摘要:

It is shown that focused wavefields satisfy a reciprocity principle concerning the roles of the focal point and the observation point. According to this principle, the diffracted field at a point P2of a converging spherical wave centred on the focal point P1is the complex conjugate of the diffracted field observed at P1if the focal point is at P2. The commonly used Debye integral representation of focused wavefields violates the reciprocity principle. Therefore we derive a new diffraction formula for focused wavefields which we call the modified Debye integral (MDI). It is constructed so as to satisfy the reciprocity principle, and may be considered as a generalization of the Debye integral and also as a refinement on a diffraction integral derived by Hopkins. For focusing of a two-dimensional perfect wave we show that the MDI gives the same f-number dependent parabolic phase factor on the geometrical focal line as is obtained numerically in the Kirchhoff approximation. For focusing of three-dimensional waves the MDI and the Kirchhoff theory are shown to predict the same focal shift in the paraxial approximation, as long as the shift is small compared with the focal distance.

ISSN:0030-3909    

DOI:10.1080/713821060

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The imaging properties of Fresnel zone plates are discussed in terms of possible manufacturing inaccuracies such as non-circularity and displacement of zones. The degradation of the on-axis intensity is used to establish manufacturing tolerances.

ISSN:0030-3909    

DOI:10.1080/713821074

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The spatial filtering property of a Fabry-Pérot interferometer is shown to transform the spatial spectrum of an arbitrary image into a form which satisfies, approximately, the conditions for self-imaging. The limitations of the technique are studied and checked experimentally.

ISSN:0030-3909    

DOI:10.1080/713821069

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

The ability of a two-wavelength lidar in the identification of natural and artificial targets is discussed. The analysis is carried out on the basis of target spectral reflectance and taking into account the atmospheric extinction at the two laser wavelengths. Preliminary results performed with a two-wavelength (0·532 and 1·064 μm) Nd-YAG lidar are reported.

ISSN:0030-3909    

DOI:10.1080/713821063

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor

摘要:

This paper considers photon correlation spectroscopy of spontaneous fluctuations in the number of particles in a small scattering volume containing interference fringes formed by counter-propagating laser beams. The ‘fringe number fluctuation’ contribution to the correlation function of the scattered intensity is predicted to decay exponentially in time. The particle diffusion constant can be obtained directly from the measured decay time and the easily calculated fringe spacing. Experiments were performed on both quasi-elastic and fluorescent scattering from dyed polystyrene particles of diameter 0·109 μm in aqueous suspension. For the quasi-elastic scattering reasonable agreement with theory was found. Some difficulties were encountered with the fluorescent scattering; nevertheless, the experimentally derived value of the diffusion constant was close to the expected value. Thus the fringe number fluctuation technique appears to provide a useful modification of the usual fluorescence correlation spectroscopy.

ISSN:0030-3909    

DOI:10.1080/713821075

出版商:Taylor & Francis Group    

年代:1983

数据来源: Taylor