Size-of-source Effect Research Articles

On the characterization and correction of the size-of-source effect in radiation thermometers

A new high-resolution automatic scanning method is developed for measuring the size-of-source effect (SSE) of radiation thermometers. The method has several advantages over the traditional direct and indirect methods for measuring the SSE, and is much easier to implement. A mathematical model of the SSE is developed and is used to relate the new method to the direct and indirect methods, demonstrating that all three are equivalent. Experimental results are presented that confirm the equivalence of these methods to a high degree of accuracy. Equations are also given for correcting the SSE for measurements on extended non-uniform sources.

Optical Diffraction Corrections in Radiometric Thermodynamic Temperature Determination

One of the main components of uncertainty in high-temperature thermometry arises because of the size-of-source effect (SSE). This effect makes the temperature measurement sensitive to the geometry of the radiating environment. It is caused by optical diffraction and especially by light scattering off/from, and inter-reflections between, optical components inside the pyrometer. The LNE-INM/CNAM is involved in extending the thermometry temperature scale to very high temperatures (T > 2000 °C) and has developed eutectic-based fixed points (Sadli et al. (in: Zvizdic (ed.) Proceedings of TEMPMEKO 2004, 9th International Symposium on Temperature and Thermal Measurements in Industry and Science, 2004)) and a thermodynamic temperature measurement capability based on absolute radiometric methods (Briaudeau et al. (in: D. Zvizdic (ed.) Proceedings of TEMPMEKO 2004, 9th International Symposium on Temperature and Thermal Measurements in Industry and Science 2004)). A new measurement technique that uses an optical fiber has been developed and tested, allowing the determination of the SSE at any defocusing plane, with high resolution. A model based on optical diffraction has been developed to simulate the SSE in a real situation, considering the contribution to the pyrometer signal of the whole “3D” optical scene inside the blackbody furnace. Using the same approach, it has been demonstrated that optical scattering in a simple radiance meter can be estimated from accurate optical diffraction measurement.

A High-Emissivity Blackbody with Large Aperture for Radiometric Calibration at Low-Temperature

A newly designed high-emissivity cylindrical blackbody source with a large diameter aperture (54 mm), an internal triangular-grooved surface, and concentric grooves on the bottom surface was immersed in a temperature-controlled, stirred-liquid bath. The stirred-liquid bath can be stabilized to better than 0.05°C at temperatures between 30 °C and 70 °C, with traceability to the ITS-90 through a platinum resistance thermometer (PRT) calibrated at the fixed points of indium, gallium, and the water triple point. The temperature uniformity of the blackbody from the bottom to the front of the cavity is better than 0.05 % of the operating temperature (in °C). The heat loss of the cavity is less than 0.03 % of the operating temperature as determined with a radiation thermometer by removing an insulating lid without the gas purge operating. Optical ray tracing with a Monte Carlo method (STEEP 3) indicated that the effective emissivity of this blackbody cavity is very close to unity. The size-of-source effect (SSE) of the radiation thermometer and the effective emissivity of the blackbody were considered in evaluating the uncertainty of the blackbody. The blackbody uncertainty budget and performance are described in this paper.

Development of a Multiband near-to-far Infrared Radiance COmparator (MIRCO)

In recent years, there has been a growing demand to calibrate industrial blackbodies both at short wavelengths for lower temperatures and at long wavelengths for higher temperatures. User requests cover a very wide temperature range, from −20°C to 1,500°C in the infrared bands used by thermal cameras or as defined by specific applications (especially the 1–3 μm, 3–5 μm, and 8–12 μm bands). Therefore, LNE (Laboratoire National de Metrologie et d’Essais) has developed a radiance comparator with a mirror-based optical system, an associated set of interference filter wheels, a modular holder for several infrared detectors, and a lock-in amplifier. This setup is designed to be very versatile in terms of wavelength and temperature. Targeted performances have a thermal resolution better than 0.05°C, and a known and controlled size-of-source effect (SSE). A silicon detector and a visible-to-near infrared integrating sphere were used to assess the stray light inside the housing, and supplementary baffles and stops were used to reduce it to an acceptable level. The investigation included measurement of the SSE for this comparator layout. Then, the performance in the 3–5 μm and 8–12 μm bands, using, respectively, indium antimonide (InSb) and mercury cadmium telluride (MCT) detectors, was evaluated using a water heat-pipe blackbody. This paper describes the modeling and the technical solutions implemented to optimize the optical system. Preliminary results are presented for the short-term stability, the thermal resolution between −20°C and 960°C, and also the SSE up to 60 mm in these bands.

