Flat mirror tilt and piston measurement based on structured light reflection

Simultaneous measurement of displacement, pitch, and yaw angles of a cavity output mirror based on phase measuring deflectometry

Accurate and high dynamic range measurement of the three attitude variations (i.e., displacement, pitch, yaw angles) has been widely researched based on various approaches to meet different application requirements. However, none of the existing methods can detect these three attitude variations at the same time. This paper proposes a method based on a classical phase measuring deflectometry (PMD) system and a double iteration algorithm to simultaneously measure the three variations of the output mirror in a laser resonator. The relationship between the attitude variations and the lateral displacement parameters (LDP) of the imaged structural fringes is derived in the PMD system. Through the established relationship, the attitude variations could be calculated once the LDP are detected. Furthermore, a two-step measuring scheme combining the four-step shifting phase and the color-coded fringe is proposed to effectively detect the LDP. Finally, an experiment is conducted to verify the feasibility of the proposed method.

Spatial Multiplexing Technique for Improving Dynamic Range of Speckle Correlation based Optical Lever

Speckle correlation based optical levers (SC-OptLev) possess attractive characteristics suitable for sensing small changes in the angular orientations of surfaces. In this study, we propose and demonstrate a spatial multiplexing technique for improving the dynamic range of SC-OptLev. When the surface is in its initial position, a synthetic speckle intensity pattern, larger than the area of the image sensor is created by transversely shifting the image sensor and recording different sections of a larger speckle pattern. Then, the acquired images are stitched together by a computer program into one relatively large synthetic speckle pattern. Following the calibration stage, the synthetic speckle intensity pattern is used to sense changes in the surface's angular orientation. The surface is monitored in real-time by recording part of the speckle pattern which lies within the sensor area. Next, the recorded speckle pattern is cross-correlated with the synthetic speckle pattern in the computer. The resulting shift of the correlation peak indicates the angular orientations of the reflective surface under test. This spatial-multiplexing technique enables sensing changes in the angular orientation of the surface beyond the limit imposed by the physical size of the image sensor.

Simultaneous multi-segmented mirror orientation test system using a digital aperture based on sheared Fourier analysis

This paper presents a simultaneous multi-segmented mirror orientation test system (SMOTS) using localized sheared images. A CMOS camera captures images of reflected 2D sinusoidal patterns from the test mirrors as their orientation changes. Surface orientation is measured to within 0.8 µrad (0.16 arcseconds) for a flat mirror. In addition, we measure the variation of seven mirror segments simultaneously. Furthermore, SMOTS is applied to measure the orientation of two concave mirrors with an accuracy of 2.7 µrad (0.56 arcseconds). The measurement time for seven segments is 0.07 s. This technique can monitor the mirror segment orientation in an open/closed-loop for various optical setups.

The application of multi-baseline digital close-range photogrammetry in three-dimensional imaging and measurement of dental casts

OBJECTIVE

To explore a new technique for reconstructing and measuring three-dimensional (3D) models of orthodontic plaster casts using multi-baseline digital close-range photogrammetry (MBDCRP) with a single-lens reflex camera.

STUDY DESIGN

Thirty sets of orthodontic plaster casts that do not exhibit severe horizontal overlap (>2 mm) between any two teeth were recorded by a single-lens reflex camera with 72 pictures taken in different directions. The 3D models of these casts were reconstructed and measured using the open source software MeshLab. These parameters, including mesio-distal crown diameter, arch width, and arch perimeter, were recorded six times on both the 3D digital models and on plaster casts by two examiners. Statistical analysis was carried out using the Bland-Altman method to measure agreement between the novel method and the traditional calliper method by calculating the differences between mean values.

RESULTS

The average differences between the measurements of the photogrammetric 3D models and the plaster casts were 0.011-0.402mm. The mean differences between measurements obtained by the photogrammetric 3D models and the dental casts were not significant except for the lower arch perimeter (P>0.05), and all the differences were regarded as clinically acceptable (<0.5 mm).

CONCLUSIONS

Measurements obtained by MBDCRP are compared well with those obtained from plaster casts, indicating that MBDCRP is an alternate way to store and measure dental plaster casts without severe horizontal overlap between any two teeth.

Active-target-based calibration of relative poses of mirrors in intraoral scanners

This paper describes a practical method using an active target to calibrate relative poses of mirrors in intraoral scanners. Intraoral scanning is a fast-growing technology. Mirrors are widely used in intraoral scanners to increase the measuring area of a single view. The relative poses of these mirrors must be calibrated for accurate three-dimensional profiling. We present and analyze the geometric model of multiview mirrors. A calibration method for relative poses of mirrors is developed based on fringe projection. Vertical and horizontal fringes are displayed on an active target, and every pixel can be viewed as a calibration marker. This calibration method allows the mirrors to have a narrow common field of view. A cell phone display is chosen as the active target, and experimental results demonstrate the effectiveness of the proposed method compared to traditional methods.