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A comparison circulating a standard infrared thermometer, a handy infrared thermometer, and a thermal imager was conducted among eleven participants from the industrial user, academic user, and calibration laboratory communities of these thermometers in Japan. The result revealed that the scatter width of the reported value for the handy type thermometer was as large as 31 °C at 450 °C. Investigation into the cause of the discrepancy showed that the treatment of emissivity of the source was one large factor, which is relevant when using contact thermometer or a near infrared radiation thermometer as the reference. Large SSE of the handy thermometer linked to the definition of the targeting distance of the thermometer on the blackbody cavity also played a major role. The comparison successfully identified the shortcomings of the current calibration practices, and the result can be utilized to greatly improve the reliability of the infrared thermometer scale in future.

In numerous applications, including current body temperature monitoring in viral pandemic management, thermal imaging cameras are used for quantitative measurements. These require determination of the measurement accuracy (error) and its traceability (measurement uncertainty). Within error estimation, the size-of-source effect (SSE) is an important error source. The SSE is the relation between the physical size of a target and the instrument’s nominal target size. This study presents a direct evaluation of the error due to the SSE. A stable and uniform temperature, generated by blackbodies, was measured by a high-quality thermal imager. To limit the generated radiation, custom-made blocking tiles with different apertures were used. Effects of aperture shapes and positions, camera-target distances and temperature levels on the error were investigated. The study findings suggest that due to the SSE the measured temperatures are too low, especially at longer camera-target distances. The SSE error depends on the number of pixels available and included into the region of interest, for which the accurate measurement is about to be performed. For an accurate temperature measurement, an array of at least 10 × 10 pixels should be exposed to the observed target radiation, while 3 × 3 central pixel area should be included in the temperature calculation.

A high-power light-emitting diode (HPLED) is proposed as a new light source for the measurement of the size-of-source effect (SSE) in radiation thermometers. The HPLED is a more compact, simple, and inexpensive light source and is suitable for SSE measurement by the scanning method. An experiment has been done using a 3 W HPLED with a hemispherical front lens and a peak emission wavelength of 660 nm. A linear pyrometer (Model LP4 manufactured by KE) was used for the measurements. The scanning process was carried out by using a computerized linear translation stage spanning 500 mm. To confirm the measurement result, the SSE of the LP4 was also measured by the conventional indirect method using an integrating sphere with a 65 mm diameter exit port. The same trends of the SSE value of the scanning and indirect method were found in all ranges of the indirect method measurement. The results show that the light source has enough brightness and a sufficiently wide angular distribution to provide a dynamic measurement range of up to 10 $$^{8}$$ and allow the measurement of the SSE up to a radius of 500 mm. These results support the application of HPLED as an alternative light source for SSE measurements by the scanning method. [ABSTRACT FROM AUTHOR]

New Asia Pacific Metrology Programme (APMP) comparisons of radiation thermometry standards, APMP TS-11, and -12, have recently been initiated. These new APMP comparisons cover the temperature range from $$156\,^{\circ }\hbox {C}$$ to $$2800\, ^{\circ }\hbox {C}$$ . Three radiation thermometers with central wavelengths of 1.6 $$\upmu \hbox {m}$$ , 0.9 $$\upmu \hbox {m}$$ , and 0.65 $$\upmu \hbox {m}$$ are the transfer devices for the radiation thermometer scale comparison conducted in the so-called star configuration. In parallel, a compact fixed-point blackbody furnace that houses six types of fixed-point cells of In, Sn, Zn, Al, Ag, and Cu is circulated, again in a star-type comparison, to substantiate fixed-point calibration capabilities. Twelve APMP national metrology institutes are taking part in this endeavor, in which the National Metrology Institute of Japan acts as the pilot. In this article, the comparison scheme is described with emphasis on the features of the transfer devices, i.e., the radiation thermometers and the fixed-point blackbodies. Results of preliminary evaluations of the performance and characteristic of these instruments as well as the evaluation method of the comparison results are presented. [ABSTRACT FROM AUTHOR]

Application of radiation thermometry in industrial scenes is rapidly increasing with the widespread use of low-cost infrared thermometers and thermal imagers. However, their performances are not always up to the users' expectations. This is often due to lack of appropriate information on the limitations of the instrument performance and of radiation thermometry itself. In this article, these limitations are disclosed, namely the targeting capabilities of the thermometers including the size-of-source effect of thermal imagers, reflection errors, and unknown emissivity of the measurement object. Attempts made at the NMIJ are introduced, which aim at alleviating the effect of these difficulties. Two-color radiation thermometers have been neglected from the traceability chain and from standardization efforts due to their technical complexity. Recent activities to incorporate them effectively in the calibration chain and to establish international standards are presented. Calibration of low-cost thermometers with a fixed instrumental emissivity setting has been an issue for calibration laboratories. Simple apparatus that enables calibration of such instruments is described. Methods to compensate for unknown emissivities are presented utilizing auxiliary sources to realize a blackbody condition, which is applied to thermal imagers to overcome the problem of the size-of-source effect and reflection error at the same time. Extensions of the technique to objects with specular and scattering surfaces are described. Such efforts are encouraged in the thermometry community since they are essential in establishing an unbroken chain of traceability to the industrial front. [ABSTRACT FROM AUTHOR]

