Multispectral Pyrometer Research Articles

Understanding coaxial photodiode-based multispectral pyrometer measurements at the overhang regions in laser powder bed fusion for part qualification

Coaxial photodiode monitoring sensors provide a digital signature at the melt pool level in laser powder bed fusion (L-PBF), facilitating faster part qualification. However, current signatures are plagued by significant noise and signal variation and show counterintuitive trends such as a decrease in measured temperature near overhang surfaces. This paper investigates the behavior of coaxial photodiode-based melt pool monitoring (PD-MPM) thermal measurements at overhang regions by conducting a combination of experiments and multiphysics simulations. High-speed in situ synchrotron X-ray imaging is coupled with a coaxial photodiode system to enable comparison of the observed melt pool phenomena and monitoring signals during double-track AlSi10Mg experiments. A multiphysics model is developed to simulate the melt pool dynamics and concurrent sensor signals throughout the process. We propose a surrogate model that clarifies the correlation between sensor signals and melt pool temperature. Both experimental and simulation results emphasize the significant impacts of laser energy and keyhole formation on solid-liquid interface discontinuities and PD-MPM signals. Results reveal that a boiling region with a relatively smaller projected area, as near an overhang, can lead to a decrease in the measured melt pool temperature, even when the true peak temperature remains constant, meaning that PD-MPM temperature measurements cannot be used to estimate absolute melt pool temperature. Consequently, in overhang regions, interaction between the melt pool and underlying powder results in an abruptly deforming melt pool and a pronounced decrease in both individual photodiode intensities and overall measured melt pool temperature. This work illustrates how to correctly interpret the coaxial photodiode monitoring signals in L-PBF by uncovering the intricate dynamics within the melt pool, particularly in challenging overhang regions, and pave the foundation for data-driven part qualification.

Generalized inverse matrix-graphic deep learning algorithm for multispectral pyrometer temperature inversion.

The multispectral radiometric temperature measurement technique is affected by the unknown emissivity, and there is no multispectral radiometric temperature inversion algorithm applicable to any scene or target. To address the above problems, this paper converts the multispectral radiometric temperature inversion problem into an image recognition problem containing the temperature information to be measured, and proposes a graphical multispectral radiometric temperature adaptive inversion algorithm. In this paper, we use the difference between spectral channels to convert the one-dimensional radiation data into a two-dimensional radiation map; use the generalized inverse to obtain the spectral emissivity distribution features, fuse them with the two-dimensional radiation map, and use an improved deep learning network to achieve adaptive temperature inversion. It is experimentally verified that the algorithm proposed in this paper can achieve simultaneous inversion of temperature and emissivity for any scene or target with sufficient data set.

Development of multi-spectral pyrometer for measuring cathode surface temperature of pulsed vacuum arc discharge

The cathode surface temperature is a vital parameter in the pulsed vacuum arc discharge process, which has an important influence on the formation of vacuum arc plasma, electrode corrosion prediction and ion source lifetime. Currently, the research of cathode surface temperature measurement is mainly based on a high-speed charge coupled instrument (CCD) camera. However, there are no other temperature measuring instruments to verify the results of a high-speed camera. For this reason, a new type of multi-spectral pyrometer is developed. The developed pyrometer adopts optical filter instead of traditional prism splitting, which simplifies the calibration process of pyrometer. An optical sampling system is redesigned to ensure that a tiny target of 100 μm can be measured. On the built experimental platform, the synchronous experiment of multi-spectral pyrometer and high-speed camera are carried out. The experimental results show that the developed multi-spectral pyrometer can verify the measurement results of the multi-spectral pyrometer based on high-speed camera.

