EMISSIVITY AND OTHER INFRARED-OPTICAL PROPERTIES MEASUREMENT METHODS DIRECT RADIOMETRIC TECHNIQUES Measuring principle The principle of direct radiometric techniques is shown schematically in the figure below. Based on the definition of emissivity, the radiances of the specimen and of a reference blackbody cavity, which are maintained at a same tempe-rature, are compared under the same viewing and wavelength conditions by a radiometric system. The emissivity is given by the ratio of the measured radiances. Principle of direct radiometric techniques The main practical problem that is encountered in the application of this technique is related to the attainment of equal temperatures for the specimen and the blackbody cavity. Indeed, the temperature error is the main source of error in the measured emissivity. The graph below shows the relative emissivity error generated by a temperature error of 1%. 16 Emissivity error/% 14 12 10 8 6 4 2 Temperature error: 1% The curve in the graph shows that for a same relative temperature error the emissivity error is larger for low values of the product λt. This suggests that the temperature determination must be particularly accurate at short wavelengths and/or low temperatures. It also suggests that a radiometric technique is more suited for measurements at long wavelengths and/or high temperatures. The problem of ensuring equal temperatures for specimen and blackbody can be solved in many different ways as illustrated by the examples below. 0 0 3000 6000 9000 12000 15000 λt/µm K Emissivity error generated by a temperature error of 1% The examples will show that the reference blackbody may be either an integral blackbody cavity, whose walls are formed by the specimen or a separate blackbody controlled at the temperature of the specimen. The integral blackbody is generally preferred at high temperatures, where temperature measurement and control may be difficult, or at very
short wavelengths where very good temperature control is required for accurate measurements. Example 1: Technique for ceramic materials at high temperature This example refers to an apparatus for measuring the total normal emissivity and normal spectral emissivity of ceramic oxides from 1200 K to 1800 K is shown in the figure below. In this application the specimen furnace and the reference blackbody furnace are operated separately. A schematic drawing of the former is shown in the figure below. The equipment for specimen heating The specimen in the form of a cylinder is supported by an alumina tube and located at the center of a vertical tubular furnace provided with a side window for observation. To avoid temperature gradients generated by the heat loss across the window, the specimen is rotated at a speed that can be varied from 1 to 300 rpm. The design of the furnace shell is such that the furnace may be operated in an inert atmosphere as well as in air. The temperature of the specimen is measured by a thermocouple located as shown in the drawing. The separate reference blackbody furnace is of a traditional type and the cavity temperature is measured with a thermocouple. The temperatures of the specimen and blackbody are maintained equal to within ± 1 C. This equipment was used to measure the emissivity of various ceramic materials including aluminum oxide, zirconium oxide, thorium oxide, magnesium oxide, calcium zirconate, aluminum silicate, magnesium aluminate, and others. H. E. Clark, D. G. Moore, Method and Equipment for Measuring Thermal Emittance of Ceramic Oxides from 1200 K to 1800 K, in Thermal Radiation of Solids, NASA SP-55, S. Katzoff (ed.), 1965, pp. 241-257
Example 2: Technique with separate blackbody and specimen A technique for measuring the directional spectral emissivity where the equality of the temperatures of the specimen and reference blackbody is not necessary was implemented using the equipment shown in the figure below. The specimen and the reference blackbody are provided with independent heaters and are located inside a sphere that is connected to a vacuum pump. The internal wall of the sphere is coated with a highly absorbing black coating and is water cooled. The measurement procedure includes the measurement of the radiant fluxes from the specimen, the reference blackbody and the sphere wall. The temperatures of the specimen and blackbody are measured with thermocouples. The vacuum sphere with specimen and blackbody If S s, S b and S w are the signals from the radiometric system corresponding to the fluxes from specimen, blackbody and sphere wall, respectively, and T s and T b are the temperatures of the specimen and blackbody, it is possible to obtain ε λ (θ) from the following relationship: ε λ (θ) = {[S s - S w exp (c 2 /λt w ) - exp (c 2 /λt b )] exp (c 2 /λt b )}/{[S b S w exp (c 2 /λt w ) - exp (c 2 /λt s )] exp (c 2 /λt s )} R. C. Birkebak, Rev. Sci. Instr. 43, 1027 (1972)
Example 3 :Technique with specimen and blackbody in the same thermal region A technique suitable for measuring normal total or spectral emissivity is shown in the draving below. The specimen is positioned over a rotating mount inside a heated blackbody cavity that is provided with two sight tubes for collecting radiation from the blackbody and specimen separately. The specimen sight tube is extended down to near the sample surface so as to prevent the specimen from receiving radiation from the cavity walls. This tube is water cooled to prevent its becoming a source of radiation and its interior is blackened to avoid channeling effects on the radiation emitted by the specimen. Rotation of the specimen is necessary to present a uniform freshly heated surface to the specimen sight tube for radiation measurement. Schematic view of the specimen and blackbody assembly A. S. Kjelby, Emittance Measurement Capability for Temperatures up to 3000 F, in Measurement of Thermal Radiation of Solids, NASA SP 31, J. C. Richmond (ed.), 1963, pp. 499-503
