The purpose of this paper is to describe a new proved multi-channel laser-radar technology which enables several thousands of high-sensitive laser-radar or lidar receivers to be integrated on a fingernail-sized CMOS-chip for fast 3D-perception and, furthermore, to explain the huge number of resulting applications and to estimate the desirable scientific, economic and society impacts.
These extraordinary capabilities rely on the revolutionary improvements introduced by a smart inherently-mixing photodiode with two controllable photo-current outputs [1]. We call it PMD (Photonic Mixer Device) because the opto-electronic mixing process is accomplished directly in the photonic state, followed by an integration process to get OE-correlation and the delay of the optical echo and the modulation signal.
The PMD-principle provides an unbelievable simplification, size-reduction and improvement in Multi-Channel Light Detecting and Ranging as a MC-PMD-Lidar or 3D-PMD-camera without scanner. Thanks to the competence and merit of the PMDTechnologies GmbH in cooperation with the INV of the University of Siegen finally brought the PMD in big steps to reliability and to large pixel numbers and to products with today about 20.000 lidar receivers in a 120x160 PMD-matrix, which exhibits homogenous and exquisite specifications like very constant mean value and low standard deviation compared with conventional radar receivers.
This innovation may be seen as a breakthrough in the history of camera development. The "3D-camera" of today comprises more 3D-pixels in a PMD-array than, about 1970, the first CCD-cameras contained 2D-pixel in a CCD-array. Both are of similar size aside from the modulated sender with integrated LED's or laser diodes required for a homogenous illumination of the field-of-view.
KEYWORDS: Signal processing, Signal detection, Photonic devices, Receivers, Free space optics, Data transmission, Phase shift keying, Frequency modulation, Electronic filtering, Signal generators
Working on optical distance measurement a new optical correlator was developed at the Institute for Data Processing of the University of Siegen in the last years. The so called Photonic Mixer Device (PMD), to be meant originally for laser ranging systems, offers a lot of advantages for wireless optical data communication like high speed spatial light demodulation up to the GHz range and inherent backlight suppression. This contribution describes the application of such PMDs in a free space interconnect based on the principle of Multi Dimensional Multiple Access (MDMA) and the advantages of this new approach, starting from the MDMA principle and followed by the fundamental functionality of PMDs. After that an Optical MDMA (O-MDMA) demonstrator and first measurement results will be presented.
A series of micro-robots (MERLIN: Mobile Experimental Robots for Locomotion and Intelligent Navigation) has been designed and implemented for a broad spectrum of indoor and outdoor tasks on basis of standardized functional modules like sensors, actuators, communication by radio link. The sensors onboard on the MERLIN robot can be divided into two categories: internal sensors for low-level control and for measuring the state of the robot and external sensors for obstacle detection, modeling of the environment and position estimation and navigation of the robot in a global co-ordinate system. The special emphasis of this paper is to describe the capabilities of MERLIN for obstacle detection, targets detection and for distance measurement. Besides ultrasonic sensors a new camera based on PMD-technology is used. This Photonic Mixer Device (PMD) represents a new electro-optic device that provides a smart interface between the world of incoherent optical signals and the world of their electronic signal processing. This PMD-technology directly enables 3D-imaging by means of the time-of-flight (TOF) principle. It offers an extremely high potential for new solutions in the robotics application field. The PMD-Technology opens up amazing new perspectives for obstacle detection systems, target acquisition as well as mapping of unknown environments.
Many concepts for incoherent optical distance measurement, based on the time-of-flight (TOF) principle, are discussed in the past, but they differ in complexity and accuracy. The used modulation techniques and evaluations methods require different signal sources, which are controllable in frequency or phase delay in high precision. Development effort and outlay of TOF-systems will be reduced with the use of standard logic devices. The restrictions of these devices permit a limited number of phase or frequency steps, but the combination of standard logic devices and the principle of Phase-Shift Interferometry (PSI) offers the possibility to design a plain and precise system, at very low cost. Over the past 20 years many evaluation algorithms for PSI have been presented in different applications. The phase angle of an ideal interferogram is determinable with only three or four sampling values, but the usage of more sampling values will suppress emitter and detector non-linearities, phase shift errors and noise generally. This paper will present the design of the optimal phase-shift algorithm based on Fourier analysis of the complete recorded interferogram.