The spectral‐voltage calibration technology of multispectral pyrometers based on Gauss–Newton‐genetic algorithm method for nonsource temperature regions

AbstractTo improve the accuracy of the nonsource temperature calibration method, a new method based on a Gauss–Newton‐genetic algorithm (GN‐GA) for the nonsource calibration of a multispectral pyrometer is proposed. Based on Planck's law, a temperature–voltage power function model was established based on constraint optimization, and the temperature–voltage function relationship of each spectral channel was obtained using a GN‐GA algorithm. When the GN‐GA algorithm extrapolated at 1000°C with 3000°C as the starting point, theoretical simulation results showed that, compared with the derivative least squares method, when the wavelengths were 0.4, 0.6, 0.8, and 1.0 μm, the extrapolation accuracy increased by 54.35%, 63.96%, 51.99%, and 44.05%, respectively. The black‐body experiment results showed that this method was feasible in multispectral, pyrometer, extrapolation, temperature–calibration technology. Compared with the results obtained by the derivative least squares method, the accuracy was improved for a given extrapolation temperature region, and the extrapolation interval was longer under the same accuracy.

The development of a multispectral pyrometer for achievable true temperature field measurements of the explosion flame

The temperature measurement of explosion flames is a key challenge in the field of weapons research. Many pyrometers have been developed to determine the accurate temperature of the explosion flame. However, these pyrometers can only measure the true temperature curve and the brightness temperature field of explosion flames, not their true temperature field. A multispectral pyrometer that measures the true temperature field of the explosion flame is developed in this paper. The multispectral pyrometer acquires brightness temperature field images at four different wavelengths of the explosion flame at the same moment using synchronous spectral-splitting technology. The pixels of the obtained four brightness temperature field images are then aligned using an edge feature-based image matching algorithm. The true temperature value is finally calculated for each pixel using multispectral radiation thermometry to construct the true temperature field of the explosion flame. Based on the experimental results, the true temperature field of the explosion flame can be measured with the proposed multispectral pyrometer.

True temperature inversion for a multi-spectral pyrometer based on the inner penalty function constrained optimization algorithm

A data processing algorithm based on the inner penalty function for constrained optimization is presented for a multi-wavelength pyrometer. It can directly obtain the true temperature and spectral emissivity simultaneously without requiring for the spectral emissivity model beforehand. The simulation results show that the algorithm is suitable for continuous thermal radiation materials, and the absolute error is less than 15 K when the true temperature is 1800 K. The experimental results also verify the reliability of the algorithm. This data processing algorithm resolves the true temperature measurement by thermal radiation problem which was due to the unknown emissivity model.

Research on the calibration method of non-source temperature area of multispectral pyrometer based on a new exponential curve model

For the temperature measurement of high temperature targets, the use of multispectral pyrometers has a great advantage. However, with the increase of the measured target temperature, the research on the high-precision calibration method in the non-source temperature area falls far behind. Therefore, in order to break through the bottleneck of the calibration method of the non-source temperature area for high temperature targets, a new non-source temperature area calibration method is proposed in this paper. In this method, a temperature-gray/voltage (T-G/U) model based on the modified exponential function is established by using the close relationship between the change trend of the extrapolated temperature curve and the modified exponential curve model. There will be a large error in the prediction of the high temperature section in the non-source temperature area, so a quadratic polynomial term with error compensation is introduced on the basis of this model. The method of fitting estimation is employed to obtain the estimated temperature value of the predicted point in the high temperature section of the non-source temperature area. For the prediction of the entire non-source temperature area, the genetic algorithm is used to adaptively select the model parameter t, and the high-precision extrapolation calibration of the high temperature target in the entire non-source temperature area is realized. Both theoretical and experimental data verify the effectiveness and feasibility of the new method.

Measurement performance analysis for a charge-coupled-device-based near-infrared multi-spectral pyrometer

The handheld charge coupled device (CCD) spectrometer can detect emitted, reflected or transmitted light spectrum rapidly, and so it was widely used in the fields of scientific research and industrial applications. In this paper, based on a CCD array spectrometer, a multi-spectral pyrometer is also developed, and the dependence of the spectral radiation intensity and the analog to digital converter (ADC) counts is determined by means of a blackbody furnace. Meanwhile, the noise model of the multi-spectral pyrometer is established and the influences of the readout noise, photoelectron noise and dark noise on temperature measurement accuracy are evaluated quantitatively. Experimental results show that temperature measurement accuracy is mainly affected by the readout noise. In the temperature range from 800 °C to 1200 °C and the near-infrared region around the spectral band of 780–1030 nm, the maximum relative temperature measurement deviation caused by the readout noise is 1.79% when the exposure time is set to 9 ms. And under the same setting conditions, the maximum relative temperature measurement deviation caused by the photoelectron noise and the dark noise is 1.00% and 0.24%, respectively.

