Large depth-of-field 3D shape measurement using an electrically tunable lens

Accurate 3D Measurement of Complex Texture Objects by Height Compensation Using a Dual-Projector Structure

Fringe projection profilometry is a widely used technique for 3D measurement due to its high accuracy and speed. However, the accuracy significantly decreases when measuring complex texture objects, especially in the junction of different colors. This paper analyzes the causes of errors resulting from complex textures and proposes a height compensation method to revise the error by employing a dual-projector structure. Moreover, the dual-projector is capable of acquiring a pair of errors with opposite signs, which can be utilized to calculate the accurate 3D information after determining the ratio of this pair of errors. Experiments provide significant improvement in measuring complex texture objects, demonstrating the proposed method's ability.

Reliability of tunable lenses: feedback sensors and the influence of temperature, orientation, and vibrations

We compare different aspects of the robustness to environmental conditions of two different types of piezo-actuated fluid-membrane lenses: a silicone membrane lens, where the piezo actuator indirectly deforms the flexible membrane through fluid displacement, and a glass membrane lens, where the piezo actuator directly deforms the stiff membrane. While both lenses operated reliably over the temperature range of 0°-75°C, there was a significant effect on their actuation characteristics, which can be well described through a simple model. The silicone lens in particular showed a variation in focal power of up to 0.1m^{-1 ∘}C^-1. We demonstrated that integrated pressure and temperature sensors can provide feedback for focal power, however, limited by the response time of the elastomers in the lenses, with polyurethane in the support structures of the glass membrane lens being more critical than the silicone. Studying the mechanical effects, the silicone membrane lens showed a gravity-induced coma and tilt, and a reduced imaging quality with the Strehl ratio decreasing from 0.89 to 0.31 at a vibration frequency of 100 Hz and an acceleration of 3g. The glass membrane lens was unaffected by gravity, and the Strehl ratio decreased from 0.92 to 0.73 at a vibration of 100 Hz, 3g. Overall, the stiffer glass membrane lens is more robust against environmental influences.

Large DOF microscopic fringe projection profilometry with a coaxial light-field structure

Fringe projection profilometry (FPP) has been widely researched for three-dimensional (3D) microscopic measurement during recent decades. Nevertheless, some disadvantages arising from the limited depth of field and occlusion still exist and need to be further addressed. In this paper, light field imaging is introduced for microscopic fringe projection profilometry (MFPP) to obtain a larger depth of field. Meanwhile, this system is built with a coaxial structure to reduce occlusion, where the principle of triangulation is no longer applicable. In this situation, the depth information is estimated based on the epipolar plane image (EPI) of light field. In order to make a quantitative measurement, a metric calibration method which establishes the mapping between the slope of the line feature in EPI and the depth information is proposed for this system. Finally, a group of experiments demonstrate that the proposed LF-MFPP system can work well for depth estimation with a large DOF and reduced occlusion.

Electrically Tunable Lens (ETL)-Based Variable Focus Imaging System for Parametric Surface Texture Analysis of Materials

Electrically tunable lenses (ETLs) are those with the ability to alter their optical power in response to an electric signal. This feature allows such systems to not only image the areas of interest but also obtain spatial depth perception (depth of field, DOF). The aim of the present study was to develop an ETL-based imaging system for quantitative surface analysis. Firstly, the system was calibrated to achieve high depth resolution, warranting the accurate measurement of the depth and to account for and correct any influences from external factors on the ETL. This was completed using the Tenengrad operator which effectively identified the plane of best focus as demonstrated by the linear relationship between the control current applied to the ETL and the height at which the optical system focuses. The system was then employed to measure amplitude, spatial, hybrid, and volume surface texture parameters of a model material (pharmaceutical dosage form) which were validated against the parameters obtained using a previously validated surface texture analysis technique, optical profilometry. There were no statistically significant differences between the surface texture parameters measured by the techniques, highlighting the potential application of ETL-based imaging systems as an easily adaptable and low-cost alternative surface texture analysis technique to conventional microscopy techniques.

Parametric dog-bone-shaped tunable cylindrical fluidic lens

Tunable spherical fluidic lenses are among the most essential components in adaptive optics. However, fabricating cylindrical tunable lenses has proven more challenging, mainly due to the difficulty in eliminating the defocus component. We demonstrate a parametric approach to minimize the defocus in cylindrical tunable fluidic lenses. We theoretically model and experimentally verify that a dog-bone-shaped tunable cylindrical fluidic lens exhibits almost pure cylindrical performance within the range of ${\pm{\rm 5D}}$ of astigmatism. We anticipate these results will facilitate the use of tunable cylindrical fluidic lenses in adaptive optics applications and particularly ophthalmic devices, where rapid and reliable wavefront correction is required.

Large depth-of-field three-dimensional shape measurement with the focal sweep technique

Three-dimensional (3D) shape measurement based on the fringe projection technique has been extensively used for scientific discoveries and industrial practices. Yet, one of the most challenging issues is its limited depth of field (DOF). This paper presents a method to drastically increase DOF of 3D shape measurement technique by employing the focal sweep method. The proposed method employs an electrically tunable lens (ETL) to rapidly sweep the focal plane during image integration and the post deconvolution algorithm to reconstruct focused images for 3D reconstruction. Experimental results demonstrated that our proposed method can achieve high-resolution and high-accuracy 3D shape measurement with greatly improved DOF in real time.

Calibration method for the electrically tunable lens based on shape-changing polymer

In this paper, a calibration method for the camera system with electrically tunable lens (ETL) based on shape-changing polymer (SCP) is proposed to improve the accuracy, robustness and practicality of the system. The camera model of the ETL based on SCP is proposed based on the analyses of its optical properties. The calibration strategy, including initial estimation of camera parameters and bundle adjustment is presented. To eliminate the influence of temperature on ETL in machine vision applications, a real-time temperature compensation method is proposed. The proposed method makes use of the existing calibration hardware without adding new components to the system. Both simulations and experiments are conducted to evaluate the effectiveness and accuracy of the proposed camera model and calibration method. The measurement error with the proposed calibration method is below 20 microns at high magnification, whose measurement accuracy is improved by five times than the existing method at high magnification. With the proposed calibration method for the camera system with ETL based on SCP, the calibration workload is reduced and accurate calibration at high magnification is achieved. It also benefits the development of autofocusing 3D measurement technology.

State-of-the-art active optical techniques for three-dimensional surface metrology: a review [Invited]

This paper reviews recent developments of non-contact three-dimensional (3D) surface metrology using an active structured optical probe. We focus primarily on those active non-contact 3D surface measurement techniques that could be applicable to the manufacturing industry. We discuss principles of each technology, and its advantageous characteristics as well as limitations. Towards the end, we discuss our perspectives on the current technological challenges in designing and implementing these methods in practical applications.

Autofocusing method for a digital fringe projection system with dual projectors

This paper presents a novel technique to achieve autofocusing for a three-dimensional (3D) profilometry system with dual projectors. The proposed system uses a camera that is attached with an electronically focus-tunable lens (ETL) that allows dynamic change of camera's focal plane such that the camera can focus on the object; the camera captures fringe patterns projected by each projector to establish corresponding points between two projectors, and two pre-calibrated projectors form triangulation for 3D reconstruction. We pre-calibrate the relationship between the depth and the current being used for each focal plane, perform a 3D shape measurement with an unknown focus level, and calculate the desired current value based on the initial 3D result. We developed a prototype system that can automatically focus on an object positioned between 450 mm to 850 mm.