TABLE OF CONTENTS The major advantage in using a wavelength modulating technique over amplitude modulation is the immunity of the system components to attenuation in the fibres or connectors. One of the most widely adopted measurement techniques involving wavelength or colour remote sensing is the use of an optical radiation pyrometer. This is simply a sapphire fibre enclosed in a black body cavity whose function is to transmit the reflected broad-band radiation from the cavity to a spectrometer that computes the temperature of the cavity by examining two narrow bands in the received spectrum.
Using a neodyminum doped fibre, a differential absorption characteristic with temperature can be observed, and the ratio of the responses at two wavelengths is a function of temperature. Optical components are not always necessary to produce the spectral pattern, and thermochromic substances such as cobalt chloride in an isopropyl-alcohol/water solution that changes colour between 25 C and 75 C can be spectrographically measured using a two wavelength detector system. However, these systems suffer from long equilibration time constants.
In another technique, light from an LED transmitted by a fibre is absorbed by a GaAlAs crystal, and is then re-emitted at another wavelength (photoluminescence) which is temperature dependent, and analysed by a two detector wavelength system.
Wavelength modulation can also be achieved using white light and a displacement technique, such as by rotating a prism, using a diffraction grating or by linear displacement of the receiving fibre from an emitting fibre followed by an achromatic lens. Alternatively, a...