In the last years 3D-Vision systems based on the Time-Of-Flight (TOF) principle have gained more importance than Stereo Vision (SV). TOF offers a direct depth-data acquisition, whereas SV involves a great amount of computational power for a comparable 3D data set. Due to the enormous progress in TOF-techniques, nowadays 3D cameras can be manufactured and be used for many practical applications. Hence there is a great demand for new accurate algorithms for 3D object recognition and classification. This paper presents a new strategy and algorithm designed for a fast and solid object classification. A challenging example - accurate classification of a (half-) sphere - demonstrates the performance of the developed algorithm. Finally, the transition from a general model of the system to specific applications such as Intelligent Airbag Control and Robot Assistance in Surgery are introduced. The paper concludes with the current research results in the above mentioned fields.
Optical distance measurement, based on the time-of-flight (TOF) principle, suffer from the unfavorable ratio of desired accuracy and the small received signal power, which is reflected by the target. Many receiver concepts are discussed in the past, but only any concepts are putted into practice. The concept of Photonic Mixer Devices (PMD) offers any interesting features for laser ranging systems, like fast photo-detection with inherent mixing, accumulation of a adequate signal power by a following integration, flicker noise suppression, background illumination, and interference rejection of two-channel designs. The combination of PMD and phase-lock techniques additionally enables a continuous tracking of the object distance, his relative velocity to the measurement system, and the restoration of the received optical signal. Different design concepts of the Photonic Mixer Device (PMD) with their characteristics and advantages as the key component in an Electro-Optical Phase-Locked Loop (OE-PLL) and an Electro-Optical Delay-Locked Loop (OE-DLL) will be discussed.
This contribution describes a ranging system based on a new kind of optical sensor, called the Photonic Mixer Device (PMD). This unit combines fast optical sensing and modulation of incoherent light signals in one component part by its unique and powerful principle of operation. The key feature of this new sensor is the direct detection of the correlation value between the light and an electrical reference signal as an analog voltage value. The realized ranging system is based on the following concept: a laser diode or LED is modulated with a PN-code sequence and the reference signal consists of the same PN-code sequence. With a delay unit the phase of the reference signal is shifted until the PMD-sensor returns an expected correlation value. The delay between the reference and the reflected light signal implies information concerning the distance to the reflecting object. An array of these sensors provides a 3D- ranging system that could be used for, e.g., collision avoidance in new traffic control systems or in product monitoring systems. Fast modulation of the laser and the PMD provides a high distance resolution. Nowadays PMD-realizations are based on standard CMOS technology, but we also keep investigation in a new kind of PMD based on MSM technology (metal- semiconductor-metal).
KEYWORDS: Modulation, Ranging, 3D surface sensing, Sensors, Signal processing, Electro optics, 3D metrology, Photonic devices, Sensing systems, 3D acquisition
With this paper we present new 3D sensing technique based on the novel Photonic Mixer Device (PMD), a new generation of smart 3D sensor, which provides a brilliant interface between the world of incoherent light and the world of electronic signal processing. As a new semiconductor device, it combines fast optical sensing and mixing in one component of pixel size by its unique and powerful principle of operation. Based on standard CMOS-technology, it can be easily integrated into PMD sensing arrays, providing both 3D depth and intensity information of the scene. The presented 3D TOF ranging system based on PMD measures the phase and time delay of the back scattered optical signal. The RF- modulated light reflected from the 3D-scene represents the total 3D depth information within the aperture of the PMD receiver. Since the whole 3D-scene is illuminated simultaneously by using intensity-modulated light, the PMD- array on the receiver side performs parallel electro-optical mixing and correlation and delivers an optimal evaluation of time-of-flight and the optical power for each PMD pixel. So there is no scanner required in contrast to the conventional 3D-laser radar systems. The introduction of the PMD into the 3D range sensing technique offers very attractive solutions for the realization of flexible, extremely fast and robust low-cost 3D solid-state smart ranging systems.
