Novel techniques for laser doppler vibrometry for vibrating shiny and mirror surfaces

A laser Doppler vibrometer (LDV) is a versatile non-contact optical instrument used to measure the velocity at a single point on the surface of a moving object. The instrument works most efficiently when used for an object whose surface roughness is sufficient to allow scattered light to be received at the device sensor head. For an object with a mirror-like surface, which allows only specular reflection, precise alignment is a necessity, as a small tilt change can cause the light to diverge away from the sensor. Surface enhancements such as retroreflective tapes can mitigate the loss of light signal detection by improving the light scattering from the surface. This solution does not work for delicate or small objects, as altering them may change their measurement properties. This study addresses these gaps in LDV-based methods by introducing novel external retroreflection techniques. These new methods allow recovery of the light signal lost due to a vibrating mirror with a large tilt or rotation angle. However, the Doppler-shifted frequency observed using this new method does not directly yield the target velocity without correction, because the oblique retroreflection itself also contributes to the Doppler frequency shift. In this study, an equation describing the contribution of the oblique retroreflection to the detected Doppler frequency shift in the signal is derived. With the help of this equation and the oblique retroreflection method, a conventional LDV can be used to characterize a vibrating mirror with a large tilt angle. Furthermore, the new technique has also been used for unprecedented measurements of large deflections of a cantilever mirror with dynamic rotation. For a scanning mirror with a large rotational angle, the oblique retroreflection model is inadequate as it does not consider the translation of the target point during the rotational motion. Therefore, an in-line velocity model is developed to overcome this limitation and enable the LDV with the external retroreflection method to characterize the velocity of the dynamic target point of a scanning mirror along the direction of the laser beam. In addition, the in-line velocity model includes a unique trough-shaped retroreflector design, superior to the design used in the external retroreflection method. With the trough design, the retroreflector can recover reflected light from the mirror at a wider angle of rotation. This second technique has been used successfully to characterize a rotating mirror driven by a torsional spring at a large rotational angle (up to 30°), a performance beyond the capability of a conventional LDV by itself. Additionally, an off-axis detection using an in-plane LDV is developed and tested to measure the in-plane motion vibration of shiny metallic samples, which a conventional in-plane LDV is not capable of doing. This off-axis detection method offers a great potential value for the in-plane motion characterization of micro-electro- mechanical systems (MEMS) devices with shiny metallic surfaces.Doctor of Philosoph

Improving photorealism of virtual objects for augmented reality using common illumination

Use of recent techniques from image-based rendering to bridge the gap between rendering quality and speed. It uses prefiltered high-dynamic range environment maps of the real scene, containing real lighting information, to render diffuse and glossy virtual objects with a novel multi-pass rendering framework.Master of Engineering (MPE

Effect of oblique retroreflection from a vibrating mirror on laser Doppler shift

A laser Doppler vibrometer (LDV) fails to measure a large out-of-plane vibration of a rotating mirror when the mirror obliquely reflects the laser beam away, causing a signal loss from being detected. To solve this problem, an external retroreflective tape was used to recover the oblique reflection. However, the reading of LDV obtained from the recovered signal is not right because the retroreflection adds extra Doppler frequency shifts to the oblique reflection. Here, we first derive the relationship of Doppler shift to the oblique angle of retroreflection. For the first time with the help of retroreflection, a standard LDV can measure the largely vibrating mirror as well as a high-speed camera, albeit without the need for heavy computation.Economic Development Board (EDB)Accepted versionThis work is supported by Nanyang Technological University and Sunny Instruments Singapore as part of the Industrial Postgraduate Programme (IPP) from Economic Development Board (EDB) of Singapore

Directional-sensitive differential laser Doppler vibrometry for in-plane motion measurement of specular surface

A new method for measuring in-plane vibration velocity of glossy and specular surface using differential laser Doppler vibrometer (LDV) is proposed in this work. A standard tangential LDV using similar differential configuration is only able to measure in-plane velocity of objects with rough surface, due to its inherent on-axis optical design that collects backscatter light along its optical axis. The proposed method adopts an off-axis detection scheme, in which the photodetector is decoupled from LDV, and placed along the dominant direction of the scattered light. For optimal placement, the bidirectional reflectance distribution function (BRDF) of the sample must be considered ideally, but in our measurement tests, the off-axis detection along the direction of specular reflection is sufficient to obtain good measurement results. Another advantage with this setup is that it also works with the objects with rough surface. Experimental works using the standard tangential LDV and a prototype of this method were conducted to measure the in-plane motion of four different samples representing rough, glossy and mirror-like surface. An electrodynamic shaker was used to provide the in-plane motion of the samples at three different frequencies. A single point axial vibrometer was used to validate the in-plane velocity of the measurement from both in-plane LDVs. Some preliminary results showed that the in-plane motion of the object with glossy and specular surface can be measured using the proposed method.Published versio

Dex-ray: augmented reality neurosurgical navigation with a handheld video probe

OBJECTIVE: We developed an augmented reality system that enables intraoperative image guidance by using 3-dimensional (3D) graphics overlaid on a video stream. We call this system DEX-Ray and report on its development and the initial intraoperative experience in 12 cases. METHODS: DEX-Ray consists of a tracked handheld probe that integrates a lipstick-size video camera. The camera looks over the probe's tip into the surgical field. The camera's video stream is augmented with coregistered, multimodality 3D graphics and landmarks obtained during neurosurgical planning with 3D workstations. The handheld probe functions as a navigation device to view and point and as an interaction device to adjust the 3D graphics. We tested the system's accuracy in the laboratory and evaluated it intraoperatively with a series of tumor and vascular cases. RESULTS: DEX-Ray provided accurate and real-time video-based augmented reality display. The system could be seamlessly integrated into the surgical workflow. The see-through effect revealing 3D information below the surgically exposed surface proved to be of significant value, especially during the macroscopic phase of an operation, providing easily understandable structural navigational information. Navigation in deep and narrow surgical corridors was limited by the camera resolution and light sensitivity. CONCLUSION: The system was perceived as an improved navigational experience because the augmented see-through effect allowed direct understanding of the surgical anatomy beyond the visible surface and direct guidance toward surgical targets