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A special issue of Sensors (ISSN 1424-8220). This special issue belongs to the section "Optical Sensors".

Deadline for manuscript submissions: 15 October 2024 | Viewed by 15130

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Special Issue Editors

Prof. Dr. Jürgen Hartmann

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Institute Digital Engineering (IDEE), University of Applied Sciences Würzburg-Schweinfurt, Ignaz-Schön-Straße 11, 97421 Schweinfurt, Germany
Interests: sensor technology; metrology; thermo-physics; radiometry; in-line process controll
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Dr. Jochen Manara

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Guest Editor

Bavarian Center for Applied Energy Research (ZAE Bayern) Magdalene-Schoch-Str. 3, 97074 Würzburg, Germany
Interests: thermophysics; infrared-optical characterization; non-contact measurements

Special Issue Information

Dear Colleagues,

As a result of the restriction in fossil energy resources and the increasing demand for efficiency and waste control, the performance and control of energy conversion processes are gaining more attention. The measurement of process parameters, in particular temperature, is necessary in order to operate the process at its most efficient range, thus producing less waste. The parameters of the materials used for such high-temperature processes must be known to have sufficient accuracy at the relevant high temperatures in order to optimize the efficiency of the process, to simulate the process in advance to prevent unnecessary waste, and to ensure a safe and efficient process. Additionally, to save resources, the efficiency of high-temperature energy conversion processes has to be increased by increasing the operation temperatures.

As these processes operate at elevated temperatures above 1000 °C, contactless measurement methods, in particular optothermal methods, have to be used. Such methods are also helpful for understanding and controlling other high-temperature processes like casting, and modern, laser-based additive manufacturing methods, such as selective laser melting. The present Special Issues aims to present basic research regarding optothermal sensor technology. In addition, dedicated applications of such optothermal sensor technology in the above mentioned or other fields are also welcome.

We are particularly interested in (but not limited to) contributions that focus on topics such as:

Optothermal sensor technology concepts;
Application of optothermal sensor technologies in material science;
Application of optothermal sensor technologies in energy technologies;
Application of optothermal sensor technologies in process technologies.

Prof. Dr. Jürgen Hartmann
Dr. Jochen Manara
Guest Editors

Manuscript Submission Information

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Keywords

thermal radiation
radiation thermometry
non-destructive testing
high-temperature measurements
thermophysics
online process control
additive manufacturing
gas turbines
process technologies
energy conversion processes

Published Papers (7 papers)

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Research

11 pages, 3528 KiB

Open AccessCommunication

Process Monitoring Using Synchronized Path Infrared Thermography in PBF-LB/M

by Dennis Höfflin, Christian Sauer, Andreas Schiffler and Jürgen Hartmann

Sensors 2022, 22(16), 5943; https://doi.org/10.3390/s22165943 - 09 Aug 2022

Cited by 3 | Viewed by 1676

Abstract

Additive manufacturing processes, particularly Laser-Based Powder Bed Fusion of Metals (PBF-LB/M), enable the development of new application possibilities due to their manufacturing-specific freedom of design. These new fields of application require a high degree of component quality, especially in safety-relevant areas. This is currently ensured primarily via a considerable amount of downstream quality control. Suitable process monitoring systems promise to reduce this effort drastically. This paper introduces a novel monitoring method in order to gain process-specific thermal information during the manufacturing process. The Synchronized Path Infrared Thermography (SPIT) method is based on two synchronized galvanometer scanners allowing high-speed and high-resolution observations of the melt pool in the SWIR range. One scanner is used to steer the laser over the building platform, while the second scanner guides the field of view of an IR camera. With this setup, the melting process is observed at different laser powers, scan speeds and at different locations with respect to the laser position, in order to demonstrate the positioning accuracy of the system and to initially gain thermal process data of the melt pool and the heat-affected zone. Therefore, the SPIT system shows a speed independent overall accuracy of ±2 Pixel within the evaluated range. The system further allows detailed thermal observation of the melt pool and the surrounding heat-affected zone. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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13 pages, 3770 KiB

Open AccessCommunication

Opto-Thermal Investigation of Additively Manufactured Steel Samples as a Function of the Hatch Distance

by Dennis Höfflin, Maximilian Rosilius, Philipp Seitz, Andreas Schiffler and Jürgen Hartmann

