Infrared temperature sensors for measurement on plastics are subject to certain requirements in terms of transmission and wavelength. The transmission levels of plastic films vary as a function of the wavelength. They are reciprocally proportional to the thickness, with thinner materials being more light-transmissive than thick plastics. Optimum temperature measurements can be made at wavelengths at which the transmission level is approximately zero irrespective of the thickness. Polyethylene, polypropylene, nylon and polystyrene, for example, are non-transmissive to infrared radiation at 3.43 µm, while polyester, polyurethane, teflon, FEP and polyamide are non-transmissive at 7.9 µm. In case of thicker (>0.4 mm) and pigmented films, a wavelength between 8 and 14 µm can be used for temperature measurement. The optimum spectral range for the measurement can be determined using a sample of the material. The reflection factor of nearly all plastic materials is between 5 and 10%.
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