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Indirect time of flight cameras are increasingly being used in a variety of applications to provide real-time full field of view range measurements. Current generation cameras suffer from systematic linearity errors due to the influence of harmonics in the system and motion errors due to the requirement of taking multiple measurements. This paper demonstrates that replacing the standard phase detection algorithm with the windowed discrete Fourier transform can improve the root mean square (RMS) axial motion error with distance from 0.044 ± 0.002 m to 0.009 ± 0.004 m and the range from 0.112 ± 0.007 m to 0.03 ± 0.01 m for an object with a velocity of 2 m/s using a measurement time of 125 ms. This algorithm also improves the linearity of the camera by removing systematic errors due to harmonics, decreasing the RMS linearity error from 0.018 ± 0.002 m to 0.003 ± 0.001 m. This paper establishes the robustness of the windowed discrete Fourier transform, demonstrating that it effectively eliminates axial motion error over a variety of velocities and modulation frequencies. The potential for tailoring phase detection algorithms to specific applications is also demonstrated.

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This paper outlines a method for improving the precision of indirect time of flight range imaging cameras in high contrast scenes. Indirect time of flight cameras have potential applications in many fields including mobile robotics. However, current generation cameras are limited by their dynamic range. A method is proposed where, at the end of each measurement, the signal amplitude is compared to a threshold value on a pixelby-pixel basis with low quality pixels being integrated over multiple integration periods. This method sacrifices the uniformity of the frame rate over the image in return for maintaining a high frame rate in bright areas while still measuring quality data in darker areas. Experimental data are shown demonstrating the efficacy of this method.

<p>This thesis describes the development of a compact and modularised indirect time of flight range imaging camera. These cameras commonly use the Amplitude Modulated Continuous Wave (AMCW) technique. For this technique, an entire scene is illuminated with light modulated at a high frequency. An image sensor is also modulated and the phase shift introduced between the two modulation signals, due to the transit time of the light reflecting off objects in the scene and returning to the camera, is used to measure the distance.  The system constructed for this thesis is controlled by a Cyclone III FPGA and is capable of producing full field of view range images in real time with no additional computational resources. A PMD19K-2 sensor is used as the modulatable image sensor, and is capable of modulation frequencies up to 40 MHz.  One significant issue identified with this range imaging technology is that the precision of the range measurements are often dependent on the properties of the object being measured. The dynamic range of the camera is therefore very important when imaging high contrast scenes. Variable Frame Rate Imaging is a novel technique that is developed as part of this thesis and is shown to have promise for addressing this issue. Traditional theory for indirect time of flight cameras is expanded to describe this technique and is experimentally verified. A comparison is made between this technique and traditional High Dynamic Range Imaging. Furthermore, this technique is extended to provide a constant precision measurement of a scene, regardless of the properties of the objects in the scene.  It is shown that the replacement of the standard phase detection algorithm with a different algorithm can both reduce the linearity error in the phase measurements caused by harmonics in the correlation waveform and ameliorate axial motion error caused by relative motion of the camera and the object being measured. The new algorithm requires a trivial increase in computational power over the standard algorithm and can be implemented without any significant changes to the standard hardware used in indirect time of flight cameras.  Finally, the complete system is evaluated in a number of real world scenarios. Applications in both 3D modelling and mobile robotics are demonstrated and tests are performed for a variety of scenarios including dynamic scenes using a Pioneer 2 robot.</p>