This contribution describes a new kind of ranging system based on a recently developed optical sensor called the Photonic Mixer Device (PMD). This new device combines fast optical sensing and modulation of incoherent light signals in one component part by its unique and powerful principle of operation. Due to this, the device is blind for any kind of uncorrelated light. An line or array of these smart sensors provides a multi-dimensional ranging system as will be shown in our contribution. We propose the advantages of this new approach, including the useful qualities of using a PN-code sequence. Especially the results of the extremely high resolution ranging system, including a new kind of PMD elements, are the main focus of this contribution.
The PMD-technology opens up a wide area of solutions for all detector problems where a high phase accuracy is required. One important application of the new Photonic Mixer Device is optical distance measurement based on the time-of-flight principle. The high integration of PMD smart-pixels in an array based on CMOS-technology means a breakthrough in optical 3D-imaging. Additionally to the inherent mixing feature, even more functionality could be integrated according to the well-known active pixel sensor concept.
A time-of-flight (TOF) system based on a novel Photonic Mixer Device (PMD), which detects and demodulates light simultaneously, has demonstrated that it is feasible to realize a 3-D imaging system with compactness, low cost and sufficient resolution for many applications. Active illumination is used in the TOF imaging system. Because of the limited available light source with large bandwidth and in consideration of eye safety, LEDs or low power laser diodes are used as preferable light sources. The received light may be strongly damped in an order of 40- 50dB, depending on the surface characteristics of the objects. Therefore the optical transmissoin in the light transmission chain of light source-sender lens-object- reciever lens-(PMD) detector array need to be simulated and optimized in order to achieve better performance and maximal power transmission, by the design of such a 3-D camera. Based on the modeling of the light source (laser diodes and LEDs), lens system and statistical characteristics of objects one is able to simulate the light transmission and the light distribtion on the object and the PMD detector array. By the use of time-of-flight of the sensing light it is necessary to keep the phase uniformity of the illumination light, because every detector can possibly receive light from different light ways. This would result in a reduction of detection resolution. The optical-path-differences (OPD) representing the time differences from different paths have been simulated from the source to the related pixel for the designed transmitter lens and the receiver lens. Only one source is usually used by direct current modulation of laser diode or LED for uniform modulation phase. The simulatoin of the light distribution has given estimation of the light gathering efficiency of every pixel and of possible cross talk between adjacent pixels.
With the development of free space interconnection and optical computing different optics for these approaches have been designed, which are usually difficult to realize with conventional optical systems. The concept of an electrooptical interface based on the Photonic Mixer Device (PMD), which detects and mixes and optical signal simultaneously, has been proposed in previous publications. An array of vertical cavity surface-emitting lasers (VCSELs) is driven by input signals from different channels. The optical signals are then transformed by a lens system and a diffractive optical element (DOE) to a PMD array for processing and multiplexing. In this paper the DOE to realize the optical transformation in the electrooptical interface is presented. The DOE is composed an array of small segments, which correspond to the VCSELs with a pitch of about 250micrometers . The phase functions of the DOE are designed separately for each VCSEL to generate a shifted uniform distribution on the PMD-array. For compactness the segments on the edge work at a large diffraction angle, which leads to an equivalent quasi-constant phase gradient added to them and results in high frequency structure. Therefore, for simplification of the production of the DOE, a prism is combined with the DOE to generate the desired optical transformation. The optical transformation is investigated with the simulation tool ZEMAX. The simulation results show that the DOE with the designed phase function works well for the PMD based optical interface, and it is possible to realize a parallel multi- channel electrooptical interface for code division multiplex access (CDMA) communications.
