Blackbody Furnace Research Articles

Direct observation of plateau progression in fixed-point blackbodies via X-ray imaging

In this study, we address the challenge of calibrating radiation thermometers using blackbody furnaces that employ a metal melting/freezing plateau. Here, we first present the observation results utilizing X-rays, enabling us to monitor the entire temperature cycle, from heating to cooling, during the calibration process. Through this approach, we directly observe the emergence of voids within deteriorating metal cells, which may give insights into the underlying mechanisms occurring within enclosed graphite cells.

Multispectral Thermometry Method Based on Optimisation Ideas.

Multispectral thermometry is based on the law of blackbody radiation and is widely used in engineering practice today. Temperature values can be inferred from radiation intensity and multiple sets of wavelengths. Multispectral thermometry eliminates the requirements for single-spectral and spectral similarity, which are associated with two-colour thermometry. In the process of multispectral temperature inversion, the solution of spectral emissivity and multispectral data processing can be seen as the keys to accurate thermometry. At present, spectral emissivity is most commonly estimated using assumption models. When an assumption model closely matches an actual situation, the inversion of the temperature and the accuracy of spectral emissivity are both very high; however, when the two are not closely matched, the inversion result is very different from the actual situation. Assumption models of spectral emissivity exhibit drawbacks when used for thermometry of a complex material, or any material whose properties dynamically change during a combustion process. To address the above problems, in the present study, we developed a multispectral thermometry method based on optimisation ideas. This method involves analysing connections between measured temperatures of each channel in a multispectral temperature inversion process; it also makes use of correlations between multispectral signals at different temperatures. In short, we established a multivariate temperature difference correlation function based on the principles of multispectral radiometric thermometry, using information correlations between data for each channel in a temperature inversion process. We then established a high-precision thermometry model by optimising the correlation function and correcting any measurement errors. This method simplifies the modelling process so that it becomes an optimisation problem of the temperature difference function. This also removes the need to assume the relationships between spectral emissivity and other physical quantities, simplifying the process of multispectral thermometry. Finally, this involves correction of the spectral data so that any impact of measurement error on the thermometry is reduced. In order to verify the feasibility and reliability of the method, a simple eight-channel multispectral thermometry device was used for experimental validation, in which the temperature emitted from a blackbody furnace was identified as the standard value. In addition, spectral data from the 468-603 nm band were calibrated within a temperature range of 1923.15-2273.15 K, resulting in multispectral thermometry based on optimisation principles with an error rate of around 0.3% and a temperature calculation time of less than 3 s. The achieved level of inversion accuracy was better than that obtained using either a secondary measurement method (SMM) or a neural network method, and the calculation speed achieved was considerably faster than that obtained using the SMM method.

Adaptive Compensation Method for the Infrared Temperature Measurement Error Based on 3-D Thermal Imaging

Infrared thermal imager can provide two-dimensional temperature information of the measured object on-line without contact, which is one of the most commonly used temperature measurement methods. However, the measuring distance and viewing angle between the thermal imager and the measured object will cause the infrared temperature measurement error. In order to automatically compensate for the temperature measurement error caused by measuring distance and viewing angle when the infrared thermal imager is applied in actual measurement scenarios, this paper proposes an adaptive compensation method for the infrared temperature measurement error based on 3D thermal imaging. Firstly, based on the polynomial regression and infrared temperature measurement principle, a compensation model is established to compensate for the temperature measurement error caused by measuring distance and viewing angle. Secondly, the parameters of the compensation model are determined by using a blackbody furnace and a reference cylinder. Then, a 3D thermal imaging system is constructed with a depth camera and an infrared thermal imager to quantify the measuring distances and viewing angles between all temperature pixels and the thermal imager. Finally, the surface temperature of the measured object can be adaptively compensated using the established compensation model. The experimental results of the blackbody furnace and resin cylinder show that the proposed method can adaptively compensate for the infrared temperature measurement error caused by measuring distance and viewing angle.