A Bayesian approach for the estimation of the thermal diffusivity of aerodynamically levitated solid metals at high temperatures

The flash method has been the standard technique for the measurement of thermal diffusivity since it was first developed by Parker and co-workers. Several modifications of the original method have been proposed in the literature, to deal with different physical situations and materials. In this work, we extend the front-face flash method for the simultaneous estimation of the thermal diffusivity and thermal conductivity of aerodynamically levitated spherical solid metal samples at high temperatures, with laser excitation and contactless temperature measurements taken with a pyrometer. The mathematical model associated to the physical problem is formulated as an axisymmetric heat conduction problem, with heat losses by radiation and convection. Since the measurements of a pyrometer are in fact radiative fluxes, the transfer function of the pyrometer is integrated in the mathematical model. A sensitivity analysis is performed and reveals that the thermal diffusivity and the thermal conductivity of the sample can be simultaneously estimated, provided that the other model parameters are known. The solution of the inverse problem is obtained with algorithms within the Bayesian framework of statistics, by using a heat conduction reduced model with linear boundary conditions, for which an analytical solution is developed for fast computations. The Bayesian Approximation Error Model is used to compensate for errors between a complete heat conduction model with nonlinear boundary conditions and a reduced heat conduction model with linear boundary conditions. Moreover, errors related to the inaccuracy and the uncertainties of all model parameters are accounted for in the inverse analysis. Simulated transient flux measurements are assumed available from a multi-spectral pyrometer and are used for the inverse problem solution.

Multispectral pyrometer for high temperature measurements inside combustion chamber of gas turbine engines

Currently, the solution for the problem of temperature measurement of the gas phase of combustion products inside the combustion chambers of gas turbine engines is still in progress. Conventional thermocouples, which are the industry standard, are intrusive and provide spatially and temporally averaged data. In this work, a spectral system for the temperature measurement of gas flow inside the combustion chamber has been developed. The system is based on the method of multi-wave pyrometry of soot in the visible optical range and comprises a multispectral sensor-on-chip solution. Field tests on the gas turbine engine confirmed that the presented system allows measuring temperature in the range from 810 to 1120 °C with a measurement error of 3%.

Subpixel Temperature Measurements in Plasma Jet Environments Using High-Speed Multispectral Pyrometry

A high-speed (2 kHz) near-infrared (1.0–1.65 μm) multispectral pyrometer was used for noninvasive measurements of the subpixel temperature distribution near the sharp leading edge of a wing exposed to a supersonic plasma jet. The multispectral pyrometer operating in the field measurement mode was able to measure the spatial temperature distribution. Multiple spectra were used to determine the temperature distributions in the measurement region. The spatial resolution of the multispectral pyrometer was not restricted to one “pixel” but was extended to subpixel accuracy (the temperature distribution inside one pixel in the image space corresponding to the point region in the object space). Thus, this system gives high-speed, multichannel, and long working time spatial temperature measurements with a small data stream from high-speed multispectral pyrometers. The temperature distribution of the leading edge of a ceramic wing was investigated with the leading edge exposed to extreme convective heating from a high-enthalpy plasma flow. Simultaneous measurements with a multispectral pyrometer and an imaging pyrometer verify the measurement accuracy of the subpixel temperature distribution. Thus, this multispectral pyrometry can provide in situ noninvasive temperature diagnostics in supersonic plasma jet environments.

VIS-NIR multispectral synchronous imaging pyrometer for high-temperature measurements.

