Abstract. Emissivity measurements on Nextel Velvet Coating 811-21 between ÿ36 8C and 82 8C are

presented. The method used is a periodic one. This method involves sample temperature modulation

and signal processing of the sample temperature and the output signal of an infrared detector facing

the sample. The Fourier transform at the modulation frequency of the infrared signal is proportional

to the emissivity and to the Fourier transform of the temperature raised to the fourth power. As

with ordinary methods, two measurements are necessary to obtain the emissivity: one on the sample

and the other on a reference. Instead of a cold reference, the hemisphere method has been employed.

The sample is glued onto a modulated sample holder, the mean temperature of which can be

chosen between ÿ36 8C and 82 8C. This sample and the optical detection system are placed in a

vacuum chamber. The pressure inside the chamber is less than 10ÿ6 mbar. At each temperature,

ten measurements have been carried out. The accuracy obtained is better than 1%. The emissivity

of the coating studied remains constant over the temperature range explored. This coating can be

employed as a reference in this temperature range. The values obtained are consistent with values

from the literature.

1 Introduction

Design and control of thermal systems require modelling of the radiative transfer and

thus knowledge of radiative parameters of the various elements concerned, particularly

the total emissivity. Most emissivity measurement methods demand the use of a reliable

reference. Moreover, highly emissive black paints are used to control the radiative environment.

One paint widely used is Nextel Velvet Coating 811-21. Thus, its characteristics

must be known with a high degree of accuracy. We designed and built a device to measure

the normal total emissivity of this paint in the temperature range from ÿ36 8C to

82 8C. As a reference, this method uses a hemispherical mirror placed over an opaque

coating, the latter being diffusing and highly emissive.

2 Principle of emissivity measurements at low temperature

The direct emissivity measurement requires the measurement of the emitted flux. At low

temperature the flux emitted by a surface S may be hidden by the reflected ambient

flux. However, an infrared detector can measure only the sum of the emitted and

reflected fluxes. If the surface temperature is modulated, then only the emitted flux is

modulated, and by spectral analysis of the infrared signal, it is possible to extract the

component of the signal proportional to the emission and thus to the surface emissivity

(Matte|« et al 1994). Indeed, if the surface temperature is modulated, then the emitted

flux can be written as: