nstrument which is most suited to a and the resonant modes of a laser cavity. particular application. 7.5.6 Accuracies Blackbody sources and electrical substitution radiometers are used as primary standards for the calibration of radiometric instruments. Absolute calibrations are transferred from these primary standards to the broad variety of sources and power meters used in working laboratories with special incandescent lamps. Errors that can build up with transfer of calibrations result in field instruments with calibration accuracies which are typically around the 5 per cent FIXED MIRROR Figure 7.9 Recoder output from the set up of Figure 7.8 Fourier Transform Spectroscopy is another application of the interferometer and has emerged as a direct alternative to the disper- BEAM SPLITTER sive prism or grating spectrometer previously discussed. Its main application is in the field of organic chemical analysis. The basic MOVING MIROR optical element in the Fourier Transform Spectrometer is normally SOURCE a Michelson interferometer as shown in Figures 7.10 and 7.11. As the mirror in one of the arms of the interferometer is moved the PATH DIFFERENCE detector records the interference pattern produced by the superposi- SAMPLE IS PLACED IN EITHER SOURCE tion of the two beams which are split and recombined by the beam OR DETECTOR ARM splitter. The detector is fixed at the centre of the ring pattern of the interferometer. The recorded interference pattern is called the inter- ferogram. The interferogram is digitised and sent to a computer. The computer reduces the interferogram to a spectrum by means of a Fourier Transform algorithm. DETECTOR Although more sophisticated than the grating instrument the in- strument has benefited greatly from advances in digital computing. Figure 7.10 Michelson interferometer It does have an energy throughput advantage over dispersion instru- The human eye 7/9 FOURIER RELATIVE DETECTOR TRANSFORM SOURCE INTERFEROGRAM INTENSITY SIGNAL production WAVE NUMBER ) PATH DIFFERENCE Figure 7.11 Int