SERIES 205 SPECTRAL EMISSOMETER
Figure 1. Series 205 Spectral Emissometer.
An automated bench-top emissometer which measures spectral emissivity over a broad spectral range from the near to mid IR while simultaneously determining the surface temperature at the measurement location.
The Model 205 NB and Model 205 WB Emissometers are Bench Top FT-IR based instruments designed specifically to facilitate simultaneous measurements of surface spectral emittance and temperature using optical techniques over the Near and/or Mid-IR spectral range at temperatures ranging from 50 to 2000°C. This patented technology (U.S. patent # 5,239,488) can be used to measure radiative properties (emittance, reflectance, transmittance) as a function of temperature for a wide range of materials such as:
The system can also provide measurements of radiance as well as directional-hemispherical reflection and transmission over a wide spectral range from 12,500 to 500 cm-1 (0.8 to 20 microns) for the Model 205 WB, and from 6,000 to 500 cm-1 (1.7 to 20 microns) for the Model 205 NB.
The Model 205 WB Emissometer has additional extended capabilities to operate as a stand-alone FT-IR spectrometer covering the Near and Mid-IR spectrum from 12,500 cm to 500 cm-1 with up to 1 cm-1 resolution. The Model 205 NB covers the Mid-IR spectrum from 6,000 to 500 cm-1 .
The Series 205 Emissometers significantly advance the state-of-the-art in emissivity measurements. Previous methods and instruments for measuring spectral emittance at elevated temperature required the precise knowledge of the sample temperature. The Model 205 overcomes these difficulties and provides all information necessary to simultaneously determine the precise temperature and emissivity for the same target spot on the sample.
The Emissometer is shown schematically in Fig. 2. All optical components, including the FT-IR spectrometer, are mounted on a 3 foot x 4 foot optical bench. The hemi-ellipsoidal mirror enables the measurement of radiation in a hemispherical-directional mode. The sample can be heated with an oxy/acetylene torch, CO2 laser, or other means. The FT-IR spectrometer is utilized in the emission mode and can accept radiation from either side of the sample by positioning the selector mirror. The design of the spectrometer's interferometer allows for the incoming beam to be modulated and split into two outgoing beams. In the Model 205 WB Emissometer, two separate detectors are utilized to measure near and mid-IR energy in these two beams simultaneously. A room temperature indium-gallium-arsenide detector is used for the Near-IR (12,500 to 6,000 cm-1), and a liquid nitrogen cooled mercury-cadmium-telluride detector is used for Mid-IR (6,000 to 500 cm-1). For the Model 205 NB Emissometer, only the MCT detector is required.
The hemi-ellipsoidal mirror has both foci inside the mirror. A near-blackbody source is located at one of the foci and the sample is located at the other focus. This mirror geometry, combined with the radiating characteristics of the near-blackbody source, provides a means of measuring the hemispherical-directional reflectance of the front surface of the samples. Likewise, for transmissive samples, the hemispherical -directional transmittance can be measured from the back side.
Figure 2. Schematic of Benchtop Emissometer.
An integral part of the optical system is the rotating chopper system which moves either an aperture or a cold near-blackbody element in front of the source. The FT-IR data collection system is synchronized with these two states, and allows for the distinction of sample radiation from reflected/transmitted radiation as follows. For the reflectance measurement, the IR beam originates at the near-blackbody source at one focus of the hemi-ellipsoidal mirror. The radiation reflects from the hemi-ellipsoidal mirror and is focused onto the sample at the other focus where it is reflected (scattered) by the sample into the interferometer. The reflectance and the sample radiance are measured together when the aperture on the chopper rotor is in place over the source (chopper open condition). This is shown by the top curve in Figure 3. When a cold near-blackbody is substituted for the aperture over the source (chopper closed condition), it is the sample radiance alone which is measured (bottom curve in Fig. 3). Both the radiance (r) and directional-hemispherical reflectance (R) can be obtained from these two spectra. Transmission measurements (T) are similarly obtained by repositioning the selector mirror.
The bidirectional scanning ability of the spectrometer's interferometer allows collection of sample radiance (chopper closed) in the "forward" scan, and sample radiance plus reflectance (chopper open) in the "reverse scan". At 32 cm-1 spectral resolution, a complete forward and reverse motion of the interferometer is accomplished in ~0.5 seconds, corresponding to a chopper rate of 2 Hz. Signal processing automatically separates forward motion scans from reverse motion scans and allows for signal averaging from sequential collection of data for each motion. Typically, a data set consists of 16 co-added scans of each component of the front surface measurement. For non-opaque samples, the selector mirror is flipped and 16 co-added scans of each component of the back surface are collected.
Figure 3. Spectral Measurements Performed with the Emissometer.
Figure 4. Determination of Spectral Emittance from the Measured Hemispherical Reflectance (R) and Hemispherical Transmittance (T).
Figure 5. Determination of Temperature by Matching (r/E) with a Blackbody Curve.
Figure 6. The Radiative Property Change of a Metal Alloy as it is Heated in Air.
Once the spectra are acquired, Spectral emittance (E) of the hot sample is determined by conservation of energy: E = 1 - R - T (Fig. 3). The precise sample temperature is simultaneously determined by the Planck "Blackbody" (BB) relationship: r/E = BB. Hundreds of spectral data points or "colors" are used to match the shape and amplitude of the Planck temperature curve as shown in Figure 4. As shown in Figure 5, the emissometer can rapidly monitor spectral emittance as a function of temperature and time.
Samples are mounted into position with a clamping devices which is arranged so as not to contribute extraneous radiation into the measurement. Three intersecting visible lasers beams are utilized to ensure correct positioning of the sample. Sample sizes on the order of 1 to 1-1/2 inches square or diameter are convenient, although samples as small as 1/4" square have been measured. The measurement spot on the sample is variable from 1 millimeter diameter to 3 millimeters in diameter. Samples have been heated by a variety of methods. These include: i) torch flames (propane, hydrogen, oxy/acetylene), ii) focussed infrared radiation from high intensity lamps, and iii) infrared radiation from a CO2 laser (25 W, continuous wave). Sample thickness of less than 1/4" is con-venient for sample heating in order to achieve uniform sample surface temperatures when heating on the back surface. Torch heating allows the sample to be heated from the back, the front, or both surfaces. The influence on the measurements due to the radiation contributions from the combustion products of a torch flame is not a problem as it is limited to narrow spectral regions.