The Physikalisch-Technische Bundesanstalt determined the directional spectral emissivities of several widely used black coatings: Nextel 811-21, Herberts 1534, Aeroglaze Z306 and Acktar Fractal Black. These are and were often applied in different industrial and scientific applications. The measurements are taken angularly resolved over a range from 10 • to 70 • . They cover the temperature range typical for the application of the respective coating and a wide wavelength range from 4 µm to 100 µm. The respective directional total emissivities and hemispherical total emissivities are given as well. The measurements were taken under vacuum at the reduced background calibration facility to achieve low uncertainties and avoid atmospheric interferences. Additionally, some measurements were taken with the emissivity measurement setup in air.
Blackbody sources at near-ambient temperature are routinely used to calibrate infrared instruments used in remote sensing and thermal imaging applications to measure radiance and radiation temperature. The measured temperature of the blackbody and its calculated effective emissivity determine its radiance and radiation temperature according to Planck's law. The temperature measurement is generally accomplished with a contact thermometer which is calibrated against the International Temperature Scale (ITS-90). The ammonia heat-pipe blackbody of the Physikalisch-Technische Bundesanstalt (PTB) in Germany is a primary source standard working over a wide spectral range with low uncertainties, i.e. less than 33 mK at 10 µm in the temperature range from –60 °C to 50 °C. A more direct method of absolute radiance measurement is to use an absolutely calibrated radiometer, calibrated against a primary detector standard, the cryogenic radiometer. AMBER (Absolute Measurements of Blackbody Emitted Radiance) is an absolutely calibrated radiometer of the Optical Measurement Group of the National Physical Laboratory (NPL) in the UK which was specially designed to determine the radiance and hence the radiation temperature of near-ambient-temperature blackbodies. When AMBER is operated at short wavelengths, where photodetectors offering good long-term stability exist, it derives its traceability through the cryogenic radiometer. However, available photodetectors operating in the 8 µm to 12 µm wavelength range offer poor long-term stability so when AMBER is used in this wavelength range, the NPL radiance temperature scale is based on a gallium fixed-point blackbody operating at 29.7646 °C (ITS-90). At other radiance temperatures, the NPL scale also relies on the gallium fixed-point blackbody but requires the calibration of the relative spectral irradiance responsivity of the AMBER radiometer (done against NPL spectral responsivity standards), measurement of the radiometric zero, as well as the use of Planck's equation. This paper gives the results of a comparison of the radiation temperature scale of the PTB with the radiation temperature scale of NPL in a temperature range from −57 °C to 50 °C for wavelengths around 10 µm.
Due to several excellent material properties, fiber-reinforced plastic (FRP) composites are expediently used in many applications, e.g., in the field of renewable energy and in oil, gas, and transportation applications. They show excellent mechanical stability, low weight and fatigue, and high corrosion resistance. However, their full potential for exploitation, as well as the lifetime of FRP-based structures, is limited due to certain defects and damage mechanisms. One of the most important methods used to ensure the quality of FRPs is non-destructive testing and active thermography. A prerequisite for quantitative active thermography is accurate knowledge of the optical properties of the investigated material (i.e., its spectral emissivity, reflectivity, and transmissivity). The objective of PTB as a partner organization within the European EMRP project titled "Validated Inspection Techniques for Composites in Energy Applications" was to improve the state of the art of this technique. One of the goals in doing so was to significantly reduce the uncertainty of emissivity measurements of FRP materials in the visible and infrared wavelength ranges. Achieving a target value of lower than 0.01 of the emissivity of partially transmitting materials is very challenging, especially at temperatures close to room temperature. Different experimental setups at PTB were employed for these measurements: one setup for spectral emissivity measurements in air and the other for diffuse reflectivity and transmissivity measurements. In this paper, we give a review of PTB measurements on emissivity, reflectivity and transmissivity of semitransparent FRP composites. Part of this work has already been published in Adibekyan et al. (emissivity, reflectivity and transmissivity of semitransparent fiber-reinforced plastic composites. https://www.ndt.net/article/dgzfp-irt-2017/ papers/17.pdf, 2018). Here, we present the complete set of data for seven technical relevant materials and compare the results. The directional spectral emissivity was determined at a nominal sample temperature of 40°C, at angles of observation from 10°to 70°with respect to the surface normal and in a wavelength range from 5 µm to 25 µm. In addition, these spectrally and directionally resolved measurements allow to calculate the total directional emissivity and the hemispherical emissivity. For the Extended author information available on the last page of the article
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