Nicolas Blanc

An all-solid-state optical range camera for 3D real-time imaging with sub-centimeter depth resolution (SwissRanger)

A new miniaturized camera system that is capable of 3-dimensional imaging in real-time is presented. The compact imaging device is able to entirely capture its environment in all three spatial dimensions. It reliably and simultaneously delivers intensity data as well as range information on the objects and persons in the scene. The depth measurement is based on the time-of-flight (TOF) principle. A custom solid-state image sensor allows the parallel measurement of the phase, offset and amplitude of a radio frequency (RF) modulated light field that is emitted by the system and reflected back by the camera surroundings without requiring any mechanical scanning parts. In this paper, the theoretical background of the implemented TOF principle is presented, together with the technological requirements and detailed practical implementation issues of such a distance measuring system. Furthermore, the schematic overview of the complete 3D-camera system is provided. The experimental test results are presented and discussed. The present camera system can achieve sub-centimeter depth resolution for a wide range of operating conditions. A miniaturized version of such a 3D-solid-state camera, the SwissRanger 2, is presented as an example, illustrating the possibility of manufacturing compact, robust and cost effective ranging camera products for 3D imaging in real-time.

Novel pixel architecture with inherent background suppression for 3D time-of-flight imaging

The time-of-flight (TOF) principle is a well known principle to acquire a scene in all three dimensions. The advantages of the knowledge of the third dimension are obvious for many kinds of applications. The distance information within the scene renders automatic systems more robust and much less complex or even enables completely new solutions. A solid-state image sensor containing 124 x 160 pixels and the corresponding 3D-camera, the so-called SwissRanger camera, has already been presented in detail in [1]. It has been shown that the SwissRanger camera achieves depth resolutions in the sub-centimeter range, corresponding to a measured time resolution of a few tens of picoseconds with respect to the speed of light (c~3•108 m/s). However, one main drawback of these so-called lock-in TOF pixels is their limited capacity to handle background illumination. Keeping in mind that in outdoor applications the optical power on the sensor originating from background illumination (e.g., sun light) may be up to a few 100 times higher than the power of the modulated illumination, the sensor requires new pixel structures eliminating or at least reducing the currently experienced restrictions in terms of background illumination. Based on a 0.6 µm CMOS/CCD technology, four new pixel architectures suppressing background illumination and/or improving the ratio of modulated signal to background signal at the pixel-output level were developed and will be presented in this paper. The theoretical principle of operation and the expected performance are described in detail, together with a sketch of the implementation of the different pixel designs at silicon level. Furthermore, test results obtained in a laboratory environment are published. The sensor structures are characterized in a high background-light environment with up to sun light conditions. The distance linearity over a range of a few meters with the mentioned light conditions is measured. At the same time, the distance resolution is plotted as a function of the target distance, the integration time and the background illumination power. This in-depth evaluation leads to a comparison of the various background suppression approaches; it also includes a comparison with the traditional pixel structure in order to highlight the benefits of the new approaches. The paper concludes by providing parameter estimations which enables the outlook to build a sensor with a high lateral resolution containing the most promising pixel.

Miniature 3d TOF camera for real-time imaging

In the past, measuring the scene in all three dimensions has been either very expensive, slow or extremely computationally intensive. The latest progresses in the field of microtechnologies enable the breakthrough for time-of-flight (TOF) based distance-measuring devices. This paper describes the basic principle of the TOF measurements and a first specific implementation in a state-of-the-art 3D-camera ”SwissRanger SR-3000” [T. Oggier et al., 2005]. Acquired image sequences will be presented as well.

Demonstration of a novel drift field pixel structure for the demodulation of modulated light waves with application in three-dimensional image capture

A new pixel structure for the demodulation of intensity modulated light waves is presented. The integration of such pixels in line and area array sensors finds application in time-of-flight three-dimensional imaging. In 3D range imaging an illumination module sends a modulated optical signal to a target, where it is reflected back to the sensor. The phase shift of the reflected signal compared to the emitted signal is proportional to the distance to one point of the target. The detection and demodulation of the signal is performed by a new pixel structure named drift field pixel. The sampling process is based on the fast separation of photogenerated charge due to lateral electrical fields below a high-resistive transparent poly-Si photogate. The dominant charge transfer phenomenon of drift, instead of diffusion as in conventional CCD pixels, allows much higher modulation frequencies of up to 1 GHz and a much higher ultimate distance accuracy as a consequence. First measurements performed with a prototype pixel array of 3x3 pixels in a 0.8 micron technology confirm the suitability of the pixels for applications in the field of 3D-imaging. Depth accuracies in the sub centimeter range have already been achieved.

