Image intensifier characterization

An image intensifier forms an integral part of a full-field image range finder under development at the University of Waikato. Operating as a high speed shutter with repetition rates up to 100 MHz, a method is described to characterise the response, both temporally and spatially, of the intensifier in order to correct for variations in the field of view and to optimise the operating conditions. A short pulse of visible light is emitted by a laser diode, uniformly illuminating the image intensifier, while a CCD camera captures the output from the intensifier. The phase of the laser pulse is continuously varied using a heterodyne configuration, automatically producing a set of samples covering the modulation cycle. The results show some anomalies in the response of our system and some simple solutions are proposed to correct for these

Development and characterisation of an easily configurable range imaging system

Range imaging is becoming a popular tool for many applications, with several commercial variants now available. These systems find numerous real world applications such as interactive gaming and the automotive industry. This paper describes the development of a range imaging system employing the PMD-19 k sensor from PMD technologies. One specific advantage of our system is that it is extremely customisable in terms of modulation patterns to act as a platform for further research into time-of-flight range imaging. Experimental results are presented giving an indication of the precision and accuracy of the system, and how modifying certain operating parameters can improve system performance

A synchronised Direct Digital Synthesiser

We describe a Direct Digital Synthesiser (DDS) which provides three frequency-locked synchronised outputs to generate frequencies from DC to 160 MHz. Primarily designed for use in a heterodyning range imaging system, the flexibility of the design allows its use in a number of other applications which require any number of stable, synchronised high frequency outputs. Frequency tuning of 32 bit length provides 0.1 Hz resolution when operating at the maximum clock rate of 400 MSPS, while 14 bit phase tuning provides 0.4 mrad resolution. The DDS technique provides very high relative accuracy between outputs, while the onboard oscillator’s stability of ±1 ppm adds absolute accuracy to the design

Maximizing precision over extended unambiguous range for TOF range imaging systems

The maximum unambiguous range for time-of-flight range imaging systems is inversely proportional to the chosen modulation frequency. However, increasing the unambiguous range by decreasing the modulation frequency will generally also degrade the range measurement precision. We describe a technique that significantly extends the range of a time-of-flight imaging system without compromising range precision. This is achieved by employing two modulation frequencies simultaneously. The chosen frequencies can be a combination of high and low frequency, or two similarly high frequencies. In this paper we present experimental results comparing single frequency; dual high and low frequency; and dual high frequency operation and demonstrate that range precision need not be appreciably compromised to achieve an extended unambiguous range

Heterodyne range imaging in real-time

A versatile full-field range imaging system has previously been constructed. This system is configurable in software to produce either high precision or fast acquisition range images. Indicatively a 10 second exposure has been shown to produce a range image of sub-millimeter precision, whilst video frame rate (30 fps) acquisition provides for centimetre precision. Currently the acquisition time of the system is to a large degree constrained by the off-line processing of the frames by an external computer. This paper presents an alternative to the off-line PC image processing utilising an Altera Stratix II FPGA. Processing rates up to 30 frames per second have been achieved with the added advantage that many of the previous systempsilas existing digital electronics can also be accommodated, providing for an even more compact and flexible system

Interest-Based Self-Organizing Peer-to-Peer Networks: A Club Economics Approach

Improving the information retrieval (IR) performance of peer-to-peer networks is an important and challenging problem. Recently, the computer science literature has attempted to address this problem by improving IR search algorithms. However, in peer-to-peer networks, IR performance is determined by both technology and user behavior, and very little attention has been paid in the literature to improving IR performance through incentives to change user behavior. We address this gap by combining the club goods economics literature and the IR literature to propose a next generation file sharing architecture. Using the popular Gnutella 0.6 architecture as context, we conceptualize a Gnutella ultrapeer and its local network of leaf nodes as a "club" (in economic terms). We specify an information retrieval-based utility model for a peer to determine which clubs to join, for a club to manage its membership, and for a club to determine to which other clubs they should connect. We simulate the performance of our model using a unique real-world dataset collected from the Gnutella 0.6 network. These simulations show that our club model accomplishes both performance goals. First, peers are self-organized into communities of interest - in our club model peers are 85% more likely to be able to obtain content from their local club than they are in the current Gnutella 0.6 architecture. Second, peers have increased incentives to share content - our model shows that peers who share can increase their recall performance by nearly five times over the performance offered to free-riders. We also show that the benefits provided by our club model outweigh the added protocol overhead imposed on the network for the most valuable peers

Heterodyne range imaging as an alternative to photogrammetry

Solid-state full-field range imaging technology, capable of determining the distance to objects in a scene simultaneously for every pixel in an image, has recently achieved sub-millimeter distance measurement precision. With this level of precision, it is becoming practical to use this technology for high precision three-dimensional metrology applications. Compared to photogrammetry, range imaging has the advantages of requiring only one viewing angle, a relatively short measurement time, and simplistic fast data processing. In this paper we fist review the range imaging technology, then describe an experiment comparing both photogrammetric and range imaging measurements of a calibration block with attached retro-reflective targets. The results show that the range imaging approach exhibits errors of approximately 0.5 mm in-plane and almost 5 mm out-of-plane; however, these errors appear to be mostly systematic. We then proceed to examine the physical nature and characteristics of the image ranging technology and discuss the possible causes of these systematic errors. Also discussed is the potential for further system characterization and calibration to compensate for the range determination and other errors, which could possibly lead to three-dimensional measurement precision approaching that of photogrammetry

The Waikato range imager

We are developing a high precision simultaneous full-field acquisition range imager. This device measures range with sub millimetre precision in range simultaneously over a full-field view of the scene. Laser diodes are used to illuminate the scene with amplitude modulation with a frequency of 10MHz up to 100 MHz. The received light is interrupted by a high speed shutter operating in a heterodyne configuration thus producing a low-frequency signal which is sampled with a digital camera. By detecting the phase of the signal at each pixel the range to the scene is determined. We show 3D reconstructions of some viewed objects to demonstrate the capabilities of the ranger

Design and construction of a configurable full-field range imaging system for mobile robotic applications

Mobile robotic devices rely critically on extrospection sensors to determine the range to objects in the robot’s operating environment. This provides the robot with the ability both to navigate safely around obstacles and to map its environment and hence facilitate path planning and navigation. There is a requirement for a full-field range imaging system that can determine the range to any obstacle in a camera lens’ field of view accurately and in real-time. This paper details the development of a portable full-field ranging system whose bench-top version has demonstrated sub-millimetre precision. However, this precision required non-real-time acquisition rates and expensive hardware. By iterative replacement of components, a portable, modular and inexpensive version of this full-field ranger has been constructed, capable of real-time operation with some (user-defined) trade-off with precision

Characterizing an image intensifier in an full-field range image system

We are developing a high precision full-field range imaging system. An integral component in this system is an image intensifier, which is modulated at frequencies up to 100 MHz. The range measurement precision is dictated by the image intensifier performance, in particular, the achievable modulation frequency, modulation depth, and waveform shape. By characterizing the image intensifier response, undesirable effects can be observed and quantified with regards to the consequence on the resulting range measurements, and the optimal operating conditions can be selected to minimize these disturbances. The characterization process utilizes a pulsed laser source to temporally probe the gain of the image intensifier. The laser is pulsed at a repetition rate slightly different to the image intensifier modulation frequency, producing a continuous phase shift between the two signals. A charge coupled device samples the image intensifier output, capturing the response over a complete modulation period. Deficiencies in our measured response are clearly identifiable and simple modifications to the configuration of our electrical driver circuit improve the modulation performance