Sensors 2022, 22(1), 46; https://doi.org/10.3390/s22010046 - 22 Dec 2021

Cited by 5 | Viewed by 2131

Abstract

Nowadays, additive manufacturing processes are becoming more and more appealing due to their production-oriented design guidelines, especially with regard to topology optimisation and minimal downstream production depth in contrast to conventional technologies. However, a scientific path in the areas of quality assurance, material and microstructural properties, intrinsic thermal permeability and dependent stress parameters inhibits enthusiasm for the potential degrees of freedom of the direct metal laser melting process (DMLS). Especially in quality assurance, post-processing destructive measuring methods are still predominantly necessary in order to evaluate the components adequately. The overall objective of these investigations is to gain process knowledge make reliable in situ statements about component quality and material properties based on the process parameters used and emission values measured. The knowledge will then be used to develop non-destructive tools for the quality management of additively manufactured components. To assess the effectiveness of the research design in relation to the objectives for further investigations, this pre-study evaluates the dependencies between the process parameters, process emission during manufacturing and resulting thermal diffusivity and the relative density of samples fabricated by DMLS. Therefore, the approach deals with additively built metal samples made on an EOS M290 apparatus with varying hatch distances while simultaneously detecting the process emission. Afterwards, the relative density of the samples is determined optically, and thermal diffusivity is measured using the laser flash method. As a result of this pre-study, all interactions of the within factors are presented. The process variable hatch distance indicates a strong influence on the resulting material properties, as an increase in the hatch distance from 0.11 mm to 1 mm leads to a drop in relative density of 57.4%. The associated thermal diffusivity also reveals a sharp decrease from 5.3 mm²/s to 1.3 mm²/s with growing hatch distances. The variability of the material properties can also be observed in the measured process emissions. However, as various factors overlap in the thermal radiation signal, no clear assignment is possible within the scope of this work. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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17 pages, 6209 KiB

Open AccessArticle

Development and Evaluation of an Improved Apparatus for Measuring the Emissivity at High Temperatures

by Mariacarla Arduini, Jochen Manara, Thomas Stark, Hans-Peter Ebert and Jürgen Hartmann

Sensors 2021, 21(18), 6252; https://doi.org/10.3390/s21186252 - 17 Sep 2021

Cited by 4 | Viewed by 2424

Abstract

An improved apparatus for measuring the spectral directional emissivity in the wavelength range between 1 µm and 20 µm at temperatures up to 2400 K is presented in this paper. As a heating unit an inductor is used to warm up the specimen, as well as the blackbody reference to the specified temperatures. The heating unit is placed in a double-walled vacuum vessel. A defined temperature, as well as a homogenous temperature distribution of the whole surrounding is ensured by a heat transfer fluid flowing through the gap of the double-walled vessel. Additionally, the surrounding is coated with a high-emitting paint and serves as blackbody-like surrounding to ensure defined boundary conditions. For measuring the spectral directional emissivity at different emission angles, a movable mirror is installed in front of the specimen, which can be adjusted by a rotatable arrangement guiding the emitted radiation into the attached FTIR-spectrometer. The setup of the emissivity measurement apparatus (EMMA) and the measurement procedure are introduced, and the derived measurement results are presented. For evaluating the apparatus, measurements were performed on different materials. The determined emissivities agree well with values published in literature within the derived relative uncertainties below 4% for most wavelengths. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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10 pages, 1912 KiB

Open AccessCommunication

Temperature-Dependent Broadening of the Ultraviolet Photoelectron Spectrum of Au(110)

by Tomonari Nishida, Ikuo Kinoshita and Juntaro Ishii

Sensors 2021, 21(17), 5969; https://doi.org/10.3390/s21175969 - 06 Sep 2021

Cited by 2 | Viewed by 1869

Abstract

To determine the thermodynamic temperature of a solid surface from the electron energy distribution measured by photoelectron spectroscopy, it is necessary to accurately evaluate the energy broadening of the photoelectron spectrum and investigate its temperature dependence. Broadening functions in the photoelectron spectrum of Au(110)’s surface near the Fermi level were estimated successfully using the relationship between the Fourier transform and the convolution integral. The Fourier transform could simultaneously reduce the noise of the spectrum when the broadening function was derived. The derived function was in the form of a Gaussian, whose width depended on the thermodynamic temperature of the sample and became broader at higher temperatures. The results contribute to improve accuracy of the determination of thermodynamic temperature from the photoelectron spectrum and provide useful information on the temperature dependence of electron scattering in photoelectron emission processes. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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13 pages, 1861 KiB