This paper proposes a new optoelectronic delay locked loop (OE-DLL) and its use in optical ranging systems. The so called PMD-DLL receiver module is based on a novel electro-optical modulator (EOM), called the Photonic Mixer Device (PMD). This sensor element is a semiconductor device, which combines fast optical sensing and mixing of incoherent light signals in one component part by its unique and powerful principle of operation. Integration of some simple additional on-chip components offers a high integrated electro-optical correlation unit. Simulations and experimental results have already impressively verified the operation principle of PMD structures, all realized in CMOS technology so far. Although other technologies are also promising candidates for the PMD realization they should not be further discussed in this contribution. The principle of the new DLL approach is intensively discussed in this paper. Theoretical analysis as well as experimental results of a realized PMD-DLL system are demonstrated and judged. Due to the operation principle of sophisticated PMD devices and their unique features, a correlation process may be realized in order to synchronize a reflected incoherent light wave with an electronic reference signal. The phase shift between both signals represents the distance to an obstacle and may be determined by means of the synchronization process. This new approach, avoiding so far needed critical components such as broadband amplifiers and mixers for the detection of small photo currents in optical distance measurement, offers an extremely fast and precise phase determination in ranging applications based on the time- of-flight (TOF) principle. However, the optical measurement signal may be incoherent -- therefore a laser source is not needed imperatively. The kind of waveform used for the modulation of the light signal is variable and depends on the demands of every specific application. Even if there are plenty other alternatives (e.g., heterodyne techniques), in this contribution only so called quasi-heterodyne techniques - - also known as phase shifting methods -- are discussed and used for the implementation. The light modulation schemes described in this contribution are square-wave as well as pseudo-noise modulation. The latter approach, inspired by the wide spread use in communication as well as in position detection (e.g., IS-95 and GPS), offers essential advantages and is the most promising modulation method for the ranging approach. So called CDMA (code division multiple access) systems form a major task in communication technology investigations since the third generation mobile phone standard is also partly based on this principle. Fast and reliable synchronization in direct sequence spread spectrum communication systems (DSSS) differs hardly from the already mentioned ranging approach and will also be discussed. The possibility to integrate all components in a monolithic PMD based DLL design is also presented and discussed. This method might offer the feature to integrate complete lines or matrixes of PMD based DLLs for highly parallel, multidimensional ranging. Finally, an outlook is given with regard to further optimized PMD front ends. An estimation of the expected characteristics concerning accuracy and speed of the distance measurement is given in conclusion.
The huge and increasing need of information in the industrial world demands an enormous potential of bandwidth in telecommunication systems. Optical communication provides all participants with the whole spectrum of digital services like videophone, cable TV, video conferencing and online services. Especially fast and low cost opto-electrical receivers are badly needed in order to expand fiber networks to every home (FTTH--fiber to the home or FTTD--fiber to the desk, respectively). This paper proposes a new receiver structure which is designed to receiver optical data which are encoded by code division multiple access techniques (CDMA). For data recovery in such CDMA networks phase locked loops (PLL) are needed, which synchronize the local oscillator with the incoming clock. In optical code division multiple access networks these PLLs could be realized either with an electrical PLL after opto-electrical converting or directly in the optical path with a pure optical PLL.
This paper proposes a high integrated optoelectronic phase locked loop for the use in optical data transmission as well as in optical ranging systems. The so called PMD-PLL receiver module is based on a novel electro-optical modulator, called the Photonic Mixer Device (PMD). The sensor is a semiconductor device, which combines fast optical sensing and modulation of incoherent light signals in one component part by its unique and powerful principle of operation. Simulations and experimental results have already verified the operation principle of PMD structures, in CMOS-technology so far. But also other technologies may be suitable for the PMD realization.
A complete range camera system, working with the time-of- flight principle, is introduced. This ranging system uses modulated LEDs as active illumination source and a new lock-in CCD sensor as demodulator and detector. It requires no mechanically scanning parts because every pixel of the sensor contains a lock-in amplifier, enabling both intensity and range measurement for all pixels in parallel. Two such lock-in imagers are realized in 2.0 micrometer CMOS/CCD technology, (1) a line sensor with 108 pixels and an optical fill factor of 100% and (2) a 64 X 25 pixel image sensor with 20% fill factor. The basics of time-of-flight ranging are introduced with a detailed description of the elements necessary. Shot noise limitation to ranging resolution is deduced and confirmed by simulation. An optical power budget is offered giving the relation between the number of photons in a pixel depending on the light source, the observed object, and several camera parameters. With the described lab setup, non- cooperative targets can be measured over a distance of several meters with a resolution of some centimeters.