A visible-infrared multispectral synchronous imaging pyrometer was developed for simultaneous, multispectral, two-dimensional high temperature measurements. The multispectral image pyrometer uses prism separation construction in the spectrum range of 650-950 nm and multi-sensor fusion of three CCD sensors for high-temperature measurements. The pyrometer had 650-750 nm, 750-850 nm, and 850-950 nm channels all with the same optical path. The wavelength choice for each channel is flexible with three center wavelengths (700 nm, 810 nm, and 920 nm) with a full width at half maximum of the spectrum of 3 nm used here. The three image sensors were precisely aligned to avoid spectrum artifacts by micro-mechanical adjustments of the sensors relative to each other to position them within a quarter pixel of each other. The pyrometer was calibrated with the standard blackbody source, and the temperature measurement uncertainty was within 0.21 °C-0.99 °C in the temperatures of 600 °C-1800 °C for the blackbody measurements. The pyrometer was then used to measure the leading edge temperatures of a ceramics model exposed to high-enthalpy plasma aerodynamic heating environment to verify the system applicability. The measured temperature ranges are 701-991 °C, 701-1134 °C, and 701-834 °C at the heating transient, steady state, and cooling transient times. A significant temperature gradient (170 °C/mm) was observed away from the leading edge facing the plasma jet during the steady state heating time. The temperature non-uniformity on the surface occurs during the entire aerodynamic heating process. However, the temperature distribution becomes more uniform after the heater is shut down and the experimental model is naturally cooled. This result shows that the multispectral simultaneous image measurement mode provides a wider temperature range for one imaging measurement of high spatial temperature gradients in transient applications.

Development of a new fiber-optic multi-target multispectral pyrometer for achievable true temperature measurement of the solid rocket motor plume

Abstract This paper describes the development of a novel multi-target multispectral pyrometer to remotely measure the true temperature of the plume of a solid fuel rocket engine in a harsh environment ground test. Using an optical fiber transmission technique, the core part of the instrument can be located 100 m away from the test site, so the influence of the harsh environment can be reduced and the instrument reliability improved. Optics are separated from prisms to accurately locate targets. Based on preamplifier circuits and parallelly adjacent pixels of the photodiode detectors, the measurement lower limit can reach 900 °C. The pyrometer is validated against experimental data from field measurements of a solid fuel rocket motor plume. The results demonstrate that the true temperature of the solid fuel rocket engine plume can be acceptably measured with the proposed pyrometer.

Inverse analysis of non-uniform temperature distributions using multispectral pyrometry

Optical diagnostics can be used to obtain sub-pixel temperature information in remote sensing. A multispectral pyrometry method was developed using multiple spectral radiation intensities to deduce the temperature area distribution in the measurement region. The method transforms a spot multispectral pyrometer with a fixed field of view into a pyrometer with enhanced spatial resolution that can give sub-pixel temperature information from a “one pixel” measurement region. A temperature area fraction function was defined to represent the spatial temperature distribution in the measurement region. The method is illustrated by simulations of a multispectral pyrometer with a spectral range of 8.0–13.0μm measuring a non-isothermal region with a temperature range of 500–800K in the spot pyrometer field of view. The inverse algorithm for the sub-pixel temperature distribution (temperature area fractions) in the “one pixel” verifies this multispectral pyrometry method. The results show that an improved Levenberg–Marquardt algorithm is effective for this ill-posed inverse problem with relative errors in the temperature area fractions of (–3%, 3%) for most of the temperatures. The analysis provides a valuable reference for the use of spot multispectral pyrometers for sub-pixel temperature distributions in remote sensing measurements.

Research on the Working-wavelength Selection Method of the MSP Non-source Temperature Calibration Method Based on Cuvre Similarity Principle

In the field of multi-spectral temperature measurement, the high-temperature blackbody furnace is applied to calibrate the multi-spectral pyrometer (MSP). The material of the blackbody furnace is graphite and its melting point is 3000℃. As a result, the MSP is not able to be calibrated above 3000℃ (non-source temperature) and the measurement range of MSP is limited to lower than 3000℃. Currently, the needs of the temperature measurement above 3000 ℃ are gradually increasing. To solve the problem above, the non-source temperature calibration method based on curve similarity principle (NCCSP) is proposed. The core idea of the NCCSP is that the mathematical model is formed based on curve similarity of the MSP output and the power function and then the derivative fitting which can control the curve trend well is applied to obtain model parameters to achieve the purpose that apply the calibration data below 3000 ℃ to predict the calibration data above 3000 ℃. The NCCSP is significance to expand multispectral pyrometer measurement range and supplement multispectral temperature measurement theory. The working-wavelength number of MSP is from a few to hundreds, under which the NCCSP is more accurate needs to be further researched. Therefore, in this paper, the impacts on the NCCSP accuracy of the working-wavelength selection are researched. Combine with Planck's law by theoretical simulation method to get the working-wavelength selection basis.

