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Stress formation during the recording and bleaching of photopolymer holograms results in distortion of the internal diffractive structure and causes many undesirable effects. This is especially important in the case of holographic solar concentrators and similar holographic optical elements that are supposed to meet stringent specifications. We report a new digital holographic method to measure and study stress formation in photopolymers during hologram recording. A glass substrate, with one of its sides diffusely reflecting, is effectively applied to facilitate the recording of good digital holograms captured using a CMOS camera. A green-sensitive photopolymer is used for the study. Digital holograms recorded in successive time intervals of exposure were overlaid and numerically reconstructed. High-contrast stress fringes were generated, and the preliminary results are presented.

We previously presented a technique calibrating a multiaperture array projector (MAAP) for performing high-speed three-dimensional (3-D) surface topology measurements with aperiodic sinusoidal fringe structured illumination. Although a stereo-camera setup used to be required for triangulation, the presented MAAP calibration technique re-enables photogrammetric 3-D measurements to be made using a single CCD or CMOS camera. For such a monocular-view photogrammetric system, intrinsic camera calibration, intrinsic projector calibration, and extrinsic system calibration are vital to obtain high-measurement performance. However, there is currently no such technique that can comprehensively perform the required extrinsic system calibration when using an MAAP and a single-camera setup. A comprehensive optimization-based method is proposed and the resulting measurement performance is evaluated. With the new calibration technique, we are able to achieve a surface standard deviation of ∼50 mm.

This work employs the complementary metal–oxide–semiconductor (CMOS) camera to acquire images in a scanning manner for laser line scan (LLS) underwater imaging to alleviate backscatter impact of seawater. Two operating features of the CMOS camera, namely the region of interest (ROI) and rolling shutter, can be utilized to perform image scan without the difficulty of translating the receiver above the target as the traditional LLS imaging systems have. By the dynamically reconfigurable ROI of an industrial CMOS camera, we evenly divided the image into five subareas along the pixel rows and then scanned them by changing the ROI region automatically under the synchronous illumination by the fun beams of the lasers. Another scanning method was explored by the rolling shutter operation of the CMOS camera. The fun beam lasers were turned on/off to illuminate the narrow zones on the target in a good correspondence to the exposure lines during the rolling procedure of the camera’s electronic shutter. The frame synchronization between the image scan and the laser beam sweep may be achieved by either the strobe lighting output pulse or the external triggering pulse of the industrial camera. Comparison between the scanning and nonscanning images shows that contrast of the underwater image can be improved by our LLS imaging techniques, with higher stability and feasibility than the mechanically controlled scanning method.

The use of dual cameras in parallax in order to detect and create 3-D images in mobile devices has been increasing over the last few years. We propose a concept where the second camera will be operating in the short-wavelength infrared (SWIR—1300 to 1800 nm) and thus have night vision capability while preserving most of the other advantages of dual cameras in terms of depth and 3-D capabilities. In order to maintain commonality of the two cameras, we propose to attach to one of the cameras a SWIR to visible upconversion layer that will convert the SWIR image into a visible image. For this purpose, the fore optics (the objective lenses) should be redesigned for the SWIR spectral range and the additional upconversion layer, whose thickness is

The high-precision detection for surface defect of ceramic balls, widely used in precise bearings, is a challenging task. Due to the fact that some defects cannot be directly detected by the gray-level of images, we propose a method for surface defect detection based on fringe reflection. Based on specular characteristics of ceramic balls, this method utilizes flat screens with fringes drawn on them. If a ceramic ball is nondefective, the image, formed by reflection on its surface, presents even fringes. The distortion of fringes designed by a reverse exact ray-tracing method occurs at the defective region of ceramic balls. During experiments conducted with silicon nitride (Si3N4) ceramic balls, images are captured by a CMOS camera with high resolution and processed by specific algorithms. Experimental results demonstrate the feasibility of this method, which can be applied for high-precision detection for surface defect of ceramic balls and other objects with similar characteristics.

