Optical Calibration Research Articles

Accuracy evaluation of optical distortion calibration by digital image correlation

Due to its convenience of operation, the camera calibration algorithm, which is based on the plane template, is widely used in image measurement, computer vision and other fields. How to select a suitable distortion model is always a problem to be solved. Therefore, there is an urgent need for an experimental evaluation of the accuracy of camera distortion calibrations. This paper presents an experimental method for evaluating camera distortion calibration accuracy, which is easy to implement, has high precision, and is suitable for a variety of commonly used lens. First, we use the digital image correlation method to calculate the in-plane rigid body displacement field of an image displayed on a liquid crystal display before and after translation, as captured with a camera. Next, we use a calibration board to calibrate the camera to obtain calibration parameters which are used to correct calculation points of the image before and after deformation. The displacement field before and after correction is compared to analyze the distortion calibration results. Experiments were carried out to evaluate the performance of two commonly used industrial camera lenses for four commonly used distortion models.

Dual-comb mid-infrared spectroscopy with free-running oscillators and absolute optical calibration from a radio-frequency reference.

By using free-running independent femtosecond OPOs with a repetition-rate difference of 500 Hz we demonstrate methane absorption spectroscopy with spectral coverage simultaneously spanning the methane P, Q and R branches and with a resolution of 0.5 cm-1. Absolute optical frequency calibration with an accuracy of 0.25 cm-1 (0.27 nm) is achieved from simultaneous repetition-rate and carrier-envelope-offset frequency measurements, without the need for any optical reference. The calibration technique allows registration and averaging of consecutively acquired dual-comb spectra, leading to a high quality and low-noise absorbance measurement in good agreement with the HITRAN database.

A multichannel visible spectroscopy system for the ITER-like W divertor on EAST.

To facilitate long-pulse high power operation, an ITER-like actively cooled tungsten (W) divertor was installed in Experimental Advanced Superconducting Tokamak (EAST) to replace the original upper graphite divertor in 2014. A dedicated multichannel visible spectroscopic diagnostic system has been accordingly developed for the characterization of the plasma and impurities in the W divertor. An array of 22 lines-of-sight (LOSs) provides a profile measurement of the light emitted from the plasma along upper outer divertor, and the other 17 vertical LOSs view the upper inner divertor, achieving a 13 mm poloidal resolution in both regions. The light emitted from the plasma is collected by a specially designed optical lens assembly and then transferred to a Czerny-Turner spectrometer via 40 m quartz fibers. At the end, the spectra dispersed by the spectrometer are recorded with an Electron-Multiplying Charge Coupled Device (EMCCD). The optical throughput and quantum efficiency of the system are optimized in the wavelength range 350-700 nm. The spectral resolution/coverage can be adjusted from 0.01 nm/3 nm to 0.41 nm/140 nm by switching the grating with suitable groove density. The frame rate depends on the setting of LOS number in EMCCD and can reach nearly 2 kHz for single LOS detection. The light collected by the front optical lens can also be divided and partly transferred to a photomultiplier tube array with specified bandpass filter, which can provide faster sampling rates by up to 200 kHz. The spectroscopic diagnostic is routinely operated in EAST discharges with absolute optical calibrations applied before and after each campaign, monitoring photon fluxes from impurities and H recycling in the upper divertor. This paper presents the technical details of the diagnostic and typical measurements during EAST discharges.

Stent optical inspection system calibration and performance.

Implantable medical devices, such as stents, have to be inspected 100% so no defective ones are implanted into a human body. In this paper, a novel optical stent inspection system is presented. By the combination of a high numerical aperture microscope, a triple illumination system, a rotational stage, and a CMOS camera, unrolled sections of the outer and inner surfaces of the stent are obtained with high resolution at high speed with a line-scan approach. In this paper, a comparison between the conventional microscope image formation and this new approach is shown. A calibration process and the investigation of the error sources that lead to inaccuracies of the critical dimension measurements are presented.

