Jakob J. Stamnes

Focusing at small angular apertures in the debye and Kirchhoff approximations

Abstract The classical problem of diffraction of a converging spherical wave by a circular aperture is re-examined. On the basis of the Kirchhoff approximation and the paraxial approximation a new expression is derived for the field in the focal region. The new expression differs from the classical one in that it depends not only on the angular aperture, but also on the distance from the aperture to the focal point. The classical result is shown to be based on the Debey approximation and the paraxial approximation. Compared with the classical result, the new result shows in inward shift of the point of maximum intensity on-axis. In the classical focal plane the new result is identical to the classical one, except for a parabolic phase factor.

Evaluation of Diffraction Integrals Using Local Phase and Amplitude Approximations

An efficient method for computing diffraction integrals is presented. It is based on an idea put forward by Hopkins [1]. The integration domain is divided into subdomains, in each of which the phase and amplitude are approximated by simple functions which make it possible to evaluate the resulting integral in terms of known functions. While Hopkins employed a linear approximation to the phase and a constant approximation to the amplitude, we here approximate both the phase and the amplitude by parabolas. A comparison of the results of our method with those of Hopkinss method shows that our method requires fewer subdomains and less computation time to yield a desired accuracy. Another advantage of our method is that it can be applied to apertures of a general shape without significant loss of accuracy.

Validity of diffraction tomography based on the first Born and the first Rytov approximations

Using computer simulations we examine the ranges of validity of the first Born and first Rytov approximations employed in diffraction tomography. To that end we apply the filtered backpropagation(FBP) algorithm in conjunction with the first Born approximation and the hybrid FBP algorithm in conjunction with the first Rytov approximation. We find that the range of validity of the first Born approximation is approximately 3 times smaller than that of the first Rytov approximation and that the range of validity of each approximation can be expressed in terms of the product of the refractive-index difference between the object and the background and the size of the object. Also, we establish precise criteria for the validity of diffraction tomography within each of these two approximations. For the first Rytov approximation the validity of the hybrid FBP algorithm is found to be limited by phase-unwrapping problems.

Focusing of two-dimensional waves

Two different methods are presented for efficient computation of two-dimensional wave fields in focal regions. Both methods are valid for arbitrarily large relative apertures. One method is based on the impulse-response integral and the other on the angular-spectrum representation. The latter method is used to analyze the discrepancy between applying the Kirchhoff or the Debye assumption to obtain an approximation for the field in the aperture. Two cases of idealized incident waves are analyzed in detail. First, we treat the case of a perfect incident wave, i.e., a wave that, in the limit of an infinitely large aperture, would produce a δ-function field distribution on the focal line if account were taken of evanescent waves. Second, the incident wave is taken to be the field radiated by a point source and subsequently focused by a lens that delays the phase of the incoming wave in a perfect manner without influencing its amplitude. The latter wave has the same phase distribution over the aperture as the perfect wave, but a different amplitude distribution.

Scattering of electromagnetic waves by spheroidal particles: a novel approach exploiting the T matrix computed in spheroidal coordinates.

A method other than the extended-boundary-condition method (EBCM) to compute the T matrix for electromagnetic scattering is presented. The separation-of-variables method (SVM) is used to solve the electromagnetic scattering problem for a spheroidal particle and to derive its T matrix in spheroidal coordinates. A transformation is developed for transforming the T matrix in spheroidal coordinates into the corresponding T matrix in spherical coordinates. The T matrix so obtained can be used for analytical calculation of the optical properties of ensembles of randomly oriented spheroids of arbitrary shape by use of an existing method to average over orientational angles. The optical properties obtained with the SVM and the EBCM are compared for different test cases. For mildly aspherical particles the two methods yield indistinguishable results. Small differences appear for highly aspherical particles. The new approach can be used to compute optical properties for arbitrary values of the aspect ratio. To test the accuracy of the expansion coefficients of the spheroidal functions for arbitrary arguments, a new testing method based on the completeness relation of the spheroidal functions is developed.

