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Modeling of the coherent imaging of small aerosol particles
Abstract
A theoretical modeling technique has been developed for calculation of defocused aerosol images formed in a laser imaging system. Two cases of interest have been examined. On-axis imaging occurs for situations in which an incident Gaussian beam propagates along the axis of the imaging system. The more general off-axis imaging case allows an incident plane wave to propagate at any angle with respect to the imaging system axis. The modeling technique has made use of the arbitrary beam theory to calculate the electromagnetic field present at the aperture of the imaging system. Propagation through the imaging lens is examined using a thin lens model and a vector approach, the latter accounting for propagation through the imaging lens exactly. The Fresnel propagation equation is then used to calculate the intensity in the image plane. The technique has no restrictions on the optical properties of the particle or on the degree or direction of defocus. Comparison to experiment has been made for defocused images of water and nickel aerosols, obtained using a nitrogen laser imaging system. Results showed good agreement in the intensity distributions for positions off the optical axis. By comparing results using the thin lens expression to those obtained using the vector approach, it was concluded that the errors appearing near the optical axis result primarily from errors introduced by the Fresnel propagation equation. Calculation of aerosol properties relevant to particle sizing have provided justification for the use of average internal intensity and boundary edge gradient for characterization of aerosol focus. Off-axis calculations for transparent and absorbing particles provided results consistent with expected physical behavior and previously verified theories. The model has correctly predicted the existence of intensity peaks occurring on both the illuminated and shadow hemispheres of particles illuminated at 90 degrees with respect to the imaging system axis. The peak on the illuminated hemisphere results from reflection at the particle surface, while the broad peak near the shadow surface is formed from light propagating within the particle.
Subject Area
Mechanical engineering|Optics
Recommended Citation
Schaub, Scott Alan, "Modeling of the coherent imaging of small aerosol particles" (1990). ETD collection for University of Nebraska-Lincoln. AAI9118474.
https://digitalcommons.unl.edu/dissertations/AAI9118474