Medical-grade Sterilizable Target for Fluid-immersed Fetoscope Optical Distortion Calibration

We have developed a calibration target for use with fluid-immersed endoscopes within the context of the GIFT-Surg (Guided Instrumentation for Fetal Therapy and Surgery) project. One of the aims of this project is to engineer novel, real-time image processing methods for intra-operative use in the treatment of congenital birth defects, such as spina bifida and the twin-to-twin transfusion syndrome. The developed target allows for the sterility-preserving optical distortion calibration of endoscopes within a few minutes. Good optical distortion calibration and compensation are important for mitigating undesirable effects like radial distortions, which not only hamper accurate imaging using existing endoscopic technology during fetal surgery, but also make acquired images less suitable for potentially very useful image computing applications, like real-time mosaicing. In this paper proposes a novel fabrication method to create an affordable, sterilizable calibration target suitable for use in a clinical setup. This method involves etching a calibration pattern by laser cutting a sandblasted stainless steel sheet. This target was validated using the camera calibration module provided by OpenCV, a state-of-the-art software library popular in the computer vision community.

Medical-grade Sterilizable Target for Fluid-immersed Fetoscope Optical Distortion Calibration.

We have developed a calibration target for use with fluid-immersed endoscopes within the context of the GIFT-Surg (Guided Instrumentation for Fetal Therapy and Surgery) project. One of the aims of this project is to engineer novel, real-time image processing methods for intra-operative use in the treatment of congenital birth defects, such as spina bifida and the twin-to-twin transfusion syndrome. The developed target allows for the sterility-preserving optical distortion calibration of endoscopes within a few minutes. Good optical distortion calibration and compensation are important for mitigating undesirable effects like radial distortions, which not only hamper accurate imaging using existing endoscopic technology during fetal surgery, but also make acquired images less suitable for potentially very useful image computing applications, like real-time mosaicing. In this paper proposes a novel fabrication method to create an affordable, sterilizable calibration target suitable for use in a clinical setup. This method involves etching a calibration pattern by laser cutting a sandblasted stainless steel sheet. This target was validated using the camera calibration module provided by OpenCV, a state-of-the-art software library popular in the computer vision community.

Optical Wavelength Meter Calibration Using Iodine Stabilized He-Ne Laser by Direct Measurement Method

Non-stabilized He-Ne lasers are widely used in calibration laboratories and industries. Wavelength calibration is a requirement to know the real wavelength of a non-stabilized He-Ne laser and its deviation. More importantly, calibration process creates a traceability link from the calibrated device to the primary standard of length measurement. One major wavelength calibration method for non-stabilized He-Ne lasers is using direct measurement method which requires a wavelength meter. The wavelength meter has to be calibrated first using a stabilizedHe-Ne laser prior to its use. The recommended radiation power of the stabilized laser in RCM-LIPI, IodineStabilized He-Ne Laser (KIM-1), is (50 ± 25) μW. Here, we examine the calibration process of the high precision wavelength meter with a measurement range of -25 dBm to +10 dBmor equals to 3.16 μW to 10 mW for a wavelength measurement range of 600 nm to 1650 nm. Several uncertainty evaluations for measurement were applied to assess the wavelength meter's performance. The results show that the wavelength meter produces a wavelength reading that is still within the desired band of measurement, thus making it a valid measurement instrument to calibrate the wavelength of non-stabilized He-Ne lasers with 10-3 nm of resolution. The wavelength meter's measurement capability can also be traced back to the length main standard, Iodine Stabilized He-Ne Laser (KIM-1).

A Study of Comparison between Window Glass and Mirror on Flatness Measurement in RCM LIPI

Optical calibration system established in RCM LIPI uses monochromatic lamp and optical flat as standard. In the calibration system, optical to be tested and optical standard are contacted together. The monochromatic lamp illuminates both optical. It helps to generate a dark light pattern called fringe. The fringe pattern can be seen through a window glass screen. The fringe pattern indicates the optical flatness. The fringe is still read using human eyes. It causes human error factor on subjectivity for determining edge of fringe. An improvement to the calibration system is presented by image processing technique. The improvement needs some trials and studies to get the right images. The previous studies review on camera setting to capture fringe pattern. This research presents to study of comparison between window glass and mirror as screen. The result will be used to design the housing system. An experiment is set using window glass and mirror to the existing system. The fringe that appears on both screen are captured by digital camera. The images captured are processed with early edge detection program that built in Matlab. It is obtained that measurement system using window glass gives the best edge detection, but the camera setting can’t be auto mode. It can capture fringe images using manual mode with some adjustment on aperture, shutter time, and ISO. On the contrary, auto mode can be applied on measurement system using mirror as screen.