We propose a practical method for the expeditious determi- nation of the performance of imaging CCD and CMOS camera systems. Special attention is paid to the operation of these devices in varying illumination conditions, which is typical to many surveillance and con- sumer electronics applications. One emerging application utilizing imag- ing sensors in variable illumination conditions is the third generation mo- bile phone, which will deliver wirelessly pictures, graphics, and video. In our method, determination of the system performance is based on the imaging of a calibrated gray scale test chart as a function of illuminance. At each level of illumination the system response is characterized by a signal to random noise figure. The signal is calculated as the difference of the system response to the lightest and darkest areas of the gray scale. The random noise is measured as the standard deviation of the gray values in a difference of two successive images of the test pattern. The proposed method is applied in three exemplary cases: 1. compari- son of inexpensive CCD and CMOS cameras, 2. analysis of the effect of wireless image transmission, and 3. comparison of the effects of auto- matic and manual gain controls. © 2001 Society of Photo-Optical Instrumentation

We propose a stress measurement system based on a projection moire method and heterodyne interferometry for thin films on a flexible substrate. In the measurement setup, a CMOS camera in which every pixel can receive a series of heterodyne moire signals by using a continuously relative displacement with a constant velocity is used. Furthermore, the phase of the optimized sinusoidal curve and the surface profile of the flexible substrate are determined using a least-squares sine fitting algorithm. The thin-film stress is obtained by representing the cross-sectional curve of the surface profile by using a polynomial fitting method, estimating the resultant curvature radii of the uncoated and coated substrates, and using these two radii in the corrected Stoney formula. The proposed measurement system has the advantages of high accuracy, high resolution, and high capacity for substrates with high flexibility and a large measurement depth.

Optical wireless communication (OWC) using visible lights, also known as visible light communication (VLC), has attracted significant attention recently. As the traditional OWC and VLC receivers (Rxs) are based on PIN photo-diode or avalanche photo-diode, deploying the complementary metal-oxide-semiconductor (CMOS) image sensor as the VLC Rx is attractive since nowadays nearly every person has a smart phone with embedded CMOS image sensor. However, deploying the CMOS image sensor as the VLC Rx is challenging. In this work, we propose and demonstrate two simple contrast ratio (CR) enhancement schemes to improve the contrast of the rolling shutter pattern. Then we describe their processing algorithms one by one. The experimental results show that both the proposed CR enhancement schemes can significantly mitigate the high-intensity fluctuations of the rolling shutter pattern and improve the bit-error-rate performance.

This study concerns the illumination system in a radial imaging capsule endoscope (RICE). Uniformly illuminating the object is difficult because the intensity of the light from the light emitting diodes (LEDs) varies with angular displacement. When light is emitted from the surface of the LED, it first encounters the cone mirror, from which it is reflected, before directly passing through the lenses and complementary metal oxide semiconductor (CMOS) sensor. The light that is strongly reflected from the transparent view window (TVW) propagates again to the cone mirror, to be reflected and to pass through the lenses and CMOS sensor. The above two phenomena cause overblooming on the image plane. Overblooming causes nonuniform illumination on the image plane and consequently reduced image quality. In this work, optical design software was utilized to construct a photometric model for the optimal design of the LED illumination system. Based on the original RICE model, this paper proposes an optimal design to improve the uniformity of the illumination. The illumination uniformity in the RICE is increased from its original value of 0.128 to 0.69, greatly improving light uniformity.

Short-wavelength infrared image sensors based on p-i-n photodiode arrays present a tremendous interest in applications such as passive and active imagery for laser detection/warning, hot spot or detection for lasers sensors, enhanced vision systems or low light level sensors. The capability to work at room temperature with dark current equivalent to silicon-based devices is another motivation for the fast development of this technology. This paper presents several modules and camera based on InGaAs photodiode arrays from the III-VLab. First, we describe the electro-optics performance in terms of dark signal, sensitivity, and particularly the visible extension capability. We also present a nucless logarithmic sensor based on a 1/2 video graphics array (VGA) format at a pitch of 25 μm initially designed for visible CMOS camera chip. We will also present the next generation of focal plane arrays based on a VGA format of 640×512 pixels with a pitch of 15 μm. This array will be associated to a CTIA readout circuit and also to an innovative CMOS logarithmic wide dynamic range ROIC, developed by New Imaging Technologies. This VGA logarithmic device developed for automotive safety will involve visible extension capability in a European project named 2Wide_sense.

