Abstract

A theoretical model is presented for coherent imaging of a spherical aerosol particle illuminated by an arbitrary incident electromagnetic field propagating obliquely with respect to the imaging system axis (i.e., off-axis imaging). Analytical simplification of the theoretical development is presented for the important special case of plane-wave illumination. The modeling technique uses the arbitrary beam theory, presented by the authors in an earlier paper [ J. Appl. Phys. 64, 1632 ( 1979)], to calculate the external electromagnetic field resulting from the interaction of the incident field with the aerosol particle. Scalar diffraction theory is used to propagate the dominant electric field component through the imaging lens and to the image plane. Glare spot calculations generated by using the theoretical model are presented for both opaque and transparent particles with size parameters (ratios of circumference to wavelength) ≈ 94 illuminated by both off-axis focused-beam- and plane-wave-incident fields. For transparent aerosols in this size range, the glare spot images are dominated by the specular reflection peak and the edge glare spot that results from light incident at grazing angles. Glare spot experimental data obtained for tightly focused pulsed laser (λ = 610–630 nm, tpulse ≈ 150 fs, 2w0 ≈ 20 μm) illumination of 70-μm-diameter water droplets showed qualitative agreement with beam calculations performed with the theoretical model. Theoretical images corresponding to partial-wave-resonance conditions showed a radial shift in the location of the glare spot peaks as well as an increase in glare spot intensities.

© 1992 Optical Society of America

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