Size-of-source Effect Research Articles

Characterizing a linear pyrometer at the National Metrology Institute of South Africa

A linear pyrometer is used to realize the International Temperature Scale of 1990 (ITS-90) for temperature ranges above 961.78 °C in most National Metrology Institute (NMI) radiation thermometry labs. The different components of the linear pyrometer system working together to measure temperature may exhibit some errors which should be accounted for. The errors can be characterized by several equipment parameters that should be measured before the radiation thermometer is used to realize ITS-90 temperatures. Relative spectral responsivity (s), size of source effect (SSE), gain ratio (GR), non-linearity (NL), distance effect (DE), temperature coefficient and zero drift are all major equipment parameters that characterize the efficiency of a linear pyrometer in realizing ITS-90. In this work, an attempt was made to describe and demonstrate these major parameters by using actual measured characterization results of a linear pyrometer. Uncertainty contributions from some of the parameters considered in scale realizations was also demonstrated using the measurement results.

Measurement, Validation and Uncertainty of an Experimental Procedure to Characterize the Size-of-Source Effect of Radiation Thermometers, in the Framework of an Industrial Calibration Laboratory.

An experimental procedure for characterizing the size-of-source effect (SSE) is proposed. Such an effect is the cause of one of the main influence variables generating uncertainty in the measurement, both in calibration and use, of direct reading radiation thermometers (RT). The procedure and uncertainty calculation described in the paper are aligned in terms of metrological traceability, with the requirements generally imposed to ensure the accuracy of measurements in industry and science. Results of application and validation of this particular procedure with equipment, including black body (BB) sources normally used in radiation thermometry calibration laboratories in the industrial field, are shown.

Realisation of the ITS-90 and thermodynamic temperature measurements above 960 °C using a monochromator-based radiance comparator

At LNE-Cnam, the international temperature scale of 1990 (ITS-90) and thermodynamic temperature measurements above the silver point, are carried out with a radiance comparator. This instrument is, more generally, devoted to any radiance comparison in temperature range from the ambient to 3000 °C. The instrument developed in the early 1990s at LNE-Cnam has the advantage of being completely adjustable. Compared to compact radiation thermometers based on lenses and a narrow-band interference filter, the radiance comparator is only made of gold coated mirrors and a Czerny–Turner monochromator to select the spectral bandwidth. The instrument offers the possibility to tune the geometric extent and the slit scattering function. In return, the radiance comparator is a complex instrument that requires a complete and a regular characterisation at the highest level of accuracy. In the first part, this paper describes the instrument and its operating principle. In a second part, a complete study of the wavelength calibration, the slit scattering function, size of source effect, out-of-band transmittance, linearity and other main sources of uncertainty are presented and discussed. Their associated uncertainties are estimated separately and are grouped together to give an example of propagation of uncertainties when realising the ITS-90.

The size-of-source effect in thermography

Abstract. In thermometry, the displayed temperature value of an object depends on the size of the object. This behaviour, also known as the size-of-source effect (SSE), might be a major cause of measurement uncertainty in a thermoscene. The SSE is caused by diffraction, scattering, reflection, aberration and digitization in the optoelectronic propagation path. The influence of diffraction and digitization (sampling and pixelization) can be described advantageously with the modulation transfer function MTF. The system MTF of an uncooled camera is determined by the diffraction in the lens (optical MTF) and the averaging of the radiation over the pixel area (pixel MTF). If the system MTF is known, the contrast reduction and, thus, the SSE can be calculated. Especially with very small objects, e.g. hotspots creating an image covering less than 4 pixel × 4 pixel on the focal plane, the displayed temperatures are much too low. When imaging large objects, such as area blackbodies, not only the edge areas are affected, but also the entire image. Finally, the contrast reduction by the MTF is explained by means of a complex scene (Siemens star).

Evaluation of the Size-of-Source Effect in Thermal Imaging Cameras.

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.

Metrological characterization and calibration of thermographic cameras for quantitative temperature measurement

Abstract. We present the metrological characterization and calibration of three different types of thermographic cameras for quantitative temperature measurement traceable to the International Temperature Scale (ITS-90). Relevant technical specifications – i.e., the non-uniformity of the pixel-to-pixel responsivity, the inhomogeneity equivalent temperature difference (IETD), the noise equivalent temperature difference (NETD), and the size-of-source effect (SSE) – are determined according to the requirements given in the series of Technical Directives VDI/VDE 5585. The measurements are performed with the camera calibration facility of the Physikalisch-Technische Bundesanstalt. The data reference method is applied for the determination and improvement of the non-uniformity, leading to an improved IETD for all three cameras. Finally, the cameras are calibrated according to the different procedures discussed in the VDI/VDE 5585 series. Results achieved with the different calibration procedures are compared for each type of camera and among the three cameras. An uncertainty budget for the calibration of each camera is given according to GUM (ISO, 1995) and VDI/VDE 5585.