Airborne Imaging Spectrometer HySpex

The Remote Sensing Technology Institute (IMF) of the German Aerospace Center (DLR) operates an airborne imaging spectrometer system called HySpex. Owing to its accurate calibration, the system is well suited for benchmark reference measurements and feasibility studies for Earth observation applications. The sensor also serves as simulator for the upcoming German satellite mission EnMAP. HySpex covers the spectral range from the visible and near infrared (VNIR) to the short wave infrared (SWIR) and it has been extensively characterised with numerous measurements in the IMF calibration laboratory (CHB). The HySpex instrument is made available to interested third party users through the user service Optical Airborne Remote Sensing and Calibration Homebase (OpAiRS).

Optical Torque Wrench Design and Calibration.

Expanding the capabilities of optical traps with angular control of the trapped particle has numerous potential applications in all fields where standard linear optical tweezers are employed. Here we describe in detail the construction, alignment, and calibration of the Optical Torque Wrench, a mode of function that can be added to linear optical tweezers to simultaneously apply and measure both force and torque on birefringent microscopic cylindrical particles. The interaction between the linear polarization of the laser and the birefringent cylinder creates an angular trap for the particle orientation, described by a periodic potential. As a consequence of the experimental control of the tilt of the periodic potential, the dynamical excitability of the system can be observed. Angular optical tweezers remain less widespread than their linear counterpart. We hope this technical guide can foster their development and new applications.

Establishing metrological traceability for radiometric calibration of earth observation sensor in Malaysia

The space borne earth observation (EO) sensor provides a continuous large spatial coverage over the earth at relatively low cost (cost-effective) and can be practically accessible worldwide. The daily synoptic view offered by instrument in earth orbit is tremendously useful in various applications, particularly long term global monitoring that needs multi-disciplinary, multi-temporal and multi-sensor data. Due to the indirect measurement nature of the EO sensor, calibration and validation (cal/val) are essentially required to establish the linkage between the acquired raw data and the actual target of interest. Ultimately, EO sensor provider must strive to deliver “the right information, at the right time, to the right people”. This paper is authored with the main aim to report the process of establishing metrological traceability for radiometric calibration of EO sensor at Optical Calibration Laboratory (OCL), National Space Agency of Malaysia (ANGKASA). The paper is structured into six sections. The first section introduces the context of EO and background of radiometric calibration. The next section discusses the requirements for metrological traceability in radiometric calibration while the following third section outlines ANGKASA efforts in setting up the metrological traceability laboratory in radiometric calibration. Meanwhile, the uncertainty estimation results is reported in the fourth section and the fifth section explains some of the continuous efforts made in order to improve the current metrological traceability set up. Lastly, the summary of this paper is provided in the last section.

Optical tweezer calibration by using a part of the intensity distribution of a trapped particle.

In this paper, we demonstrated that an optical tweezer setup can be calibrated by using a part of the symmetric intensity distribution of the trapped particle in digital video microscopy. First, we modified the radial symmetry center method, which was a recently proposed position detection algorithm. This algorithm uses gradient vectors of the particle intensity distribution, which allows us to use a part of the symmetric intensity distribution in the calculation of the particle center. We applied the modified algorithm to different camera image configurations, which are obtained by cutting the same experimental video frames. We further calibrated the trap stiffness for each camera configuration. Then we compared the trap stiffness values and the position distributions. As a result, we can conclude that optical tweezer setups can be calibrated by using a part of the intensity distribution of the trapped particle.

Optical configuration optimization and calibration for the POINT system on EAST.

Calibration of the polarimeter system is one of the key elements to determine the overall measurement accuracy. The anisotropic reflection and transmission properties of the mesh beam splitters can easily distort the polarization state of the circularly polarized beams. Using a rotating crystal quartz λ/2-waveplate to replace the plasma can effectively allow us to obtain the ratio of the measured Faraday rotation angle to the known rotation angle of the waveplate. This ratio is used to estimate the calibration factor for each chord to be accurately determined and help to minimize distortions introduced by the wire-mesh beam splitters. With the novel configuration optimization, the distortion of polarization state is effectively eliminated.

Transient measurement of thin liquid films using a Shack–Hartmann sensor

A Shack–Hartmann (SH) wavefront sensor was used to measure the thickness of thin liquid films of n–octane on silicon substrates. A SH device consists of an array of microlenses spaced from an image sensor by their effective focal length. A planar wavefront imposed on a SH device would produce uniformly spaced foci on the sensor for each lenslet. A distorted wavefront would then shift the foci by a calculable amount. Typically used for optical system alignment and calibration, the SH device was coupled with magnifying optics to determine the distortion of a planar wavefront source upon refraction through a film and reflection by a polished substrate. Geometrical optics, in the form of ray transfer matrices, were used to determine the location and orientation of rays emanating from the film surface given their positions and angles recorded by the SH device. The SH foci movements were translated into sample coordinates and film slopes, which were subsequently integrated to produce the film profile.