An Alternative Calculation of the Size-of-Source Effect Uncertainty for Non-uniform Sources

For a correct determination of the size-of-source effect (SSE), it is necessary to use a large (e.g., 50-mm diameter) radiation source of uniform spectral radiance. The non-uniformity of the source can be an important component of the uncertainty because it is difficult to obtain sources with good uniformity over the whole aperture. In the indirect method to measure the SSE, the component due to the source non-uniformity comes from the measurement of S(L,∞), usually measured outside the spot that blocks radiation from the radiation thermometer target. By expressing the SSE as an experimental sum of successive contributions of radiating rings at the source aperture, the uncertainty due to the non-uniformity can be reduced. In this article, the calculus of the SSE and its uncertainty for the usual equation is compared with the calculus based on an experimental equation that considers the SSE as a sum of the contributions from radiating rings.

Calculated Uncertainty of Temperature Due to the Size-of-Source Effect in Commercial Radiation Thermometers

The article evaluates the uncertainty in the temperature indicated by a radiation thermometer with a direct readout in temperature, due to the uncertainty in measuring the size-of-source effect (SSE) by the so-called “direct method.” Radiation thermometers of this type are the ones most frequently used in practice. The uncertainty of the SSE characteristic is usually not a useful quantity to report to users of commercial radiation thermometers. Instead, they would prefer to know the uncertainty in the measured temperature that results from the uncertainty of the SSE characteristic, and this will be the result of our analysis. The user of a direct reading radiation thermometer will be able to take into account the uncertainty of temperature due to the SSE, if a target with known dimensions is measured. The uncertainty in temperature due to the SSE of analyses based on Planck’s law and its approximation, Wien’s law is compared.

Development of a New InGaAs Radiation Thermometer at NMIJ

The first InGaAs radiation thermometer at NMIJ was developed more than ten years ago as a standard radiation thermometer operating from 150 to 1,100°C. Its size-of-source effect (SSE) was as large as 1% from 6 mm in diameter to 50 mm in diameter. The new thermometer has an SSE of 0.3%. The reason for the error in measuring the SSE of InGaAs thermometers was also found. The new thermometer at first suffered from nonlinearity and the distance effect (DE). These deficiencies arose from the misalignment of optics inside the thermometer and were solved by increasing the detector size from 1 mm in diameter to 2 mm in diameter. Unfortunately, the detector of 2 mm diameter had a smaller S/N ratio than that of the 1 mm one at the indium (In) point. The final design uses a detector of 1 mm diameter, but the radiation is focussed on a smaller area of the detector. The new thermometer is smaller and lighter than preceding designs and other standard InGaAs radiation thermometers. The temperature of the main part of the instrument, including the filter, the detector, and the preamplifier board, is controlled at 30°C. In addition to the calibration with the six fixed points of copper (Cu), silver (Ag), aluminum (Al), zinc (Zn), tin (Sn), and indium (In), the linearity from the In point to the Cu point, the SSE, the DE, and the spectral responsivity were measured.

