Reverse Fourier optics

A laser beam is passed through the measurement cell and brought to a waist of focus at the plane of the multielement detector. This is the reverse Fourier optical setup. Conventional Fourier optics arrangements are also used. 

Particles in the sample chamber scatter light out of the incident beam. Upon leaving the sample chamber the scattered light may be guided to the detector plane either by Fourier optics or by reverse Fourier optics. A typical schematic ofthese two optical configurations is shown in Figure 3.17. 

Figure 3.17. Fourier optics and reverse Fourier optics used in lasw diffraction instrumentation.

Figure 3.19. Geometrical analysis of particle scattering in reverse Fourier optics.

Figure 3.20. Angular uncertainty in reverse Fourier optics. The rising curves are calculated from Eq. 3.2 with IJI2 being 0.001,0.005,0.01,0.05 and 0.1 for the curves from bottom to top respectively (left ordinate). The parallel dashed lines are calculated from Eq. 3.3 with (/2-/ )// being 0.001,0.005, 0.01 and 0.05 for the lines from bottom to top respectively (right ordinate).

Beckman Coulter LS series uses a dark-field reticule having an array of 500 pm holes for automatic alignment purposes. The LS uses reverse Fourier lens optics incorporated in a binocular lens system to optimize light scattering over the widest dynamic size range. Results are calculated from either Fraunhofer or Mie theories. An application of this instrument to the measurement of micro-silica mixtures has been presented [154]. 

Figure 9.2. (a) UV/visible diode array spectrometer (Hewleu-Packard HP8450A) using the reversed-optics configuration (courtesy of Hewlett-Packard), (b) Fourier-transform IR spectrometer (Nicoiet brochure)... 

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