Sticker-type ECG/PPG concurrent monitoring system hybrid integration of CMOS SoC and organic sensor device.

The sticker-type sensor system is proposed targeting ECG/PPG concurrent monitoring for cardiovascular diseases. The stickers are composed of two types: Hub and Sensor-node (SN) sticker. Low-power CMOS SoC for measuring ECG and PPG signal is hybrid integrated with organic light emitting diodes (OLEDs) and organic photo detector (OPD). The sticker has only 2g weight and only consumes 141μW. The optical calibration loop is adopted for maintaining SNR of PPG signal higher than 30dB. The pulse arrival time (PAT) and SpO2 value can be extracted from various body parts and verified comparing with the reference device from 20 people in-vivo experiments.

Tuneable dual-comb spectrometer based on commercial femtosecond lasers and reference cell for optical frequency calibration

Two commercial femtosecond laser sources have been used to implement a dual-comb spectrometer tuneable across a spectral range from 1.5 to 2.2 μm. The optical linewidth of the comb modes was characterized for different time scales in order to estimate the achievable spectral resolution for an optimal acquisition time. The transmission spectra of three different gas samples were recorded, demonstrating good agreement with reference data. Frequency axis calibration was provided via the parallel monitoring of a reference sample. This technique allows an accurate calibration of the frequency axis of the spectrometer, with no need for stabilization or optical referencing of the frequency combs. Our set-up represents a good compromise for a compact and versatile dual-comb spectrometer based on commercially available parts with possible applications in trace-gas monitoring, remote sensing and spectroscopy of short-lived processes.

Photonic Torque Microscopy of the Nonconservative Force Field for Optically Trapped Silicon Nanowires.

We measure, by photonic torque microscopy, the nonconservative rotational motion arising from the transverse components of the radiation pressure on optically trapped, ultrathin silicon nanowires. Unlike spherical particles, we find that nonconservative effects have a significant influence on the nanowire dynamics in the trap. We show that the extreme shape of the trapped nanowires yields a transverse component of the radiation pressure that results in an orbital rotation of the nanowire about the trap axis. We study the resulting motion as a function of optical power and nanowire length, discussing its size-scaling behavior. These shape-dependent nonconservative effects have implications for optical force calibration and optomechanics with levitated nonspherical particles.

Spatially resolved scatter measurement of diffractive micromirror arrays.

Spatial light modulators (SLMs) support flexible system concepts in modern optics and especially phase-only SLMs such as micromirror arrays (MMAs) appear attractive for many applications. In order to achieve a precise phase modulation, which is crucial for optical performance, careful characterization and calibration of SLM devices is required. We examine an intensity-based measurement concept, which promises distinct advantages by means of a spatially resolved scatter measurement that is combined with the MMA's diffractive principle. Measurements yield quantitative results, which are consistent with measurements of micromirror roughness components, by white-light interferometry. They reveal relative scatter as low as 10^-4, which corresponds to contrast ratios up to 10,000. The potential of the technique to resolve phase changes in the subnanometer range is experimentally demonstrated.

Early commissioning calibration data sets for DEAP-3600

The DEAP-3600 experiment is a single phase dark matter detector using 1000 kg fiducial mass of liquid argon as scintillating target. An array of 255 high quantum efficiency PMTs detect the argon scintillation light. The experiment is located at SNOLAB, 2 km underground in Ontario, Canada. In February 2015 the PMTs were switched on and data were taken with vacuum and N2 in the detector. Optical and gamma calibration data have been collected to commission the data acquisition hardware and software and characterize the optical properties of the detector.

Optical model and calibration of a sun tracker

Sun trackers are widely used to investigate scattering and absorption of solar radiation in the Earth׳s atmosphere. We present a method for optimization of the optical altazimuth sun tracker model with output radiation direction aligned with the axis of a stationary spectrometer. The method solves the problem of stability loss in tracker pointing at the Sun near the zenith. An optimal method for tracker calibration at the measurement site is proposed in the present work. A method of moving calibration is suggested for mobile applications in the presence of large temperature differences and errors in the alignment of the optical system of the tracker.