Analysis of the Accuracy of Methods for the Direct Measurement of Emissivity

Abstract

Emissivity measurements are of great interest for both theoretical studies and technological applications. Emissivity is a property that specifies how much radiation a real body emits as compared to a blackbody. The emissivity determination of a sample should be an easy task: a simple comparison between the sample and blackbody radiation at the same temperature. Unfortunately, when measuring the emissivity, some practical problems arise due to the differences between the true emitted radiation and the detected quantity. To clarify this point, an analysis of different direct methods for emissivity measurement is presented. Furthermore, a method that includes multiple reflections is developed. The systematic errors associated with each method are computed theoretically as a function of wavelength, sample temperature, and emissivity, and the surrounding enclosure temperature and emissivity. In general, the error is very high for small sample enclosures, but it strongly decreases when the enclosure area increases. Although at short wavelengths all the analyzed methods produce similar errors, noticeable differences appear under other conditions, and methods considering more radiation terms do not always produce lower errors.

Environmental Radiation Change-based Emissivity Measurement Method for Field Targets

“…Tab.2 shows a 0.017 (1.9%) maximum error for object emissivity between the indirect and direct measurement methods. The result is less than 5% of the maximum error of multi-wavelength emissivity measurement in reference [8]. We observe that higher emissivity results in lower measurement error.…”

Normal and Directional Spectral Emittance Measurement of Semi-Transparent Materials Using Two-Substrate Method: Alumina

“…2 represents essentially complex spectra. The detailed derivation is described elsewhere [3,5,7]. Accordingly, the emittance of the sample is described as follows:…”

“…Equation 6 is the same as the 'black surrounding method' which gives the lowest error for direct emissivity measurements, when the surroundings are considered as a blackbody [7]. The above method is valid for opaque materials, but not for semi-transparent materials, such as alumina, since the radiance of the substrate penetrates the sample and is added to the total radiance.…”

“…Most measurement errors are well categorized by Birch and Clarke [2]. Since such problems have been well studied and resolved [3][4][5][6][7], and the FT-IR spectrometer provides a relatively straightforward calibration method [3][4][5][6][7], high resolution, and fast measurement, the direct method has been widely used for emittance measurements. However, the precise measurement of the emittance is still very difficult for semi-transparent materials since additional radiances can pass through the materials, and can be detected.…”

Construction and Measurement of Normal Spectral Emissivity Device using Fourier Transform Infrared Spectrometer

“…또한 동일 물질이라 하더라도 표면의 산화된 정도나 거칠기에 따라 복 사율은 다르게 측정되며, [3,4] 측정에 필요한 여러 장치들에 의 한 위상차, [5,6] 검출기의 비 선형성, [7] 대기 복사의 영향, 그리 [8] 과 radiometric 방법이 있으며, radiometric 방법에는 direct 방법 [9] 과 indirect 방법 [10] 이 있다. 최근에는 분광 복사율 측정을 위하여 퓨리에 변환 적외선 분광기(FT-IR)를 이용한 direct 방법 [1][2][3][4][5][6]11] 이 주로 사용되고 있으며, 그 중 실험 장치의 교정과 대기 복사의 보정에 효과적인 two-temperature 방법 [5,6,11] 이 물질의 분광 복사율을 측정에 주로 이용되고 있다. 본 논문에서는 퓨리에 변환 적외선 분광기(FT-IR)를 이용 한 적외선 분광 복사율 측정 장치를 기술하고, 물질의 수직 분광 복사율 및 표면 상태에 따른 복사율 변화를 측정하였 다.…”