Quantitative traceable temperature measurement using novel thermal imaging camera.

Conventional thermal imaging cameras, based on focal-plane array (FPA) sensors, exhibit inherent problems: such as stray radiation, cross-talk and the calibration uncertainty of ensuring each pixel behaves as if it were an identical temperature sensor. Radiation thermometers can largely overcome these issues, comprising of only a single detector element that can be optimised and calibrated. Although the latter approach can provide excellent accuracy for single-point temperature measurement, it does not provide a temperature image of the target object. In this work, we present a micromechanical systems (MEMS) mirror and silicon (Si) avalanche photodiode (APD) based single-pixel camera, capable of producing quantitative thermal images at an operating wavelength of 1 µm. This work utilises a custom designed f-theta wide-angle lens and MEMS mirror, to scan +/- 30° in both x- and y-dimensions, without signal loss due to vignetting at any point in the field of view (FOV). Our single-pixel camera is shown to perform well, with 3 °C size-of-source effect (SSE) related temperature error and can measure below 700 °C whilst achieving ± 0.5 °C noise related measurement uncertainty. Our measurements were calibrated and traceable to the International Temperature Scale of 1990 (ITS-90). The combination of low SSE and absence of vignetting enables quantitative temperature measurements over a spatial field with measurement uncertainty at levels lower than would be possible with FPA based thermal imaging cameras.

Size-of-source (SSE) effect of broadband and interference filters for Double Wavelength Thermometers

In this work we present a comparative study of an impact of several optical windows (the SCHOTT glass filters (KG1 to KG5) with a broadband transmittance in the visible spectral range) and an interference filter (with a peak transmittance at 650 nm) on the size-of-source effect (SSE) of a radiation thermometer. The investigations were performed in the scope of the development of a Double Wavelength Radiation (DWT) thermometer at UME. There are two radiometric channels in the DWT, where one of the channels will comprise of a broadband transmittance filter, while the other - an interference filter. In order to exclude the optical path effects on the SSE measurements the investigations were performed on a single channel radiation thermometer. The SSE measurements were performed by means of the indirect method. The SSE measurement results are illustrated and discussed.

Quantitative thermal imaging using single-pixel Si APD and MEMS mirror.

Accurate quantitative temperature measurements are difficult to achieve using focal-plane array sensors. This is due to reflections inside the instrument and the difficulty of calibrating a matrix of pixels as identical radiation thermometers. Size-of-source effect (SSE), which is the dependence of an infrared temperature measurement on the area surrounding the target area, is a major contributor to this problem and cannot be reduced using glare stops. Measurements are affected by power received from outside the field-of-view (FOV), leading to increased measurement uncertainty. In this work, we present a micromechanical systems (MEMS) mirror based scanning thermal imaging camera with reduced measurement uncertainty compared to focal-plane array based systems. We demonstrate our flexible imaging approach using a Si avalanche photodiode (APD), which utilises high internal gain to enable the measurement of lower target temperatures with an effective wavelength of 1 µm and compare results with a Si photodiode. We compare measurements from our APD thermal imaging instrument against a commercial bolometer based focal-plane array camera. Our scanning approach results in a reduction in SSE related temperature error by 66 °C for the measurement of a spatially uniform 800 °C target when the target aperture diameter is increased from 10 to 20 mm. We also find that our APD instrument is capable of measuring target temperatures below 700 °C, over these near infrared wavelengths, with D* related measurement uncertainty of ± 0.5 °C.

Size-of-source Effect in Infrared Thermometers with Direct Reading of Temperature

The size-of-source effect (SSE) for six infrared (IR) thermometers with direct reading of temperature was measured in this work. The alternative direct method for SSE determination, where the aperture size is fixed and the measurement distance is varied, was used in this study. The experimental equivalence between the usual and the alternative direct methods is presented. The magnitudes of the SSE for different types of IR thermometers were investigated. The maxima of the SSE were found to be up to 5 %, 8 %, and 28 % for focusable, closed-focus, and open-focus thermometers, respectively. At $$275\,^{\circ }\hbox {C}$$ , an SSE of 28 % corresponds to $$52\,^{\circ }\hbox {C}$$ , indicating the severe effect on the accuracy of this type of IR thermometer. A method to realize the calibration conditions used by the manufacturer, in terms of aperture size and measurement distance, is discussed and validated by experimental results. This study would be of benefit to users in choosing the best IR thermometer to match their work and for calibration laboratories in selecting the technique most suitable for determining the SSE.

Light Source for Scanning Method of Size-of-Source Effect Measurement

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.

Size-of-Source Effect Sensitivities in Radiometers

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.

Determining size-of-source effect using the scanning slit method

Recently a scanning method for determining the size-of-source effect (SSE) in radiation thermometry was proposed. It was shown that by scanning the radiation thermometer (RT) across a narrow slit with a slit width similar to the spot size of the RT, results comparable to the indirect method can be obtained. However, due to the low signal levels (almost at the noise level) detected by the RT, the method is susceptible to noise. We examine further this scanning method by determining the influence of the relative position of the slit, and slit size on the resulting SSE. We also present a simple model for the radiation propagation relevant to this method. Lastly, we compare the results of the scanning experiments to the conventional direct method for SSE and show that the scanning method is also comparable to the direct method, as long as the parameters of the scanning method (e.g., slit position, slit size) are adjusted properly.

Realization of a radiation temperature scale from 0 °C to 232 °C by a thermal infrared thermometer based on a multiple-fixed-point technique

The radiation temperature scale for a pyroelectric detector based thermal infrared thermometer with its spectral response from 8 µm to 14 µm was realized in the temperature range from 0 °C to 232 °C by using four fixed-point blackbodies (ice, Ga, In and Sn). The Planck version of the Sakuma–Hattori equation was used to interpolate the scale between the fixed-point temperatures that are corrected by considering a size-of-source effect (SSE). The expanded uncertainties (k = 2) of the scale were estimated to be 108 mK for ice, 99 mK for Ga, 175 mK for In and 234 mK for Sn.

Study on Reducing the Size-of-Source Effect of Pyrometers

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 × 10−4 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 × 10−4.

Size-of-Source Effect Difference Between Direct and Indirect Methods of Radiation Thermometers

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 × 10−4, 1.05 × 10−3, and 2.44 × 10−3, 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.

Correcting Radiation Thermometry Measurements for the Size-of-Source Effect

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.

Improvement of melting and freezing curves for fixed-point calibration of a transfer reference thermometer by size-of-source-effect correction

The size-of-source effect (SSE) of a radiation thermometer is a critical uncertainty component in the fixed-point blackbody calibration of a transfer reference thermometer in the low-temperature range where the field of view is comparable to the size of the blackbody cavity opening. The SSE causes the signal difference between the melting and freezing plateaus to increase, and steepens the slopes of the plateaus. We propose a new method that can recover the signals of the melting and freezing curves that are deteriorated by the SSE, and thereby flatten the plateaus. The improvement has been successfully demonstrated using a tin-point blackbody. The signal difference between the melting and freezing plateaus was reduced from 81 mK to 18 mK, and the freezing-point signal was corrected by 1.30 K. In addition, the slopes on the plateaus have been dramatically flattened.

적외선 복사온도계의 복사원 크기효과 및 자기복사효과

All radiation thermometers have a size-of-source effect (SSE) and a self-radiation effect (SRE). The SSE,defined as dependence of the detector signal of a radiation thermometer on the diameter of a source, is critically dependent on the wavelength since diffraction is the main cause. In this paper, we have measured the SSE and the SRE of TRT2 (Transfer Radiation Thermometer 2, HEITRONICS) widely used as a transfer standard in low and middle temperature range. At , The radiation temperature difference between the 60 mm diameter blackbody and 10 mm diameter blackbody due to the SSE was estimated to be in low temperature mode () and in middle temperature mode (). In addition, the measured radiation temperature difference of the blackbody due to the SRE was found to be 110 mK when the body temperature change of TRT2 was set at .

Analytical representations of the size-of-source effect

In this paper it is shown that the size-of-source effect (SSE) of a radiation thermometer measured for a range of source diameters Di, can be fitted by means of empirical analytical approximations F(D), selected on the basis of quite general arguments. This is necessary to analytically process SSE data and to rigorously quantify the uncertainties associated with the correction for the SSE. To test the proposed fitting scheme 35 size-of-source curves, measured by five institutes for different radiation thermometers, have been analysed. In a companion paper the uncertainty in the correction for the SSE will be elaborated in detail for two examples.