ASTM International - ASTM E408-71(2008)

Standard Test Methods for Total Normal Emittance of Surfaces Using Inspection-Meter Techniques

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Organization:	ASTM International
Publication Date:	1 May 2008
Status:	inactive
Page Count:	3
ICS Code (Properties of surfaces):	17.040.20

scope:

1.1 These test methods cover determination of the total normal emittance (Note) of surfaces by means of portable, inspection-meter instruments.

Note 1-Total normal emittance (ε_N) is defined as the ratio of the normal radiance of a specimen to that of a blackbody radiator at the same temperature. The equation relating ε_N to wavelength and spectral normal emittance [ε_N(λ)] is

where:
L _b(λ, T)	=	Planck's blackbody radiation function = c ₁π⁻¹λ⁻⁵(e^c₂/λT − 1)⁻¹,
c ₁	=	3.7415 × 10 ⁻16 W·m²,
c ₂	=	1.4388 × 10 ⁻² m·K,
T	=	absolute temperature, K,
λ	=	wavelength, m,
L _b(λ, T)dλ	=	Δπ ⁻¹T⁴, and
Δ	=	Stefan-Boltzmann constant = 5.66961 × 10 ⁻⁸ W·m²·K⁻⁴

1.2 These test methods are intended for measurements on large surfaces when rapid measurements must be made and where a nondestructive test is desired. They are particularly useful for production control tests.

1.3 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

abstract:

These test methods cover determination of the total normal emittance of surfaces by means of portable, inspection-meter instruments. At least two different types of instruments are commercially available for performing this measurement. Test Method A uses an instrument which measures radiant energy reflected from the specimen and Test Method B utilizes an instrument which measures radiant energy emitted from the specimen. Both test methods are limited in accuracy by the degree to which the emittance properties of calibrating standards are known and by the angular emittance characteristics of the surfaces being measure. Test Method A is normally subject to a small error caused by the difference in wavelength distributions between the radiant energy emitted by the two cavities at different temperatures, and that emitted by a blackbody at the specimen temperature. Test Method B also has nongray errors since the detector is not at absolute zero temperature. Test Method A is subject to small errors that may be introduced if the orientation of the sensing component is changed between calibration and specimen measurements. This type of error results from minor changes in alignment of the optical system. Test Method A is subject to error when curved specular surfaces of less than about a certain radius are measured. These errors can be minimized by using calibrating standards that have the same radius of curvature as the test surface. Test Method A can measure reflectance on specimens that are either opaque or semi-transparent in the wavelength region of interest. However, if emittance is to be derived from the reflectance data on a semi-transparent specimen, a correction must be made for transmittance losses. Test Method B is subject to several possible significant errors. These may be due to variation of the test surface temperature during measurements, differences in temperature between the calibrating standards and the test surfaces, changes in orientation of the sensing component between calibration and measurement, errors due to irradiation of the specimen with thermal radiation by the sensing component, and errors due to specimen curvature. Test Method B is limited to emittance measurements on specimens that are opaque to infrared radiation in the wavelength region of interest.

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