For advanced performances related to 3D metrology in production, security and traffic control, the Institut fur Nachrichtenverarbeitung is currently engaged in the investigation of a new 3D-camera system based on rf- modulation interferometry, other than triangulation. The new 3D-camera system proposed in this paper uses the techniques of optical rf-modulation of the active illumination source and the correlation of these signals reflected from a target within a large aperture. This method yields all phase- correlation functions, relating to each voxel of the 3D- scene in parallel. Hence, the system delivers a fast parallel measurement and evaluation process by completely using a 2D transmitting and receiving path. To fulfill this concept, we have set up a binocular system using a modified Pockels-cell arrangement and a modulated laser transmitter. Since the electro-optical modulator plays a key important role in realizing the whole system, we would discuss the modified arrangement of stacked birefringent KD*P crystals that incorporates the properties of large aperture and high frequency modulation in the region of some tenth of megahertz. Additionally, an optical reference path is introduced to compensate for the fluctuation of the light source, thus enhancing the capability of the system. The experimental results of this reformed system will be presented in this paper, e.g., the standard deviation of (lambda) /2500 being obtained.
This paper presents first hardware implementation and investigations of a new electro-optic modulator (EOM), called the Photonic Mixer Device (PMD). The PMD is a semiconductor device combining the characteristics of fast optical sensing and modulation. Arranged to a PMD-matrix it looks like a CCD-matrix but additionally provides the depth information of each image pixel using an appropriately modulated scene illumination. Besides this feature of 3D- imaging by means of time-related correlation the PMD-chip will enable on-chip 2D-image processing by means of spatial correlation. The principle of operation of the PMD, possible technologies for realization, facilities and applications will be described. This new device offers high potential for optical sensory systems due to an amazingly simple and powerful procedure of electro-optical mixing and correlation. Both CCD and CMOS are appropriate technologies. The latter will be treated in detail as a single-element Photonic Mixer Device. Here we propose several architectures--including two quadrant (2Q)-PMDs--with readout and preprocessing circuits for both, the phase resp, time-of-flight values and the pixel intensities. Arranging PMD-pixels to a PMD-line or to a PMD-matrix will provide a new generation of flexible and powerful solid-state 3D- cameras based on time-of-flight. According to the EOM- principle, the proposed 2D-mixer requires no additional optical devices, no broadband electronic amplifiers and mixers. This solid-state PMD-array offers even more unique facilities, e.g., performing high speed spatial light modulation up to the GHz range or optical CDMA- communication. The modulation characteristic of the PMD is an important aspect of this mixer. It is associated with optimizing the layout design for the PMD, which will be presented and discussed in this paper, too. PMD test chips have been realized in CMOS technology. Some simulation and first test results of the chip are also presented. The simulation results prove the operation principle of the PMD and provide us with parameters for an optimized layout design. First experimental results verified the expected operation principle of the test PMD.
The photonic mixer device (PMD) is a new electro-optical mixing semiconductor device. Integrated into a line or an array it may contribute a significant improvement in developing an extremely fast, flexible, robust and low cost 3D-solid-state camera. Three dimensional (3D)-cameras are of dramatically increasing interest in industrial automation, especially for production integrated quality control, in- house navigation, etc. The type of 3D-camera here under consideration is based on the principle of time-of-flight respectively phase delay of surface reflected echoes of rf- modulated light. In contrast to 3D-laser radars there is no scanner required since the whole 3D-scene is illuminated simultaneously using intensity-modulated incoherent light, e.g. in the 10 to 1000 MHz range. The rf-modulated light reflected from the 3D-scene represents the total depth information within the local delay of the back scattered phase front. If this incoming wave front is again rf- modulated by a 2D-mixer within the whole receiving aperture we get a quasi-stationary rf-inference pattern respectively rf-interferogram which may be captured by means of a conventional CCD-camera. This procedure is called rf- modulation interferometry (RFMI). Corresponding to first simulative results the new PMD-array will be appropriate to the RFMI-procedure. Though looking like a modified CCD-array or CMOS-photodetector array it will be able to perform both, the pixelwise mixing process for phase delay respectively depth evaluation and the pixelwise light intensity acquisition for gray level or color evaluation. Further advantageous properties are achieved by means of a four- quadrant (4Q)-PMD array which operates as a balanced inphase/quadrature phase (I/Q)-mixer and will be able to capture the total 3D-scene information of several 100,000 voxels within the microsecond(s) - to ms-range.
KEYWORDS: Ranging, Laser systems engineering, Receivers, Signal to noise ratio, Interference (communication), Linear filtering, Laser optics, Distance measurement, Signal processing, Estimation theory
This paper proposes an optimized model of the laser ranging system in the case of colored noise, which is based on the theory of maximum-likelihood estimation. With this model the ranging error is analyzed in detail for colored noise in different cases in which the optical receiver is approximated as first, second and third order low pass filter, respectively. The theoretical results are further verified by means of more detailed numerical simulations, where some characterizing parameters such as signal-noise- ratio, bandwidth of the optical receiver and laser power etc., are taken into account. According to theoretical and simulated results a ranging system with a novel delay-locked loop is experimentally realized and investigated. This paper will also present measurement results to demonstrate the practical feasibility of the optimized laser ranging system.
KEYWORDS: Modulation, CCD cameras, Signal to noise ratio, 3D modeling, Imaging systems, Interference (communication), Charge-coupled devices, Distortion, Signal attenuation, Visual process modeling
This paper focuses on simulating a model of 3D-color vision system based on synthetic nonlinear modulation. The model is set up to recover 3D and color properties from a colored object through evaluating several rf-interferograms sampled by a black-white CCD camera. Colorizing a black-white CCD camera in a 3D-vision system implies high resolution. The synthetic nonlinear modulation is different from other 3D- color vision systems. Different colored lights are synchronously modulated with characterizing rf-frequencies to detect a 3D object. Recovering colors is equally treated as recovering 3D information. Optical filters are not used. Instead, a suitable algorithm is adopted for recovering color and 3D information. Since a modulated optical rf- signal is used as a detecting probe rather than an un- modulated optical wave, higher orders of harmonic signals may be caused by electrical or optical components. Although linear matching techniques are adapted to prevent the problem, it is necessary to simulate the vision system for predicting its performances. An 8-bit black-white CCD camera with different signal to noise ratios is taken as an example in the simulation. 3D color properties are evaluated for the system in the case of nonlinearity and noise. An optimized result is obtained for realizing this vision system.
KEYWORDS: Modulation, 3D modeling, 3D vision, Modulators, Visual process modeling, Distortion, Phase shifting, Nonlinear optics, Phase shift keying, Color vision
A model of synthetic non-linear modulation is suggested to derive a fast and enhanced multi-probing recovering algorithm. The processing approach originally arises from minimizing non-linear errors caused by using optical modulators such as Pockels cells in an active vision system. The algorithm has been obtained in the case that the non- linear modulator and demodulator are identical. This paper will adapt the algorithm more flexible subject to a non- linear phase drifting due to non-identical or asymmetric modulators. The model of synthetic non-linear modulation reveals that a 3D color vision system can be realized in a compact and integral method. Various incoherent signals acting as color probes are synchronously modulated each with characterizing rf-signals. These signals are then incoherently superposed in the transmission medium. After being reflected from an object, the selectively attenuated signals are compactly demodulated by using one single broadband mixer. The colors are numerically filtered and evaluated rather than using optical filters. The rf- modulated 2D interferogram can be fast processed by the proposed algorithm to extract 3D and color properties. Considering the difficulty to obtain a broadband optical demodulator, a technique of `modulation matching' is also suggested in this paper thus improving the performance. It provides a prospect to recover 3D color properties with reduced optical elements and one black-white CCD camera at low cost and high resolution.