Multi-spectral pyrometer for narrow space with high ambient temperature

A fiber-optic multi-spectral pyrometer with high spatial and temporal resolution has been applied to measure temperatures of the range from 700 to 1200 K. In a narrow space, the important problems in temperature measurement include the unknown emissivity on target surface and the thermal radiation from the high ambient temperature. This paper analyzed several critical issues affecting the multi-spectral pyrometer and calculated the corresponding model through genetic algorithm. The experiment result showed that this method has high accuracy and the measurement error is $$<$$ 0.44 %.

Research on Temperature Points Selection of the Non-source Temperature Calibration Method Based on Pyrometer Transfer Function

The non-source temperature calibration method of the multi-spectral pyrometer (MSP), put forward in the previous article, is the non-source calibration method based on the MSP transfer function (NCCTF). In this paper, the practicability of NCCTF is researched. The situations that maybe encountered of the temperature points in the actual calibration process are simulated. The impact of the intervals temperature points of different number and unequal intervals temperature points on the NCCTF accuracy are researched theoretically to determine the temperature points select method. Simulation experimental results show that the NCCTF can meet the actual calibration process with good usability.

Validation of a multitechnical device aimed to reach the temperature of a material under shock loading

The temperature is one of the parameters which occurs in the equations of state, to describe the behaviour of materials submitted to an intense shock. A multispectral pyrometer is used to determine the temperature. This diagnostic is well known on classical experimental set-up, but it allows to obtain only one temperature measurement. The aim of this work is to validate a device able to achieve two simultaneous measurements of temperature, coupled with chronometry and velocity measurements, to improve our knowledge of the behaviour and of the temperature of a material under shock loading. It is necessary to resize and to characterize the existing diagnostics and to take into account the duration of the phenomena (700 ns), as well as mechanical and luminous disturbances generated by the shock. The true temperature of the shocked material is obtained from a radiance measurement at short wavelength. So the pyrometer is reconfigured. This assembly was validated on bismuth samples. The obtained results enable us to consider new prospects, like the addition of a reflectivity measurement, in order to reduce uncertainty on the temperature.

Multispectral precise pyrometer for measurement of seawater surface temperature

The paper presents selected problems referring to remote precise temperature measurements. A method of correction of temperature measurement of seawater, disturbed by sun radiation, and a method of elimination of power supply interference were discussed for multispectral IR pyrometer.

Effect of a thin layer on the measurement of the thermal diffusivity of a material by a flash method

The flash method has been the standard technique for the measurement of thermal diffusivity since it was first developed by Parker and co-workers. Several modifications of the original method have been proposed in the literature, to deal with different physical situations and materials. In this work, we extend the front-face flash method for the simultaneous estimation of the thermal diffusivity and thermal conductivity of aerodynamically levitated spherical solid metal samples at high temperatures, with laser excitation and contactless temperature measurements taken with a pyrometer. The mathematical model associated to the physical problem is formulated as an axisymmetric heat conduction problem, with heat losses by radiation and convection. Since the measurements of a pyrometer are in fact radiative fluxes, the transfer function of the pyrometer is integrated in the mathematical model. A sensitivity analysis is performed and reveals that the thermal diffusivity and the thermal conductivity of the sample can be simultaneously estimated, provided that the other model parameters are known. The solution of the inverse problem is obtained with algorithms within the Bayesian framework of statistics, by using a heat conduction reduced model with linear boundary conditions, for which an analytical solution is developed for fast computations. The Bayesian Approximation Error Model is used to compensate for errors between a complete heat conduction model with nonlinear boundary conditions and a reduced heat conduction model with linear boundary conditions. Moreover, errors related to the inaccuracy and the uncertainties of all model parameters are accounted for in the inverse analysis. Simulated transient flux measurements are assumed available from a multi-spectral pyrometer and are used for the inverse problem solution.