeters at shorter distances.
This work has been financed by the European Commission
under the 6th Framework Program, (contract number FP6-
IST-34107). CSEM thanks them for their support.
[1] European Project “Action Recognition and Tracking based on
Time-of-Flight Sensors (ARTTS)”, www.artts.eu
[2] R. Lange, “3D Time-of-flight distance measurement with custom
solid-state image sensors in CMOS/CCD-technology”,
(Dissertation, University Siegen, 1972)
[3] C. Gimkiewicz, M. Columbus, M. Moser, “Compact illumination
modules based on high-power VCSEL arrays“, SPIE Photonics
Europe Conference, SPIE Proc. 6997, Paper no. 6997-53, 2008
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