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.
OPTICS
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.
OPERATION
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
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.
SPECIFICATIONS
EMISSOMETER PERFORMANCE |
Model 205
WB |
Model 205
NB |
Spectral Range |
Near- and Mid-IR |
Mid-IR |
Spectral Range |
12,500 to 500
cm-1 |
6,000 to 500
cm-1 |
Emissivity Measurement
Accuracy (typical) |
± 3% |
± 3% |
Temperature Measurement
Accuracy (typical) |
± 5°C |
± 5°C |
Temperature Range |
50 to 2000°C |
50 to 2000°C |
Sample Size |
10 mm to 40 mm
|
10 mm to 40 mm |
Measurement Spot Diameter |
1 mm to 3 mm |
1 mm to 3 mm |
NEAR BLACKBODY SOURCE |
|
|
Source Surface Area |
6.45 cm-1 |
6.45 cm-1 |
Surface Temperature Control |
± 2°C |
± 2°C |
Surface Temperature
Uniformity over Full Source |
±10°C |
± 10°C |
Surface Temperature
Uniformity over Measurement Spot Diameter |
± 2°C |
± 2°C |
Chopper Type |
water-cooled
rotating shutter |
water-cooledrotating
shutter |
SAMPLE HEATING OPTIONS |
|
|
Oxy/Acetylene Torch and High
Intensity Lamps |
Standard |
Standard |
Propane, Hydrogen Torch and
25 W Continuous Wave CO2 Laser |
Optional |
Optional |
STAND-ALONE FT-IR
SPECTROMETER |
|
|
FT-IR Model |
Bomem MB 155 |
Bomem MB 100 |
Spectral Range |
12,500 to 500
cm-1 |
6,000 to 500
cm-1 |
Near IR Detector |
InGaAs 12,500
to 6,000 cm-1 |
|
Mid-IR Detector |
MCT 6,000 to
500 cm-1 |
MCT 6,000 to
500 cm-1 |
DATA SYSTEM |
|
|
Computer |
Pentium PC |
Pentium |
PHYSICAL CHARACTERISTICS |
|
|
Optical Platform |
4' x 3' x
1" |
4' x 3' x
1" |
Computer and Monitor Foot
Print |
17" x
17" |
17" x
17" |
Keyboard Foot Print |
20" x
8" |
20" x
8" |
Input Voltage |
120 VAC |
120 VAC |