In order to have an efficient design methodology and achieve high-performance inertial instruments, it is important to analyze the dynamic characteristics of micromechanical vibratory gyroscopes. In this paper, a novel optical method based on a high-speed CMOS camera and digital image spectrum correlation is developed to measure the motion of the sensitive elements. Unlike digital image correlation (DIC) in the spatial domain, correlation in the spectral domain gives direct access to the whole image under translational motion, which retains all the advantages of DIC and provides an expanded range of measurement. In addition, the proposed method is much faster in computation and capable of calculating dynamic temporal serial images in real time. The proposed method is employed to obtain the displacement of temporal series images and to extract dynamical parameters such as resonant frequency and quality factor at atmospheric pressure. During dynamic testing, the discrete wavelet transform is employed to eliminate the low-frequency components, and the continuous wavelet transform (CWT) for identification of the frequency and damping ratio. Experimental results show that the proposed method is a direct inspection tool and a practical alternative to the conventional electrical measurements of micromechanical vibratory gyroscopes.

Star trackers based on charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) sensors, use a brightness independent star identification strategy that requires highly accurate measurements of star spot position. Therefore, precise calibration of the measurement model is crucial. Generally, ground-based real night sky observation is a specific and costly approach. In this work, an optical laboratory method for star tracker calibration is proposed. This method is based on ordinary instruments, laser autocollimation adjustment, and image processing techniques. The calibration issues of the star tracker with a brightness independent star identification strategy are analyzed. A new laboratory calibration method is introduced, and its results on an active pixel sensor (APS) star tracker are discussed. This method proves to be practical and adequate in the development of a star tracker with wide field of view.

Full-field optical coherence tomography OCT using a complementary metal-oxide semiconductor CMOS camera with an in- tegrated a digital signal processor DSP is demonstrated. The CMOS- DSP camera employed is typically used in machine vision applications and is based on an array of 10241024 direct readout pixels that are randomly addressable in space and time. These characteristics enable the camera to be used as a fast full-field detector in carrier-based optical metrology systems. The integrated DSP facilitates basic signal process- ing including real-time filtering and undersampling. The optical setup used to implement this OCT method is composed of a free-space Mich- elson interferometer and a superluminescent diode SLD light source, with an electromechanical shaker for depth scanning. Unlike classical OCT approaches, however, the setup does not require any electrome- chanical device for lateral scanning. A 6430 pixel region of interest was imaged at 235 frames/s and sampled in depth, corresponding to a volumetric measurement of 875410150 m. Measurements carried out on a simple calibration specimen indicated lateral and axial resolu- tions of 14 and 22 m, respectively. The presented approach offers an inexpensive and versatile alternative to traditional OCT systems and pro- vides the basis for a functional machine vision system suitable for indus- trial applications.

Schlieren photography normally uses a background light source grid or mirror system to send light through a sample area and a second grid or a knife edge at the camera to block nondeflected light. We describe a new technique that uses the inactive areas between CCD or CMOS sensor pixels as the second grid, so only a background grid is needed. This simplifies the setup and lowers the cost of the system.

We present an empirical dark current model for CMOS active pixel sensors (APSs). The model is based on experimental data taken of a 256 3 256 APS chip fabricated via HP in a standard 0.5-mm CMOS technology process. This quantitative model determines the pixel dark current dependence on two contributing factors: the ''ideal'' dark current determined by the photodiode junction, introduced here as a stable shot noise influence of the device active area, and a leakage current due to the device active area shape, i.e., the number of corners present in the photodiode and their angles. This part is introduced as a process- induced structure stress effect. © 2002 Society of Photo-Optical Instrumentation Engineers. (DOI: 10.1117/1.1475995)

This paper demonstrates a convenient experimental tech- nique for measuring the resolution of a digital imaging system. The spa- tial frequency response of the system is obtained by deriving the modu- lation transfer function (MTF) from the measured contrast transfer function (CTF). Basic signal-processing techniques are used to establish a relationship between the MTF and the CTF. Experimental methodology is discussed with respect to avoidance of shift variation in the CTF. Dis- crete Fourier transformation is applied for a power-spectrum analysis of the MTF. Using the CTF as a measure of performance, a series of ex- periments were carried out to characterize an imaging system based on an integrated CMOS camera. The experimental results showed that this approach can be applied in general, for the assessment of spatial reso- lution for digital imaging systems. © 1998 Society of Photo-Optical Instrumenta- tion Engineers. (S0091-3286(98)00609-6)