An experimental study on the thermal radiation attenuation effect of water mist from flat fan nozzle

The factors which influence the radiation attenuation efficiency of the water mist from flat fan nozzle have been investigated in the study. The characteristic parameters of the water mist were obtained by laser-based diagnostic system for flow field. Both blackbody furnace and gasoline pool fires were used as heat sources in the experiments. The influences of system pressure, the vertical distance from the nozzle, the temperature of the radiation source and the number of the mist layers on the radiation attenuation effectiveness have been systematically studied. The experimental results showed that when the vertical distance down the nozzle increased from 20 cm to 200 cm, the radiation attenuation efficiency increased at first, then remained almost stable for a substantial distance and decreased afterwards, and three vertical zones were identified based on the attenuation characteristics; as the temperature of the blackbody furnace radiation aperture increased from 1000 ℃ to 1300 ℃, the radiation attenuation efficiency had a certain decrease, a linear correlation was found between the mitigation rate and the radiative source temperature under different system pressures. The attenuation effectiveness improved with the increase of both system pressure and the number (thickness) of water mist layer, while the augmentation rate in attenuation slowed as the system pressure and the water mist thickness increased. The findings provide useful insights into the optimization of attenuation performance of water mist and potentially contribute to the protection against thermal radiation in fields like the fluorine chemical industry.

Ratio pyrometry of emulated firebrand streaks

Firebrands can dramatically increase the spread rates of large outdoor fires. Although much is known about other aspects of firebrand behavior, their temperatures are poorly understood. In particular, the temperatures of burning airborne firebrands have never been measured. In a novel configuration, airborne firebrand motion is emulated here by rotating a camera while imaging a tethered maple ember as a streaking ember. Ratio pyrometry is performed using a color digital camera calibrated with a blackbody furnace at 600–1200 °C. Over 1600 images of 55 stationary and streaking embers were recorded and analyzed. The same ember temperatures were obtained for stationary embers as for streaking embers with either a horizontal or a vertical axis of camera rotation. The mean ember temperatures were 918 ad 955 °C for 1 and 2 m/s wind velocities. These were independent of ember distance and velocity, but generally increased with time after ignition at a mean rate of 0.77 °C/s. The diagnostic has an estimated measurement uncertainty of ±20 °C and similar accuracy for stationary and streaking embers.

Design and evaluation of a light-field multi-wavelength pyrometer.

This letter describes the design and implementation of a multi-wavelength light-field pyrometer, where six-channel radiation images were captured with one CMOS sensor. Such capability is achieved by placing a 2 × 3 filter array in front of the main lens of an unfocused light-field camera, such that discrete wavelength and radiation intensity can be simultaneously recorded. It demonstrates that through black-body furnace experiments, how multi-channel radiation images can be extracted from one raw light-field multispectral image, and how accurate 2D temperature distribution can be recovered by optimization algorithms.

Online in-situ monitoring of melt pool characteristic based on a single high-speed camera in laser powder bed fusion process

• A coaxial melt pool temperature measurement system with a single high-speed camera in the LPBF process is developed. • A dual-waveband image-matching method with sub-pixel accuracy, and an overall parameter calibration and optimization method are proposed. • The melt pool characteristics including the temperature distribution, profile, temperature gradient, and cooling rate are measured by the developed system, and the distribution of average temperature and peak temperature under different linear energy densities during single-line printing is compared and analyzed. In metal additive manufacturing (AM), the in-situ measurement of the melt pool characteristic plays a significant role in monitoring the quality of the printed components. In this work, based on dual-wavelength thermometry, a coaxial melt pool temperature measurement system with a single high-speed camera in the laser powder bed fusion (LPBF) process is developed, including the design of the relay and optical path amplification system, and the beam splitting and chromatic aberration correction system. Moreover, a dual-waveband image-matching method with sub-pixel accuracy, and an overall parameter calibration and optimization method are proposed to improve the accuracy of the coaxial temperature measurement system. Besides, the validation experiment measured by a high-temperature blackbody furnace and a standard photoelectric pyrometer indicates that the temperature measuring error of the developed system is less than 1%. The melt pool characteristics including the temperature distribution, profile, temperature gradient, and cooling rate were measured by the developed coaxial temperature measurement system, and the distribution of average temperature and peak temperature under different linear energy densities during single-line printing was also compared and analyzed. The single-line printing results of different parameters show that the higher the linear energy density, the higher the average temperature and peak temperature of the melt pool, and the optimized parameters minimize the fluctuation of melt pool temperature and are more favorable to the formation of high-quality parts. In multi-layer printing mode, the heat accumulation is strong, resulting in the slow cooling rate of the melt pool.