When performing high accuracy radiation thermometry, the size-of-source effect (SSE) of a radiometer can provide a significant contribution to the uncertainties associated with the measurements. During the development of a new radiometer designed specifically to measure the melting points of high-temperature fixed-point cells, indirect SSE measurements were performed on a prototype instrument to aid selection of optical components and their optimum positions with the aim of minimizing its SSE. As the radiometer's objective lens can produce much of the scattered light that contributes to the SSE, a set of objective lenses was compared and found to have SSEs between $$7\times 10^{-4}$$ and $${<}6\times 10^{-5}$$ . Further improvements were found by controlling the positioning and size of the stray light reducing Lyot stop. The diameter of the Lyot stop had to be set carefully: too small a diameter and it provides a low SSE but reduces the instrument's signal from the source; too large a diameter and it provides little or no reduction in the SSE. The sensitivities in the Lyot stop and collimating lens positions were tested, and the instrument's SSE was found to be tolerant of small displacements of either the lens or Lyot stop, however, larger movements yielded an increase in the SSE. The extremes in position increased the SSE to $$3\times 10^{-4}$$ for the collimating lens and $$4\times 10^{-4}$$ for the Lyot stop. [ABSTRACT FROM AUTHOR]

Over the medium temperature range (from $$156\,^{\circ }\mathrm{C}$$ to $$1000\,^{\circ }\mathrm{C}$$ ), radiation thermometry is usually established within a national metrology institute (NMI) by means of variable temperature blackbody radiation sources, whose temperature is determined using a platinum resistance thermometer or thermocouple, calibrated in terms of the International Temperature Scale of 1990 (ITS-90), positioned in close proximity to the back radiating surface of the blackbody. It is also reasonably common to establish a scale using a suitable radiation thermometer, such as an indium gallium arsenide (InGaAs) detector-based narrow band radiation thermometer, calibrated using a number of fixed-point blackbody sources from the indium (In) to silver (Ag) (or copper (Cu)) points, with the calibration results fitted using a parameterized Planckian interpolation function. During 2007 and 2008, two InGaAs-based radiation thermometers were circulated around seven NMIs within the European Association of National Metrology Institutes (EURAMET) region in order to undertake a comparison of parameters necessary for radiation thermometry over the medium temperature range. Measurements were made of the size-of-source effect and gain (range) ratios of the two thermometers along with an assessment of the effect of changes in the ambient temperature and humidity on the thermometer output. The thermometers were also calibrated using fixed-point and/or variable temperature blackbody sources at each institute. A brief overview of the results obtained by this project is presented in this paper. [ABSTRACT FROM AUTHOR]

The size-of-source effect (SSE) is an important uncertainty source in radiation thermometry when radiation targets with different sizes are measured. In this article, a test pyrometer was established to study the way to reduce the SSE. The most dominant factors that influence the SSE, include the quality and surface condition of the objective lens, the placement and diameter of the aperture stop, and the inner baffles. Through an optimized lens and redesign of the aperture stop and baffles, the SSE of the test pyrometer is reduced to 2 × 10 when a 50 mm diameter radiance source with a 4 mm diameter central obscuration is used. The improvement method is applied to the primary standard pyrometer for which the SSE is reduced to less than 1 × 10. [ABSTRACT FROM AUTHOR]

The size-of-source effect (SSE) is a major contributor to error and uncertainty in both the calibration and use of a radiation thermometer, and correcting for the SSE is necessary to achieve high accuracy. There are several recognized methods for measuring the SSE; each measures a slightly different quantity and may depend on one of various parameters, such as the diameter of the black spot or the maximum aperture diameter. When applying corrections, it is necessary, in principle, to recognize which SSE quantity has been measured and under which conditions, as the correction formulae for each quantity differ and may also be functions of the measurement parameters. In this article, the formulae for each quantity are derived, and it is shown that the differences are only second order, so that for sufficiently small SSE, a single simple approximation can be made, independent of whether the SSE values are close to 1, as in the direct method, or close to 0, as in the indirect method. It is also shown that when measuring the radiance distribution surrounding the target in order to apply SSE corrections for the target, these surrounding measurements, which are also subject to the SSE, do not themselves require corrections, even for the highest level of target temperature accuracy. [ABSTRACT FROM AUTHOR]

The size-of-source effect (SSE) of radiation thermometers is generally determined using direct and indirect methods. The present paper describes the difference in the SSE obtained using the two methods. Three thermometers at different SSE levels, namely, 1.5 × 10, 1.05 × 10, and 2.44 × 10, are selected. SSEs with 5 mm to 96 mm aperture diameters are higher in the indirect than in the direct method. Three factors contribute to this difference: the light reflection from the inside face and edge of the aperture to the black spot, the relay reflections from the thermometer and background, and the decreased radiance caused by obstruction of light by a black spot. Thus, the difference in the SSEs results from the difference in the principle of the two methods and not from the SSE levels of the thermometers themselves. The difference is negligible to be considered in a number of radiation thermometers, but they must be taken into consideration in top-level intercomparisons. [ABSTRACT FROM AUTHOR]

In numerous applications, including current body temperature monitoring in viral pandemic management, thermal imaging cameras are used for quantitative measurements. These require determination of the measurement accuracy (error) and its traceability (measurement uncertainty). Within error estimation, the size-of-source effect (SSE) is an important error source. The SSE is the relation between the physical size of a target and the instrument's nominal target size. This study presents a direct evaluation of the error due to the SSE. A stable and uniform temperature, generated by blackbodies, was measured by a high-quality thermal imager. To limit the generated radiation, custom-made blocking tiles with different apertures were used. Effects of aperture shapes and positions, camera-target distances and temperature levels on the error were investigated. The study findings suggest that due to the SSE the measured temperatures are too low, especially at longer camera-target distances. The SSE error depends on the number of pixels available and included into the region of interest, for which the accurate measurement is about to be performed. For an accurate temperature measurement, an array of at least 10 × 10 pixels should be exposed to the observed target radiation, while 3 × 3 central pixel area should be included in the temperature calculation. [ABSTRACT FROM AUTHOR]