TAILIEUCHUNG - OPTICAL IMAGING AND SPECTROSCOPY Phần 5

Tham khảo tài liệu 'optical imaging and spectroscopy phần 5', ngoại ngữ, anh ngữ phổ thông phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 198 COHERENCE IMAGING where fa arg W 2a 0 n . Note that the relative position of the two point sources affects the spectrum of the image field even though the points are unresolved. The scattered spectrum observed on the optical axis as a function of a and wavelength for a jinc distributed cross-spectral density is illustrated in Fig. . We assume that the spectrum of the illuminating source is uniform across the observed range. The scattered spectrum is constant if the two points are in the same position or if the two points are widely separated. The scattered power is doubled if the two points are at the same point as a result of constructive interference. If the two points are separated in the transverse plane by 1-2 wavelengths the spectrum is weakly modulated as illustrated Fig. b . The spectral modulation is much greater if the sources are displaced longitudinally or if the scattered light is observed from an off-axis perspective. This example is considered in Problem more general discussion of spectral modulation by secondary scattering is presented in Sections and . While the three examples that we have discussed have various implications for imaging and spectroscopy our primary goal has been to introduce the reader to analysis of cross-spectral density transformations and diffraction. Equation is quite general and may be applied to many optical systems. Now that we know how to propagate the cross-spectral density from input to output we turn to the more challenging topic of how to measure it. MEASURING COHERENCE We saw in Section that given the cross-spectral density or equivalently the mutual coherence on a boundary the cross-spectral density can be calculated over all space. But how do we characterize the coherence function on a boundary We have often noted that optical detectors measure only the irradiance I x y t over points x y and t in space and time. Coherence functions must be inferred from such irradiance .

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