Research on the normal emissivity of graphite between 150 and 500 °C by an infrared camera for nuclear fusion devices

• An emissivity measurement equipment by using an infrared camera is constructed and calibrated by a blackbody furnace with a suitable emissivity model. • By using an infrared camera with no filter (band wavelength range of 7.5 μm–13 μm), a 10.2 ± 0.1 μm filter, and a 10.8 ± 0.19 μm filter, the relationship between the temperature and emissivity of graphite and the residual between data and model are obtained respectively and compared. • The emissivity of graphite at different roughness is studied, and uncertainty quantification is used to provide an error estimate for the resulting curves. Due to the advantages of high-temperature resistance, low thermal expansion coefficient, and thermal shock resistance, etc., graphite is regarded as one of the materials for the first wall of nuclear fusion devices. The real-time monitoring of temperature distribution of the first wall based on an infrared camera is widely used to guarantee the stable operation of devices. To accurately measure the temperature of the first wall, it is important to measure the normal emissivity of graphite. However, the emissivity of graphite is affected by many factors. For example, the effect of the surface layers, like carbon-hydro layers-layers, will influence the measurement of the emissivity. The ambition of this work is to study the relationship between graphite emissivity and infrared camera with filters of different wavelengths and the roughness of graphite, and then give the proposed emissivity model to improve the accuracy of graphite surface temperature measurement. The experiment is carried out with a FLIR infrared camera and calibrated by using a blackbody furnace. The emissivity at given temperatures is systematically investigated. By using an infrared camera with no filter (band wavelength range of 7.5–13 μm), a 10.2 ± 0.1 μm filter, and a 10.8 ± 0.19 μm filter, this paper studies the relationship between the temperature and emissivity of graphite from 150 °C to 500 °C and calculates the relative error of temperature measurement respectively. A suitable fitting model which fits emissivity curves is selected. The measurement results show the use of a 10.2 μm filter to measure the graphite surface temperature has the smallest relative error, the range is 0.90%∼1.17%. The emissivity of graphite with different roughness is also studied. At the same temperature, the greater the roughness of the graphite surface, the higher the emissivity value. The uncertainty of the emissivity measurement is analyzed, and its value is 0.0099–0.0211. The graphite surface temperature measurement method based on the emissivity model proposed in this paper provides useful references for the non-contact temperature measurement of the first wall in nuclear fusion devices. The research results can be used as a reference for plasma experiments in some fusion devices and other similar applications.

Infrared Thermography with High Accuracy in a Neonatal Incubator

As the accuracy of body temperature measurement is especially critical in premature infants on admission to the neonatal intensive care unit (NICU), noninvasive measurement using infrared thermography (IRT) has not been widely adopted in the NICU due to a lack of evidence regarding its accuracy. We have established a new calibration method for IRT in an incubator, and evaluated its accuracy and reliability at different incubator settings using a variable-temperature blackbody furnace. This method improved the accuracy and reliability of IRT with an increase in percentage of data with mean absolute error (MAE) < 0.3 °C to 93.1% compared to 4.2% using the standard method. Two of three IRTs had MAE < 0.1 °C under all conditions examined. This method provided high accuracy not only for measurements at specific times but also for continuous monitoring. It will also contribute to avoiding the risk of neonates' skin trouble caused by attaching a thermistor. This study will facilitate the development of novel means of administering neonatal body temperature.