Open AccessCommunication

Influence of Atmosphere on Calibration of Radiation Thermometers

by Vid Mlačnik and Igor Pušnik

Sensors 2021, 21(16), 5509; https://doi.org/10.3390/s21165509 - 16 Aug 2021

Cited by 4 | Viewed by 1775

Abstract

Current process of calibrating radiation thermometers, including thermal imagers, relies on measurement comparison with the temperature of a black body at a set distance. Over time, errors have been detected in calibrations of some radiation thermometers, which were correlated with moisture levels. In this study, effects of atmospheric air on thermal transmission were evaluated by the means of simulations using best available resources of the corresponding datasets. Sources of spectral transmissivity of air were listed, and transmissivity data was obtained from the HITRAN molecular absorption database. Transmissivity data of molecular species was compiled for usual atmospheric composition, including naturally occurring isotopologs. Final influence of spectral transmissivity was evaluated for spectral sensitivities of radiation thermometers in use, and total transmissivity and expected errors were presented for variable humidity and measured temperature. Results reveal that spectral range of measurements greatly influences susceptibility of instruments to atmospheric interference. In particular, great influence on measurements is evident for the high-temperature radiation pyrometer in the spectral range of 2–2.7 µm, which is in use in our laboratory as a traceable reference for high-temperature calibrations. Regarding the calibration process, a requirement arose for matching the humidity parameters during the temperature reference transfer to the lower tiers in the chain of traceability. Narrowing of the permitted range of humidity during the calibration, monitoring, and listing of atmospheric parameters in calibration certificates is necessary, for at least this thermometer and possibly for other thermometers as well. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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10 pages, 4150 KiB

Open AccessCommunication

Measurements of Excavation Damaged Zone by Using Fiber Bragg Grating Stress Sensors

by Xiaorong Wan, Chuan Li, Zhengang Zhao, Dacheng Zhang, Yingna Li and Jiahong Zhang

Sensors 2021, 21(15), 5008; https://doi.org/10.3390/s21155008 - 23 Jul 2021

Cited by 6 | Viewed by 1788

Abstract

In this paper, a Fiber Bragg Grating (FBG) stress sensor is developed to measure the stress variation between the lower Excavation Damaged Zone (EDZ) and the upper undistributed rock. The disturbance brought by the environmental temperature can be differentially compensated with two FBGs mounted symmetrically on the spokes. Through finite element analysis, it can be known that the direct stress and shear stress are pointed at the angles of 45° and 60° on both sides of the coal mine roadway, respectively. The anchor ends of the sensors are installed into the upper undistributed rock and the bolt tails of the mine roadway with a depth of 700 m and fastened by nuts to secure the load sensing device on the surface of the rock. When the shallow foundation of surrounding rock is pressed and deformed toward the coal mining road, the structural modifications can be converted into the stress of rock bolt and the strain of spoke. Thus, the FBG mounted on the surface of the spoke receives the shift information of the Bragg wavelength. The monitoring results indicate that the FBG stress sensors are sensitive to the variation of the EDZ. During the blasting, the stress amplitude varies from 40.256 to 175.058 kPa, and the creep time changes from 21 to 74 min. The proposed method can be applied in the field of underground coal mines for safety condition monitoring of the EDZ and forecasting the coal mine roadway stability. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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12 pages, 18579 KiB

Open AccessCommunication

Compact Measurement of the Optical Power in High-Power LED Using a Light-Absorbent Thermal Sensor

by You-Young Kim, Jae-Young Joo, Jong-Min Kim and Sun-Kyu Lee

Sensors 2021, 21(14), 4690; https://doi.org/10.3390/s21144690 - 08 Jul 2021

Cited by 2 | Viewed by 1955

Abstract

LED (Light-Emitting Diode) presents advantages such as luminescence, reliability, durability compared with conventional lighting. It has been widely applied for life, healthcare, smart farm, industry, and lighting from indoor to the automotive headlamp. However, the LED is vulnerable to thermal damage originated from the high junction temperature, especially in high power applications. Hence, it requires precise qualification on the optical power and the junction temperature from the pilot line to secure reliability. In this study, the photo-thermal sensor is proposed by employing a sheet-type thermocouple composed of photo-absorbent metal film and thermocouple. This sensor aims low-cost qualification in pilot line for high-power luminous devices and optical monitoring of costly luminaire such as automobile LED headlamp. The sensor is designed to detect the increased temperature response of LED hot spots from the transferred thermal power and absorbed optical power. The temperature response of each sheet-type thermocouple is utilized as a signal output of the absorbed optical power and hot spot temperature based on the introduced sensor equation. The proposed thermal sensor is evaluated by comparing the experiment with the measured reference value from the integrating sphere and the attached thermocouple at a junction. The experiment result reveals 3% of the maximum error for the optical power of 645 mW. Full article

(This article belongs to the Special Issue Opto-Thermal Sensor Technologies)

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