Smart pixels for real-time optical coherence tomography

Optical Coherence Tomography (OCT) is an optical imaging technique allowing the acquisition of three-dimensional images with micrometer resolution. It is very well suited to cross-sectional imaging of highly scattering materials, such as most biomedical tissues. A novel custom image sensor based on smart pixels dedicated to parallel OCT (pOCT) is presented. Massively parallel detection and signal processing enables a significant increase in the 3D frame rate and a reduction of the mechanical complexity of the complete setup compared to conventional point-scanning OCT. This renders the parallel OCT technique particularly advantageous for high-speed applications in industrial and biomedical domains while also reducing overall system costs. The sensor architecture presented in this article overcomes the main challenges for OCT using parallel detection such as data rate, power consumption, circuit size, and optical sensitivity. Each pixel of the pOCT sensor contains a low-power signal demodulation circuit allowing the simultaneous detection of the envelope and the phase information of the optical interferometry signal. An automatic photocurrent offset-compensation circuit, a synchronous sampling stage, programmable time averaging, and random pixel accessing are also incorporated at the pixel level. The low-power demodulation principle chosen as well as alternative implementations are discussed. The characterization results of the sensor exhibit a sensitivity of at least 74 dB, which is within 4 dB of the theoretical limit of a shot-noise limited OCT system. Real-time high-resolution three-dimensional tomographic imaging is demonstrated along with corresponding performance measurements.

A time-of-flight line sensor: development and application

A new miniaturised 256 pixel silicon line sensor, which allows for the acquisition of depth-resolved images in real-time, is presented. It reliably and simultaneously delivers intensity data as well as distance information on the objects in the scene. The depth measurement is based on the time-of-flight (TOF) principle. The device allows the simultaneous measurement of the phase, offset and amplitude of a radio frequency modulated light field that is emitted by the system and reflected back by the camera surroundings, without requiring any mechanical scanning parts. The 3D line sensor will be used on a mobile robot platform to substitute the laser range scanners traditionally used for navigation in dynamic and/or unknown environments.

Recent developments on X-ray phase contrast imaging technology at CSEM

X-ray phase contrast imaging (XPCi) using the Talbot-Lau grating interferometer attracts increasing attention for its implementation in various fields of applications such as in the (bio-) medical domain, non-destructive testing or security. Since the method is compatible with laboratory X-ray tube sources as well as with large field of view digital X-ray image sensors, it has a large potential to provide XPCi for industrial and medical applications as widespread as conventional X-raying is. Here, we report on our recent results and measurements regarding the grating interferometer technology.

Low-power digital image sensor for still-picture image acquisition

This article presents the design and realization of a CMOS digital image sensor optimized for button-battery powered applications. First, a pixel with local analog memory was designed, allowing efficient sensor global shutter operation. The exposure time becomes independent on the readout speed and a lower readout frequency can be used without causing image distortion. Second, a multi-path readout architecture was developed, allowing an efficient use of the power consumption in sub-sampling modes. These techniques were integrated in a 0.5 um CMOS digital image senor with a resolution of 648 by 648 pixels. The peak supply current is 7 mA for a readout frequency of 4 Mpixel/s at Vdd equals 3V. Die size is 55 mm2 and overall SNR is 55 dB. The global shutter performance was demonstrated by acquiring pictures of fast moving objects without observing any distortion, even at a low readout frequency of 4 MHz.

Low-Power Micro-Cameras for Mobile Multimedia Devices

CMOS-based image sensors offer the unique capability of integrating all electronic circuitry of a digital camera on a single chip. The resulting system, hereafter referred to as micro-camera because of its high level of integration, has a great potential in terms of low-power consumption, rendering a wide range of new applications in the field of portable multimedia devices possible. This chapter presents the concepts for the design of such low-power micro-cameras. In a first part, the photodetection principle and the basic CMOS sensor architectures are described. In a second part, the design of low-power Analog to Digital Converters suited for video applications is presented before shortly discussing onchip control and pre-processing functionalities. Two examples of micro-camera realisations are then presented jointly to their specifications, architecture and performances. A summary followed by an outlook on future improvements and applications concludes the chapter.

CCD and APS/CMOS technology for smart pixels and image sensors

The relentless progress of semiconductor technology makes it possible to provide image sensors and pixels with additional analog and digital functionality. Growing experience with such photosensor functionality leads to the development of modular building blocks that can be employed for smart pixels, single-chip digital cameras and functional image sensors. Examples given include a non-linear pixel response circuit for high-dynamic range imaging offering a dynamic range of more than 180 dB, low-noise amplifiers and avalanche-effect pixels for high-sensitivity detection performance that approaches single-photoelectron resolution, lock-in pixels for optical time-of-flight range cameras with sub-centimeter distance resolution and in-pixel demodulation circuits for optical coherence tomography imaging. The future is seen in even higher levels of integration, such as system-on-a-chip machine vision cameras (“seeing chips”), post-processing with non-silicon materials for the extension of the detection range to the X-ray, ultraviolet and infrared spectrum, the exploitation of all properties of the incident light and imaging of fields other than electromagnetic radiation