Measuring the spectral emissivity of single particle at high temperature

It has a very broad prospect to use solid particles as the working medium in the solar collector of concentrated solar power generation system. The emissivity of a particle has an undeniable effect on the efficiency of a solid particle receiver. However, the spectral emissivity of a single particle at high temperature is rarely reported. In this work, an experimental method for measuring the spectral emissivity of a single particle at high temperature is established. This method is based on a self-designed heater, Fourier transform infrared spectrometer (FTIR) and a blackbody furnace. It can measure the spectral emissivity of a single particle at high-temperature of 500-900°C, and the test wavelength range is 2.5–25 μm. By measuring the spectral emissivity of 3–5 mm diameter particles, the influence of particle size on the emissivity was obtained. The expanded uncertainty of spectral emissivity of 3 mm SiC particles at 800°C is 1.380%.

A Novel Accuracy Validation Method of Surface Temperature Measurement by the ReFaST Pyrometer

Accuracy validation of surface temperature by optical pyrometry is always a hard task. The measurement results of pyrometers have been compared with a thermocouple which was welding or embedding on/in the tested sample in the past. However, the measurement results of the thermocouple could hardly represent the surface temperature, because thermal equilibrium is difficult to achieve between the thermocouple and tested sample when the surface temperature above 400 °C. In our previous research, the ReFaST pyrometer has been proposed, which measure the surface temperature and emissivity simultaneously. The accuracy validation of the ReFaST pyrometer is compared with a flat-type thermocouple which is fixed on the tested sample. However, the verification method by the flat-type thermocouple still has several shortcomings and need to be improved. In this paper, a novel accuracy validation method is proposed. By inserting the ReFaST pyrometer into a blackbody furnace, the multiple reflectance is formed between the reflector and the tested surface, so the tested sample and the thermocouple always maintain thermal equilibrium during the measurement. Therefore, the measurement results of the thermocouple can represent the surface temperature accurately and the uncertainty of accuracy verification is decreased. We develop the apparatus and carry out experiments. Compared with flat-type thermocouple, the uncertainty of accuracy verification is reduced by 81 % at 750 °C. The temperature difference between the ReFaST pyrometer and thermocouple is 1.76 °C on average ranging from 700 °C to 1000 °C, and the standard uncertainty is 0.69 °C. Besides, the accuracy verification method proposed in this paper is suitable for any type of pyrometer, as long as the reflector with a tube is in front of the pyrometer.

Emissivity measurement of tungsten based on a double-wavelength infrared thermometer between 140 and 500℃

At present, most Tokamak devices adopt non-contact temperature measurements for the first wall using an infrared thermometer. Tungsten is regarded as the most likely plasma-facing material, especially as the divertor target, in future Tokamka devices. Accordingly, it is important to obtain accurate emissivity of tungsten for accurate temperature measurement of the divertor target plate and ensuring steady operation of Tokamak devices. In this work, a set of emissivity measurement equipment with a self-designed double-wavelength colorimetric infrared thermometer is established, and a method of emissivity measurement based on double-wavelength colorimetry considering background radiation is proposed. The normal spectral emissivity of tungsten with wavelengths of 10.2 μm and 10.8 μm between 140 and 500℃ in the same working environment as the Tokamak devices is measured after calibrating the optical splitter and the infrared thermometer by using a blackbody furnace. The measurement results indicate that the normal spectral emissivity of tungsten ranges from 0.0936 to 0.1246 between 140 and 500℃ in a vacuum environment. The emissivity increases with the increase of temperature, and the relative increase rate is 33.12%. Finally, the uncertainty of the emissivity measurement results is calculated, which ranges from 0.0444 to 0.0767. The normal spectral emissivity of tungsten measured based on the double-wavelength colorimetric principle proposed in this paper provides a good reference for the accurate temperature measurement of the divertor target plate in Tokamak devices. It also provides a reference for the measurement of metal emissivity with background radiation under similar vacuum conditions.

