Various existing target ranging techniques are limited in terms of the dynamic range of operation and measurement resolution. These limitations arise as a result of a particular measurement methodology, the finite processing capability of the hardware components deployed within the sensor module, and the medium through which the target is viewed. Generally, improving the sensor range adversely affects its resolution and vice versa. Often, a distance sensor is designed for an optimal range/resolution setting depending on its intended application. Optical triangulation is broadly classified as a spatial-signal-processing-based ranging technique and measures target distance from the location of the reflected spot on a position sensitive detector (PSD). In most triangulation sensors that use lasers as a light source, beam divergence-which severely affects sensor measurement range-is often ignored in calculations. In this paper, we first discuss in detail the limitations to ranging imposed by beam divergence, which, in effect, sets the sensor dynamic range. Next, we show how the resolution of laser-based triangulation sensors is limited by the interpixel pitch of a finite-sized PSD. In this paper, through the use of tunable focus lenses (TFLs), we propose a novel design of a triangulation-based optical rangefinder that improves both the sensor resolution and its dynamic range through adaptive electronic control of beam propagation parameters. We present the theory and operation of the proposed sensor and clearly demonstrate a range and resolution improvement with the use of TFLs. Experimental results in support of our claims are shown to be in strong agreement with theory.
This paper has supplementary downloadable material available at http://ieeexplore.ieee.org, provided by the author. The material consists of a video, viewable with VLC Media Player, demonstrating CAOS imaging for real-time (48 POI frame/second) laser beam tracking using POI Frame=1023 CAOS pixels and POI refresh time of 20.8 ms. The size of the video is 2.22 MB. Contact
Demonstrated for the first time, to the best of our knowledge, is laser beam imaging via multiple mode operations of the digital micro-mirror device-based coded access optical sensor (CAOS) camera. Specifically, outlined are novel modes of software programmable CAOS imaging, which include the time division multiple access (TDMA) mode, the code division multiple access (CDMA) mode, the CDMA-TDMA mode, the frequency division multiple access (FDMA)-TDMA mode, the frequency modulation (FM)-CDMA-TDMA mode, FM-TDMA mode, and the FDMA-CDMA-TDMA mode. Engagement of FDMA and CDMA modes enables simultaneous multi-pixel improved signal-to-noise ratio photo detection, while use of TDMA prevents optical point detector saturation. The use of the FDMA and FM modes creates high digital signal processing gain via temporal spectrum analysis to produce extreme dynamic range pixel-specific imaging. Using an un-attenuated 633 nm He-Ne laser near-Gaussian focusing beam, experimentally acquired are 13.68 μm spatial resolution laser beam map CAOS images with dynamic ranges of 44.78 dB, 45.28 dB, 83.90 dB, and 94.9 dB for the TDMA, CDMA, FM-CDMA-TDMA, and FM-TDMA modes, respectively. The FM-TDMA mode with its extreme dynamic ranging CAOS pixel-mapping capability experimentally measures the non-Gaussian spatially oscillatory irradiance behavior predicted by Huygens-Fresnel diffraction theory. The demonstrated CAOS camera through software control allows high-flexibility, robust imaging of laser beams across different wavelength bands with widely varying beam irradiance levels and spatial signatures, thus empowering optical system designers to match overall system requirements that are highly dependent on accurate and reliable laser beam metrology.
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