Characterization of spectral emissivity via thermal conductive heating and in-situ radiance measurement using a low-e mirror
A high SNR in-situ measurement of sample radiance in a low-temperature ambient environment is used to accurately characterize sample emissivity for transmissive, low-emissivity samples. A low-e mirror is positioned behind the sample such that the sample and low-e mirror overfill the field-of-view (FOV) of the radiometer. The sample is heated via thermal conduction in an open environment. Thermal conduction heats the sample without raising the background radiance appreciably. The low-e mirror presents both a low emission background against which to measure the sample radiance and reflects radiance from the back of the sample approximately doubling the measured signal. The low-e mirror exhibits a reflectance of at least 90% and preferably greater than 98% and an emissivity of at most 7.5% and preferably less than 2% over the spectral and temperature ranges at which the sample emissivity is characterized.
1. A method of characterizing the emissivity of a sample, comprising:
a) placing a low-e mirror behind the sample, said sample and said low-e mirror positioned to overfill a field of view (FOV) of a radiometer;
b) controlling a heat source to heat the sample through thermal conduction;
c) using a radiometer to directly measure a sample radiance in the FOV over a spectral band that spans at least one of the MWIR and LWIR bands with the sample in-situ;
d) measuring the temperature of the sample;
e) computing an ideal radiance over the spectral band at the measured temperature;
f) extracting a calculated sample radiance over the spectral band from the measured sampled radiance;
g) computing the sample emissivity as the ratio of the calculated sample radiance to the ideal radiance over the spectral band; and
h) repeating steps b through g at increments over a temperature range,
wherein said low-e mirror exhibits a reflectance of at least 90% and an emissivity of at most 7.5% over the spectral band for sample temperatures over the temperature range.
2. The method of claim 1 , wherein the low-e mirror exhibits a reflectance of at least 98%.
3. The method of claim 1 , wherein the low-e mirror exhibits an emissivity of at most 2%.
4. The method of claim 1 , wherein said low-e mirror comprises a gold coating.
5. The method of claim 1 , wherein the temperature range spans 50 C to at least 300 C.
6. The method of claim 1 , wherein the radiometer comprises a Fourier Transform IR (FTIR) radiometer.
7. The method of claim 1 , wherein the sample is heated in-situ in an open environment without raising background radiance appreciably.
8. The method of claim 1 , wherein the low-e mirror is placed between and in contact with the sample and the heat source.
9. A method of characterizing the emissivity of a sample comprising:
a) placing a low-e mirror behind the sample, said sample and said low-e mirror positioned to overfill a field of view (FOV) of a radiometer;
b) controlling a heat source to heat the sample through thermal conduction, wherein the heat source comprises an annular heating element having a central void therethrough, wherein the sample is placed on the surface of the annular heating element over the central void, wherein the low-e mirror is placed behind and spaced apart from the annular heating element, wherein a portion of the sample and the low-e mirror within the central void overfill the FOV of the radiometer
c) using a radiometer to directly measure a sample radiance in the FOV over a spectral band with the sample in-situ;
d) measuring the temperature of the sample;
e) computing an ideal radiance over the spectral band at the measured temperature;
f) extracting a calculated sample radiance over the spectral band from the measured sampled radiance;
g) computing the sample emissivity as the ratio of the calculated sample radiance to the ideal radiance over the spectral band; and
h) repeating steps b through g at increments over a temperature range,
wherein said low-e mirror exhibits a reflectance of at least 90% and an emissivity of at most 7.5% over the spectral band for sample temperatures over the temperature range.
10. The method of claim 9 , further comprising:
actively cooling the low-e mirror.
11. A method of characterizing the emissivity of a sample, comprising:
a) placing a low-e mirror behind the sample, said sample and said low-e mirror positioned to overfill a field of view (FOV) of a radiometer;
b) controlling a heat source to heat the sample through thermal conduction;
c) using a radiometer to directly measure a sample radiance in the FOV over a spectral band with the sample in-situ;
d) measuring the temperature of the sample;
e) computing an ideal radiance over the spectral band at the measured temperature;
f) extracting a calculated sample radiance over the spectral band from the measured sampled radiance;
g) computing the sample emissivity as the ratio of the calculated sample radiance to the ideal radiance over the spectral band; and
h) repeating steps b through g at increments over a temperature range,
wherein said low-e mirror exhibits a reflectance of at least 90% and an emissivity of at most 7.5% over the spectral band for sample temperatures over the temperature range,
wherein the step of exacting the calculated sample radiance comprises:
characterizing the reflectance of the low-e mirror and the transmission of the sample over the spectral range;
measuring an ambient radiance emitted from an ambient source;
measuring a low-e mirror radiance emitted from the low-e mirror without the sample;
subtracting the ambient radiance from the low-e mirror radiance and the measured sample radiance; and
extracting the calculated sample radiance over the spectral band as a scaled difference between the adjusted low-e mirror radiance and adjusted measured sample radiance.