Calibration and measurement performance analysis for a spectral band charge-coupled-device-based pyrometer.

Commercial charge-coupled device (CCD) cameras are used widely in industrial applications. This paper uses a commercial CCD camera to develop a spectral band CCD-based pyrometer. To improve the calibration accuracy, a thermometry model based on the extended effective wavelength is introduced to describe the pyrometer. The Tikhonov regularization method is employed to acquire the stabilized solutions for the model parameters considering their sensitivities to the perturbations in the calibration data. Therefore, the relationships between the temperature measurement performances and the system parameters, namely, the electron gain and F-number, are analyzed in terms of the temperature measurement range and sensitivity. Meanwhile, the error and uniformity of the temperature measurements are also investigated using a blackbody furnace. The experimental results show that the temperature measurement range for the designed pyrometer is 800-1203 °C, in which the sensitivity is 0.4906-35.64 °C-1 and the average relative error and non-uniformity of the temperature measurements are 2.5‰ and 1.36%, respectively.

Temperature Measurement Method of Flame Image Fusion with Different Exposures

Fixed exposure will lead to underexposure or overexposure of collected flame radiation images using CCD, which has a great influence on the temperature measuring accuracy. A temperature measurement method was proposed by image fusion with multi-exposure, which can eliminate the influence of insufficient underexposure and overexposure. The approach was first to acquire a group of flame radiation images during different exposures. Then a partial region with good exposure effect in each radiation image was obtained by segmentation, with which the complete flame image can be spliced together. An experimental system was built to calibrate the temperature measurement parameters by two-color pyrometry through a blackbody furnace. The relation between exposure time and monochromatic gray level, as well as the relation between the temperature and temperature measurement coefficient were obtained. A candle flame was selected as the measuring object and the complete and accurate flame temperature distribution was acquired following our proposed method. The experimental results show that, compared with the temperature measurement using a single exposure time, our method can effectively avoid the measurement error caused by underexposure and overexposure, and improve the measurement accuracy.

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.

An experimental method for improving temperature measurement accuracy of infrared thermal imager

In order to improve the temperature measurement accuracy of infrared thermal imager during practical applications, an experimental system for measuring the temperature of blackbody furnace is designed in this paper. This experimental system is designed on the basis of infrared radiation theory and temperature measurement principle of thermal imager. Several contrast experiments of temperature measurement on blackbody furnace with different measuring distances, different measuring angles and different working time are completed through this experimental system. After analyzing the experimental results, the influences of the measuring distance, measuring angle and working time on the accuracy of thermal imager are found. Furthermore, the correction formulas of temperature measurement errors are obtained. The new measured temperature data are compensated by the correction formulas. The compensation results show that the temperature measurement accuracy of the thermal imager is improved effectively. The feasibility of the experimental method proposed in this paper is verified.

Fire ember pyrometry using a color camera

Firebrands can dramatically increase the hazards of wildland fires. While embers have been extensively studied, little is known about their temperatures. To address this an imaging ember pyrometer is developed here using an inexpensive digital color camera. The camera response was calibrated with a blackbody furnace at 600–1200 °C. The embers were smoldering cylindrical maple rods, 6.4 mm in diameter and 2 cm long. Temperatures were obtained from ratios of green/red pixel values and from grayscale pixel values. Ratio pyrometry is more accurate when ember emissivity times ash transmittance is below unity, but grayscale pyrometry has signal-to-noise ratios 18 times as high. Thus a hybrid pyrometer was developed that has the advantages of both, providing a spatial resolution of 17 μm, a signal-to-noise ratio of 530, and an estimated uncertainty of ±20 °C. The measured ember temperatures were between 750 and 1070 °C with a mean of 930 °C. Comparing the ratio and grayscale temperatures indicates the ember's mean emissivity times ash transmittance in the visible was 0.73. Temperatures were also measured with fine bare-wire thermocouples, which were found to underpredict the ember's mean temperature by 230 °C owing to ember quenching and imperfect thermal contact.