Example 4: Technique with dynamic measurement of the specimen s radiance The technique proposed in this example allows normal spectral emissivity measurements to be made on solid materials of any kind at temperatures from about 150 C to about 950 C. The core of the apparatus is draw schematically below. The specimen, a flat disc of about 20 mm diameter, is attached to a cylindrical nickel heater block and surrounded by a shield and shutter whose temperatures are controlled so that the whole assembly is isothermal. The sample temperature, T 0, is measured with a Pt/Pt13Rh (type R) thermocouple that is positioned between the sample and the heater block. To measure emissivity, the shutter is rapidly removed and sample surface radiance versus time data are recorded with a spectral radiometer. The radiometer voltage rises rapidly as the shutter is removed and then falls as the surface cools (see figure), To obtain the radiometer voltage, V 0, corresponding to the sample radiance at T 0, the radiometer voltage is extrapolated to zero time. The radiometer is then directed towards a blackbody cavity whose temperature, T B, has been adjusted so that the radiometer output is equal to V 0. The spectral emissivity is then calculated from the ratio of the Planckian functions at T 0 and T B Shield Insulation Specimen Heater block Shutter Schematic view of the apparatus The time behaviour of the radiometer signal s J. S. Redgrove, High Temp.- High Press. 17, 145-151 (1985) J. S. Redgrove, Measurement 8 (2), 90-95 (1990) J. S. Redgrove, M. Battuello, High Temp.- High Press. 27/28, 135-146 (1995/1996)
Example 5: Technique with auxiliary reflector The specimen itself can act as a reference blackbody if its own radiation is reflected on it by means of an auxiliary reflector. A well known example of this kind is given by the Land surface pyrometer that can be used either as a true-temperature thermometer or an emissivity meter. Another example of this kind was proposed by Iuchi and is illustrated in the figure below. The apparatus by Iuchi A cylindrical cavity with a highly reflecting inner wall is located above the specimen (not in contact with it). A rotating sectored disk with specularly reflecting blades modulates the radiation detected by the radiometer.. The latter will receive the radiation directly emitted by the specimen when an open sector is in front of the cylinder end. Instead, when a closed sector, provided with a narrow slot for observation, passes in front of the cylinder, multiple reflections occurs between the specimen and the cylinder + slot system. Due to these multiple reflections, near blackbody conditions will be attained. The ratio of the radiometer signals obtained in the two measuring conditions will give the normal spectral emissivity of the specimen. T. Iuchi, in: Applications of Radiation Thermometry, J. C. Richmond and D. P. DeWitt (eds.), Philadelphia, ASTM, STP 895, 1985, pp. 121-15
Example 6: Technique with integrated specimen and blackbody Measurements of normal spectral emissivity and of other thermophysical properties of refractory metals at high temperatures are currently performed using pulse techniques. A pulse experiment consists of the rapid heating of a specimen by the passage of an electric current pulse of subsecond duration and of the collection during heating of the physical quantities necessary to determine the property of interest. Thanks to the short heating time it is possible to avoid chemical contamination and oxidation of the specimen. In most pulse experiments a tubular specimen is used as shown in the drawing below. The tubular specimen is provided with a rectangular hole in the middle that defines a blackbody cavity with an emissivity greater than 0.98. Due to the short duration of the experiment, there is a minimum loss of heat by thermal conduction towards the electric clamps so a good temperature uniformity is obtained along the specimen. To further guarantee temperature uniformity, the cross-sectional area of the tube is equalised by grinding away a strip of material to compensate for the material removed in drilling the blackbody aperture. Measurements of emissivity are performed by measuring simultaneously the temperature of the outer surface of the specimen and of the blackbody cavity. To this purpose, two high-speed monochromatic thermometers are used. Measurements with this technique have been done up to 3700 K. Schematic diagram of the hole-in-tube apparatus s F. Righini, A. Rosso, Measurement 1, 79-84 (1983) A. Cezarliyan, in Compendium of Thermophysical Property Measurement Methods, Vol. 1, K. D. Maglic, A. Cezarliyan, V. E. Peletskii (eds.), Plenum, New York, 1984, pp. 643-668
Example 7: Techniques using a radiation thermometer In some cases a radiation thermometer can be a useful tool for measuring the normal spectral emissivity relative to the working waveband of the thermometer itself. Three examples are given below. Example 7.1: Surface temperature measured with contact thermometer Operational steps Thermometer Specimen The surface temperature of the specimen is measured with a contact thermometer (thin film surface thermometer). The emissivity corrector of the radiation thermometer is set to read the same surface temperature. The emissivity setting gives the specimen s emissivity. Example 7.2: Specimen s surface partially coated with black paint Operational steps The specimen s surface is partially coated with a black paint of known emissivity (0.95 to 0.98). The radiation thermometer is aimed at the coated surface and the emissivity is set to the emissivity value of the paint. The radiation thermometer is aimed at the uncoated portion of the surface and the emissivity is set to give the same temperature reading as before. In this condition, the value read on the emissivity corrector corresponds to the specimen s emissivity. Coating Specimen
Example 7.3: Blackbody cavity drilled on specimen s surface Operational steps The radiation thermometer is aimed at a hole drilled into the specimen. The temperature is read with the emissivity corrector set at 1. Specimen The radiation thermometer is aimed at the specimen s surface and the emissivity is set to give the same temperature reading as before In this condition, the emissivity setting gives the specimen s emissivity.