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

Abstract

The time-of-flight (TOF) principle is a well known technology 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 information-processing systems more robust and much less complex or even enables completely new solutions. A solid-state image sensor containing 124 × 160 pixels and the corresponding 3D-camera, the so-called SwissRanger camera has already been presented in detail 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 · 10 8 m s −1 ).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 such as sunlight 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 custom 0.6µm CMOS/CCD technology, a new pixel architecture suppressing background illumination and/or improving the ratio of modulated signal to background signal at the pixel-output level was developed and will be presented in this paper. The theoretical principle of operation and the expected performance measures are described. Furthermore, test results obtained in a laboratory setup are published. The sensor structure is characterized in a high background-light environment. This in-depth evaluation leads to a comparison of the background suppression approach with the traditional pixel structure in order to highlight the benefits of the new approach.

Sunlight-proof optical distance measurements with a dual-line lock-in time-of-flight sensor

“…Lower requirements on illumination sources of correlating sensors, emitting CW power of some milliwatts with lasers or watts with LEDs, are convenient, but involve stronger dependency on ambient light. Optical filters with a narrow bandwidth of roughly 100 nm help to mitigate the background illumination by a factor of 10 in the Swissranger 3D cameras [8] as well as PMD sensors which use burst-mode operation in addition to enhance the optical SNR by another factor of five [9]. Even though both mentioned sensors (Swissranger and PMD) have also implemented some kind of in-pixel background light suppression, they are only specified for operation up to *50 klx and *90 klx of ambient light, respectively.…”

High Performance CMOS Range Imaging

“…Circuits assisting the photodetectors were developed, that suppress the dc ambient-light [Ogg95]. Nevertheless, the photon noise associated with the ambient illumination cannot be compensated, what ultimately limits the resolution.…”

Three-dimensional imaging using fast micromachined electro-absorptive shutter

“…Its disadvantage is that it needs a certain distance, normally several centimeters, which makes the overall system size larger in lateral direction than a normal one-lens camera. TOF sensors [4][5][6][7][8][9] have small form factor: they use infrared (IR) light sources around the imaging lens so that they show camera-like form factors. Their disadvantage compared to pattern projection is that complex pixel structure is needed such as single photon detector 4 or phase demodulation pixel structure, [5][6][7][8][9] which is relatively expensive and delivers a low resolution compared with standard image sensors.…”

“…TOF sensors [4][5][6][7][8][9] have small form factor: they use infrared (IR) light sources around the imaging lens so that they show camera-like form factors. Their disadvantage compared to pattern projection is that complex pixel structure is needed such as single photon detector 4 or phase demodulation pixel structure, [5][6][7][8][9] which is relatively expensive and delivers a low resolution compared with standard image sensors. The depth image resolution (i.e., number of pixels in depth image) obtained by the above two technologies have been limited to video graphics array (VGA) because of either limitation of pattern (in structured light methods) or complexity of the depth pixel structure (in TOF sensors).…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13] Among the existing depth-capturing technologies, structured light method (recently well known as Kinect) 2,3 and TOF (time of flight) sensors based on the silicon image sensor [charge-couple device (CCD), complementary metal oxide semiconductor (CMOS) image sensor (CIS)] technology [4][5][6][7][8][9] are commercialized or close to market release. The structured light method basically utilizes the well-known triangulation principle: depth is calculated by analyzing the image of a particular pattern on the object projected from the projector.…”