Calibrated Measurement Setup Research Articles

Highly Efficient Calibration of Antenna Arrays by Active Targets in the Near-Field

To enhance the capabilities of high-resolution imaging or beamforming, the trend goes towards arrays with high-channel count and large aperture sizes. The operation at increasing mm-wave frequencies exacerbates the consequences of manufacturing tolerances on the array performance, further necessitating the development of high-quality calibrations. Thereby, a large number of array response vectors, addressed in the following as data points, is stored. Typically, one data point is measured at a time for each angle. To reduce the calibration effort, hereby defined as the number of required measurements, a calibration setup employing active calibration targets (ACT) is proposed. This setup allows the storage of a multitude of data points within a single measurement. In addition, this measurement setup is placed in the near-field of the array to relax the requirements on the size of the anechoic chamber. Employing multiple targets makes it challenging to compensate the near-field effects. Therefore, methods are discussed to ensure a consistent quality of the calibration considering the near-field influences from the ACTs. Exemplary measurements deploying an antenna array operating at 78.5demonstrate that this calibration measurement setup employing three ACTs placed in the near-field of the array reduces the calibration effort by a factor of 3 compared to state-of-the-art setups. The average deviation of the antenna phase relations amounts to 4.1 degree, proving that the presented setup achieves the same calibration quality as state-of-the-art setups.

Comparison of pattern VEP results acquired using CRT and TFT stimulators in the clinical practice

There are several electrophysiological systems available commercially. Usually, control groups are required to compare their results, due to the differences between display types. Our aim was to examine the differences between CRT and LCD/TFT stimulators used in pattern VEP responses performed according to the ISCEV standards. We also aimed to check different contrast values toward thresholds. In order to obtain more precise results, we intended to measure the intensity and temporal response characteristics of the monitors with photometric methods. To record VEP signals, a Roland RetiPort electrophysiological system was used. The pattern VEP tests were carried out according to ISCEV protocols on a CRT and a TFT monitor consecutively. Achromatic checkerboard pattern was used at three different contrast levels (maximal, 75, 25%) using 1° and 15' check sizes. Both CRT and TFT displays were luminance and contrast matched, according to the gamma functions based on measurements at several DAC values. Monitor-specific luminance parameters were measured by means of spectroradiometric instruments. Temporal differences between the displays' electronic and radiometric signals were measured with a device specifically built for the purpose. We tested six healthy control subjects with visual acuity of at least 20/20. The tests were performed on each subject three times on different days. We found significant temporal differences between the CRT and the LCD monitors at all contrast levels and spatial frequencies. In average, the latency times were 9.0 ms (±3.3 ms) longer with the TFT stimulator. This value is in accordance with the average of the measured TFT input-output temporal difference values (10.1 ± 2.2 ms). According to our findings, measuring the temporal parameters of the TFT monitor with an adequately calibrated measurement setup and correcting the VEP data with the resulting values, the VEP signals obtained with different display types can be transformed to be comparable.

Capabilities of Ultrasound for Monitoring and Quantitative Analysis of Polyolefin Waste Particles in Magnetic Density Separation (MDS)

A study was conducted into the potential of ultrasound as a qualitative and quantitative tool for the Magnetic Density Separator (MDS), where an ultrasound sensor array will operate from inside the MDS ferrofluid. The desired applications of the ultrasound are monitoring of polyolefin particles in flow during the separation process, and performing throughput measurements and quality inspection. The possibility to adapt off-the-shelf medical imaging technology was investigated with an eye towards the real-time requirements for industrial application. It is shown that a medical imager can be adapted for monitoring the flowing particles at speeds up to 30 cm/s, and the imaging quality can be very good under optimum viewing angle conditions. Ultrasound imaging may also allow for quantitative through-put analysis provided the flowing particles are sufficiently spaced, which puts a limit to the maximum measurable throughput. If viewing conditions are not optimum the medical imager relies on the human operator to improve image quality. This situation is undesired for industrial operation where there may be little or no time for operator intervention. To anticipate these occurrences a second imaging method is proposed that could act in conjunction, which is effective since it is based on a different physical principle. Ultrasound quality inspection is a very different technique, and its feasibility depends first of all on whether different types of polyolefin are acoustically distinctive. Using experiments and 3D acoustic computer modelling to determine wave speed and material attenuation, the potential of ultrasound for polyolefin identification under immersed conditions was investigated. With a calibrated measurement setup it is shown that polyolefins may potentially be classified into acoustically distinctive groups. The possible errors in throughput measurement and quality inspection and the requirements for these techniques to perform under operational MDS conditions are also addressed.

3D reconstruction of specular surfaces using a calibrated projector–camera setup

Structured light is widely used for shape measurement of beamless surfaces using the triangulation principle. In the case of specular surfaces deflectometry is an appropriate method. Hereby the reflection of a light pattern is observed by a camera. The distortion of the reflected pattern is evaluated to obtain information about the reflecting surface. An important requirement for a 3d reconstruction of a specular surface by deflectometry is a calibrated measurement setup. We propose a method for the overall calibration of a setup consisting of a structured light source, a projection screen and a camera. We consider all extrinsic and intrinsic parameters for the optical mapping including a distortion model for the projector and for the camera, respectively. Given the deflectometric data obtained by the calibrated setup two methods are described which allow the 3d reconstruction of points on a specular surface. This is not trivial as a reconstruction using solely deflectometric data shows ambiguity. In the first method screen displacement between two deflectometric measurements is used to overcome this ambiguity. In the second method curvature-like features on the surface are determined to serve as starting points for a region growing approach. Results of the reconstruction of a specular surface are shown and the performance of the described reconstruction methods are compared to each other.