Study of an improved two-colour method integrated with the emissivity ratio model and its application to air- and oxy-fuel flames in industrial furnaces

Inspired by the multi-wavelength method, the present work innovatively introduces an easier-to-describe emissivity ratio model to replace the customary grey body assumption, thus polishing the algorithm of the traditional two-colour method. The precision of this method was verified with a blackbody furnace, for which the relative errors were within ±1.50%. In situ experiments show that the air-coal flame in a stokerfeed boiler is significantly close to a grey body over the detected wavelength range of 600–1000 nm, except for the emission/absorption lines, and a nearly negligible disparity (25.58 K) is found between the temperature outcome of the improved method and that of the traditional method. The oxy-coke flame in a pilot-scale test facility, however, differs from a grey body to a certain degree within that range, resulting in the deviation of the temperature obtained by the traditional method from that obtained by the improved method by more than 190 K. Obviously, the traditional method is no longer valid in this case. Thus, the two-colour method is sensitive to emissivity variations of the target, and the grey body assumption incurs distinct restrictions on application of the traditional method, thereby highlighting the advantages of the improved method.

Measurement and theoretical prediction on the surface emittance of carbonized layer of silica/phenolic ablative material

As a typical fiber reinforced resin-based ablative material, silica/phenolic composite has important applications in the thermal protection of high-speed aircrafts. At high temperature, it can form a carbonized layer, whose surface emittance is directly related to its capacity of radiative heat dissipation. In this work, carbonization experiments of the silica/phenolic composite and pure phenolic resin were designed to obtain their corresponding carbonized samples. The spectral emittances in the wavelength range from 2.5 to 25 μm of the carbonized samples were measured by a Fourier transform infrared spectroscopy with a blackbody furnace and a sample heater at temperatures from 200 to 500 °C. The results showed that the spectral emittance of the carbonized ablative material is close to that of the carbonized phenolic resin. The spectral emittances of the two carbonized samples increase with temperature and almost do not change with wavelength, indicating that the carbonized samples have the nature of a grey body. A prediction model of the total emittance was constructed based on the grey body assumption and a geometric model extracted from structure characterization of the carbonized ablative material. The predicted total emittances and those obtained from the integration of the measured spectral results are all higher than 0.8, and the deviations are below 10%. Theoretical predictions of the model show that the total emittance of the carbonized ablative material increases with the increasing porosity (ϕ) and total emittance (εs) of the fiber bundle covered with carbonized phenolic resin. Both the experimental and theoretical results reveal that the high total emittance of the carbonized silica/phenolic ablative material can be mainly attributed to the high emittance of the carbonized phenolic resin.

Investigation of the normal spectral band emissivity characteristic within 7.5 to 13 μm for Molybdenum between 100 and 500 °C

Molybdenum (Mo) is an important material to construct the first wall for the Experimental Advanced Superconducting Tokamak (EAST). The real-time monitoring of temperature distribution of the first wall based on radiation thermometry is essential to guarantee the stable operation of EAST. So, it is especially important for the acquisition of emissivity property of Mo. In this work, a self-designed emissivity measurement apparatus is developed, and the relationship between the normal infrared spectral band (7.5–13 μm) emissivity of Mo against the temperature and surface roughness of material samples is experimentally investigated under the vacuum condition over the temperature ranging from 100 °C to 500 °C. Moreover, the dependence of spectral band emissivity of Mo exposed to air on the heating-duration time at a given elevated temperature is also studied. The emissivity measurement apparatus is mainly composed of a high temperature furnace and a radiation thermometer as well as a benchmark blackbody furnace. The radiation thermometer is firstly calibrated against the blackbody furnace by means of the multi-temperature methods. And then the temperature of the sample is simultaneously measured by the two highly accurate S-type thermocouples and the radiation thermometer. Finally the emissivity value of the sample is calculated based on the direct radiometric method. The developed emissivity measurement method and experimental results obtained in this work may be helpful to understand the work state the EAST and to use of Mo as an emissivity reference.