 the refractive index of the gas between the plates. Figure 7.7 Array-detector based spectroradiometer m n = 2nd receives all wavelengths at once (7.6) 7/8 Optics and vision PLATE SEPARATION VARIED BY PIEZO ELEMENT AMPLIFIER SOURCE UNDER DETECTOR INVESTIGATION RECORDER LENS L ETALON LENS 2 PARTIALLY SILVERED PINHOLE PINHOLE GLASS PLATES. SCREEN SCREEN Figure 7.8 Fabry Perot etalon for central spot scanning In the case of air, ml = 2d. For the case of the two monochromatic ments and a multiplexing advantage in that all spectral elements are lines above the intensity variation might have a form as shown in observed throughout the recording of the interferogram. However, Figure 7.9. The fringe peaks are repeated as the order of interference as we have seen the advent of detector arrays and their use in the increments by one, m, m+ 1, m+2, etc. and are equally spaced with grating instrument tends to cancel out the second advantage of the Fourier Transform Instrument. The built in computer of a rapid scan respect to or frequency. When d and a are known the wavelength commercial instrument is mainly there to perform the Fourier trans- or frequency differences of component lines lying in what is called formation but it can also be used to store reference spectra and these the 'free spectral range' can be evaluated, where the free spectral can be used in a ratio to yield solely the spectrum of the desired range is the separation between adjacent orders of interference. A component. This is equivalent to the double beam operation of a study of the width and shape of individual spectrum lines can also commercial conventional dispersive instrument where the incident be carried out. If the instrument has high resolving power the fringes beam is split into two beams one of which traverses the specimen will have a contour corresponding closely to the line itself. The and the other passes through a reference channel. Great care should etalon is particularly suited to the measurement of laser emission be taken in selecting the i