Video-rate or high-precision: a flexible range imaging camera

Abstract

A range imaging camera produces an output similar to a digital photograph, but every pixel in the image contains distance information as well as intensity. This is useful for measuring the shape, size and location of objects in a scene, hence is well suited to certain machine vision applications.Previously we demonstrated a heterodyne range imaging system operating in a relatively high resolution (512-by-512) pixels and high precision (0.4 mm best case) configuration, but with a slow measurement rate (one every 10 s). Although this high precision range imaging is useful for some applications, the low acquisition speed is limiting in many situations. The system's frame rate and length of acquisition is fully configurable in software, which means the measurement rate can be increased by compromising precision and image resolution.In this paper we demonstrate the flexibility of our range imaging system by showing examples of high precision ranging at slow acquisition speeds and video-rate ranging with reduced ranging precision and image resolution. We also show that the heterodyne approach and the use of more than four samples per beat cycle provides better linearity than the traditional homodyne quadrature detection approach. Finally, we comment on practical issues of frame rate and beat signal frequency selection.

Analysis of Errors in ToF Range Imaging With Dual-Frequency Modulation

“…This approach is performed by gain modulating the image sensor during the integration time at the same frequency as the illumination modulation, thereby implementing a homodyne downconverter. This gain modulation can be achieved with the combination of a traditional image sensor and an external modulation device [1] but is more commonly achieved with a specialized complementary metal-oxide-semiconductor (CMOS) image sensor, with the modulation function designed into the pixels [2]- [4]. Objects that are close to the camera will exhibit a small phase shift in the returned light, which will basically be in phase with the sensor modulation, producing a bright pixel.…”

Development of a Real-Time Full-Field Range Imaging System

“…Through careful selection of N and the length of the integration time, the heterodyne method minimizes the effects of harmonics better than is normally possible with a homodyne system. This enhances measurement accuracy [13].…”

Design and Construction of a Configurable Full-Field Range Imaging System for Mobile Robotic Applications

“…Employing the 8 × 8 binning, the system could generate range images between 15.7 and 68.5 Hz with a best precision of 2.5 mm [15]. The penalty for this however, was significantly reduced spatial resolution.…”

“…This provides the user with the ability to experiment with different configurations. The authors have found this to be invaluable for determining the frequencies that produce the best precision results [15,16]. It would be additionally useful if these frequencies could be changed "on the fly", in other words be configurable without requiring the downloading of new code.…”

“…Parallel work by the authors (for example [15,17]) provided a theoretical framework to select optimum frequencies within the achievable system bandwidth.…”