SSE- and Noise-Optimized InGaAs Radiation Thermometer

For measurements of radiance temperatures in the range from 150°C to 1,000°C, low uncertainties in the temperature measurements can be achieved by using near-infrared InGaAs radiation thermometers. The design and construction of the NIST near-infrared radiation thermometer (NIRT) that is optimized for low size-of-source effect (SSE) and noise-equivalent temperatures are described. The NIRT utilizes a 50 mm diameter achromatic objective lens with low scatter that images a 4.5 mm diameter spot at a distance of 50 cm from the objective in an on-axis design. A Lyot stop is implemented in the design with the aperture stop placed after the field stop resulting in a collection f/12. A 3 mm diameter InGaAs detector is cooled to − 70°C using a four-stage thermoelectric cooler to obtain high-shunt resistance for linear, low-noise operation at high transimpedance amplifier gains. For thermal and structural stability, the optical components are placed on four, 15 mm diameter graphite-epoxy rods making the optical throughput stable. Optical ray tracing with a commercial program is used to determine the Strehl ratio and other imaging parameters. A possible approach for a detector-based temperature scale in this range which could result in 10 mK (k = 2) thermodynamic temperature uncertainties at the In-point is discussed.

System for the determination of the size-of-source effect of radiation thermometers with the direct reading of temperature

The paper describes a system for the determination of the size-of-source effect (SSE) of radiation thermometers with the direct reading of temperature, which form the majority of commercial radiation thermometers. Optics in every radiation thermometer (RT) gathers radiation from a larger area than is defined by the nominal target size. Thus, a measured temperature is more or less dependent on available target area. Every radiation thermometer suffers from the SSE. We developed a system, based on a water-cooled holder, for measuring the SSE by a direct method. The system could be placed in front of any blackbody. Manufacturing of such a system was relatively easy and could be important for users of commercial radiation thermometers because the majority of those thermometers have direct reading of temperature. We measured the SSE characteristics by the direct method for a radiation thermometer with a linearized signal and direct reading of temperature. The results of the SSE were analysed for the direct method without correction of the background radiation and with correction of the background radiation. Knowledge about the SSE characteristic of a radiation thermometer is one of the key elements for correct temperature measurement.

Methods to reduce the size-of-source effect in radiometers

In radiometry, photometry and radiation thermometry, accurate measurements of the radiance, luminance or the radiance temperatures of sources requires a knowledge of the contribution from the surroundings to the measured signal from the target area. The dependence of the radiometer or the radiation thermometer on the area surrounding the target area is described as the size-of-source effect (SSE), and minimizing the radiometer's sensitivity to SSE is critical in the lowest-uncertainty optical measurements. We describe the dominant effects that influence the SSE, and show that the SSE can be reduced to <5 × 10−5 as measured using a 50 mm diameter radiance source with a 2 mm diameter, central obscuration. The SSE is found to be dependent on the internal scatter and the optical design of the radiometer. For testing the contributions to SSE, a radiometer is constructed with a 50.8 mm diameter lens in f/12 geometry with a 1 mm diameter target size. If the internal radiometer scatter is reduced, then the SSE is found to be primarily dependent on the scatter from the objective lens such as surface finish, internal lens scatter and the particulate contamination of the lens. Various combinations of objective lenses are measured for SSE, and the relative merits of increasing optical performance at the expense of additional optical elements are also discussed.

New developments in standard thermometry at the IMGC

Recent findings in precision platinum resistance and radiation thermometry at the Istituto di Metrologia "G. Colonnetti" are described. Two issues concern high-temperature platinum resistance thermometers. The first is the use of gas-controlled heat pipes for comparing these thermometers and the development of a novel approach to improve temperature stability and uniformity. The second deals with observations made at the freezing point of silver which suggest that humidity may lead to electric leakage. In radiation thermometry the main recent achievement has been the use of a photodiode-array thermometer to realize the ITS-90 above the silver point. Relative to single-detector thermometers, this new thermometer offers several advantages that are related to its spectral characteristics. Another issue of importance is the size-of-source effect (SSE). Practical rules for minimizing SSE errors in interlaboratory comparisons are given.