Increasing demands in the areas of machine, computer and robot vision strongly motivate the renovation of optical sensing devices. Based on the physical effect of Frustrated Total Reflection (FTR), the essence of this paper deals with the new design of optical modulators used in 3D-vision systems. Optical modulators commercially available do not meet some extended requirements for 3D applications. However, the device presented in this paper permits the specifications of optical 3D-measurement techniques like accuracy, robustness, compactness as well as fastness to be realized with higher capabilities. For this purpose, we suggest two different types of optical 2D mixers. Both are intended to explore the FTR effect to modulate light in a large aperture with minimal image aberration. In one case an air gap between media of higher dense is modulated by an rf- voltage applied to a piezo crystal as an rf-controlled tuning medium. In the other case the variation of the gap or modulation is achieved by using a surface acoustic wave device which acts as a surface grating medium due to the effect of a travelling periodic corrugation. The integrated optical components play a substantial role for the optical modulation and imaging. Some simulation results of this optical devices show that the inherent non-linearity of the FTR modulator may be neglected in practical applications, thus yielding a high modulation depth. Furthermore, a 3D- image system adapting this modulation optics is also depicted in the paper.
An optimized aspherical lens with large numerical aperture is demanded in many optical systems. In addressing the requirements, we have obtained analytical solutions of such a desired two-side aspherical lens by solving differential equations. Therefore geometrical parameters and index of refraction of the lens are explicitly expressed in the formulas. Lots of aspects of the aspherical lens can be seen by analyzing the obtained formulas rather than conventionally analyzing discrete data. Criteria of manufacturing the optimized lenses in different precision form uniquely depend on the refractive index of the plastics or glass material. On the basis of summarizing the work in this paper, we further derive formulas of power transmission for the optimized aspherical lens in several cases. The power transmission analysis is associated with typical radiation distribution of a source such as Lambertian or Gaussian distribution. Two polarized states of optical waves are also taken into account. Another criterion of manufacturing aspherical lenses with a specific transmittance is given according to the numerical aperture. It can be used to evaluate the power transmittance of plastic aspherical lenses without anti-reflection coating. A design according to the proper criteria allows more tolerances in the process of plastic lens manufacturing and assembling, while the analytically optimized results are more adaptive to compensation design for correcting deformation of materials.
This paper introduces an optimization method of precision aspheric lenses for advanced small sensor heads of large numerical aperture. The approach is different from the numerical method adopted in commercial software packages. Analytically optimized results of an aspheric lens of large numerical aperture are obtained from solving differential equations strictly bounded by the condition of minimum angle of divergence and without spherical aberration. It is concluded that the optimized lens of a certain type, with minimum power losses and minimum angle of divergence, uniquely depends on the refractive index of the plastic or glass material. Criteria of manufacturing aspheric lenses in different types according to the indexes are given in the paper as well. Considering the difficulty of fabricating a lens module with two aspheric surfaces, a simplified ideal form of an aspheric lens is also proposed in this paper. Obviously, the optimized results allow more tolerances especially in manufacturing integrated plastic optics with an electrooptic device. It is proved to be useful as a tool in precision optical design software packages.
Optical and therefore nontactile 3D-measurement techniques are of increasing interest in industrial automation, especially in quality control and guidance of automotive vehicles. In connection with these demands, a new type of optical modulator jacketed in rotational plastic optics is introduced in the paper. Furthermore first results obtained by simulation studies will be presented. A simple nevertheless effective way of obtaining 3D information is to illuminate the whole 3D object or scene simultaneously with rf-modulated light. This can be well achieved by using the suggested optical modulator that incorporates the properties of a high aperture and minimum aberration in the 3D-imaging process. The mentioned modulator makes use of the effect of Frustrated Total Reflection (FTR). To exploit this FTR effect in an optical 2D mixer, the gap width between media of higher dense has to be modulated by an rf-voltage applied to a piezo crystal as an rf-controlled tuning medium. Considering the limited modulation bandwidth due to the parasitic capacity of the piezo crystal, the geometrical dimension of the modulator must be made as small as possible. Therefore the spot of the light is collimated at the focal point of the jacketing rotational ellipsoid. The integrated component made of plastic optics and piezo crystal plays a substantial role for the optical modulation and imaging. Some simulation results of this optical device show that the inherent non-linearity of the FTR modulator may be neglected in practical applications, thus yielding a high modulation depth. Furthermore, a 3D-image system adopting this plastic-made optics is also depicted in the paper, which is robust and handy for several industrial applications.