Monte Carlo and discrete-ordinate simulations of irradiances in the coupled atmosphere-ocean system

We compare Monte Carlo (MC) and discrete-ordinate radiative-transfer (DISORT) simulations of irradiances in a one-dimensional coupled atmosphere-ocean (CAO) system consisting of horizontal plane-parallel layers. The two models have precisely the same physical basis, including coupling between the atmosphere and the ocean, and we use precisely the same atmospheric and oceanic input parameters for both codes. For a plane atmosphere-ocean interface we find agreement between irradiances obtained with the two codes to within 1%, both in the atmosphere and the ocean. Our tests cover case 1 water, scattering by density fluctuations both in the atmosphere and in the ocean, and scattering by particulate matter represented by a one-parameter Henyey-Greenstein (HG) scattering phase function. The CAO-MC code has an advantage over the CAO-DISORT code in that it can handle surface waves on the atmosphere-ocean interface, but the CAO-DISORT code is computationally much faster. Therefore we use CAO-MC simulations to study the influence of ocean surface waves and propose a way to correct the results of the CAO-DISORT code so as to obtain fast and accurate underwater irradiances in the presence of surface waves.

Asymptotic approximations to angular‐spectrum representations

Under rather general conditiosn a time‐harmonic wave field u (x,y,z) can be represented in a half‐space z≳0 by a double integral known as the angular spectrum of plane waves. The representation divides naturally into the sum of two double integrals, one of which (uH) is a superposition of homogeneous plane waves and the other (ui) is a superposition of inhomogeneous plane waves. We obtain asymptotic approximations to u (x,y,z), uH, and UI valid when the point of observation of the field recedes towards infinity in a fixed direction through a fixed point. The results apply when the spectral amplitude of the plane waves belongs to a specific class which arises frequently in applications. Our approach is based on the method of stationary phase, which we extend in order to permit the presence of inhomogeneous waves in the integrand. Although the analysis of u requires that we distinguish the directions that are perpendicular to the z axis from the directions pointing into the half‐space z≳0, the results for t...

Experimental examination of the quantitative imaging properties of optical diffraction tomography

Optical diffraction tomography (ODT) is examined experimentally by reconstruction of a cross section of a transparent, cylindrical object with known geometry and refractive index. The object is immersed in three liquids of different refractive index to produce varying degrees of weak scattering. In each case the reconstructed size, shape, and refractive index are in good agreement with the known characteristics. This shows that quantitative images of transparent, cylindrical objects can be obtained by ODT. The images obtained from experimental data were also shown to be in good agreement with those obtained from computer-simulated data, thus verifying our computer simulations.

Application of the extended boundary condition method to homogeneous particles with point-group symmetries

The numerical evaluation of surface integrals is the most time-consuming part of the extended boundary condition method (EBCM) for calculating the T matrix. An efficient implementation of the method is presented for homogeneous particles with discrete geometric symmetries and is applied to regular polyhedral prisms of finite length. For such prisms, an efficient quadrature scheme for computing the surface integrals is developed. Exploitation of these symmetries in conjunction with the new quadrature scheme leads to a reduction in CPU time by 3 orders of magnitude from that of a general EBCM implementation with no geometry-specific adaptations. The improved quadrature scheme and the exploitation of symmetries account for, respectively, 1 and 2 orders of magnitude in the total reduction of the CPU time. Test results for scattering by rectangular parallelepipeds and hexagonal plates are shown to agree well with corresponding results obtained by use of the discrete-dipole approximation. A model application for various polyhedral prisms is presented.

Validity of the diffusion approximation in bio-optical imaging

Accurate numerical simulations based on rigorous radiative transfer theory are used to assess the validity of the diffusion approximation that is frequently used in bio-optical imaging. These simulations show that the error is large for a non-index-matched boundary between air and tissue. This weakness of the diffusion approximation underscores the need to understand how diffusion theory can be used to extract accurate values of tissue optical properties. A validity criterion for the diffusion approximation is established on the basis of the single-scattering albedo a and the asymmetry factor g for a slab with index-matched boundaries.