Presently there is still a remarkable gap between the requirements and the capabilities of 3D- vision in the field of industrial automation, especially in manufacture integrated 100%-quality control. For these and a lot of other applications like security and traffic control a new extremely fast, precise and flexible 3D-camera concept is presented in this paper. In order to obtain the geometrical 3D information, the whole 3D object or 3D scene is illuminated simultaneously by means of rf-modulated light. This is realized by using optical modulators such as Pockels cells or FTR optical components (FTR: Frustrated Total Reflection). The back scattered light represents the depth information within the local delay of the phase front of the rf-modulated light intensity. If the reflected wave front is mixed again within the whole receiving aperture using the same optical 2D-modulation components and the same rf- frequency, an rf-interference pattern is produced. A CCD camera may be applied to sample these rf-modulation interferograms. In order to reconstruct the 3D-image a minimum of three independent interferograms have to be evaluated. They may be produced either by applying three different rf-phases or three different rf-frequencies. This procedure will be able to deliver up to some tens of high resolution 3D images per second with some hundred thousand voxels (volume elements). Such a remarkable progress can be achieved by means of three key important steps: Firstly by separating the opto-electronic receiver device from real-time requirements by using homodyne mixing of CW-modulated light. Secondly by applying the rf- modulation signal as an optical reference signal to the receiving optical mixer. And thirdly by using a throughout 2D layout of the transmitted illumination, further, of the optical mixer in the receiving aperture, and of the optoelectronic sensing element, e.g., a CCD-chip.
The paper introduces a fast and enhanced recovering algorithm and its application in an active 3D-color vision system. The algorithm is based on the processing of several non-linearly modulated optical test signals of different colors. The processing approach arises from minimizing errors caused by using non-linear modulators in an active vision system, i.e., recovering 3D properties from higher order terms of a Fourier series expansion of the non- linear modulation. Two aspects are worth mentioning: Firstly, the modulation depth of non- linear optical components such as Pockels cells can be exploited much beyond the linear region if assisted by the appropriate recovering algorithm and thus increasing the effective aperture of the optical system. Secondly, the same algorithm can be adaptive to a synthetic non-linear modulation, i.e., the various incoherent signals used as color probes are synchronously modulated each with different characterizing rf signals by means of corresponding optical modulators. These signals are then further incoherently superposed in the transmission medium. After having been reflected from and interacted with the object of interest, the selectively attenuated signals are demodulated using a single modulator. In this process phase, color and other information are simultaneously demodulated. Therefore a single black-white CCD camera may be utilized to sample the 2D-rf interferograms, which are fast and analytically processed by the proposed algorithm in order to extract 3D ranges, colors, and other properties of the interesting object.
This paper presents a novel scheme for the purpose of minimizing delay time errors of a receiver of a time-of-flight distance sensor based on the principle of correlation. The nonrandom delay time error is reduced by means of an improved reference technique using alternatively activated target and reference photo diodes within the same receiver. Thus the accuracy of the sensor is considerably increased.
Our paper presents a new optical distance sensor, which can be used in applications. The sensor measures the time interval between the emission of a modulated laser signal and the detection of a small fraction of the signal reflected from the measured object. Maximum likelihood optimization of the transmitted signal and the structure of the receiver lead to a pseudo noise (PN) signal, which modulates a cw diode laser in conjunction with a delay locked loop (DLL) receiver. The measured distance is determined from the zero crossing of the crosscorrelation of the received and the transmitted signal. The variation of time delay is realized by means of the variation of the frequency of the voltage controlled oscillator (VCO), which provides the clock signal for the PN-generator. The VCO frequency is inversely proportional to the distance to be measured so that the spatial resolution is directly proportional to the distance. The theoretical results are verified by means of more detailed numerical simulations. The simulations were also used for the optimization of the whole system as well as the particular subsystems. Results of measurements demonstrate the practical feasibility of the system.
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