Army

Contact-free holographic imaging of aerosol particles

Imaging single and multiple aerosol particles in situ using digital in-line holography

Photonics

The in situ characterization of small aerosol particles is important in the determination of atmospheric aerosol composition for climate modeling and the detection of biological or chemical weapons agents for defense applications.

Measurements and calculations of some single and multiple-particle scattering patterns are known and there is ongoing effort to infer information relating to the particles’ physical form, such as size and shape, by analyzing the angular structure of these patterns.

Unfortunately, a limitation of this approach is the absence of a definite quantitative relationship between a pattern and the corresponding particle properties. Consequently, the inference of these properties from the patterns has proven to be very difficult in practice, except for the simplest of cases. Ideally, one would prefer to image the particles directly, thus eliminating the complexity and ambiguity associated with the interpretation of the scattering patterns but this is not fully possible given the particle size and other factors.

Army researchers have developed a particle characterization technique using holography to form images of particles, rather than their scattering patterns. A particle’s far-field scattering pattern interferes across the surface of an image sensor along with a reference wave having a portion of the light incident on the particle. For example, the scattered and reference wave may overlap in an in-line configuration embodiment and be separated in an off-axis configuration.

The resulting intensity distribution recorded by the image sensor is the particle’s hologram. A 3-D image of the particle can then be generated computationally from the hologram. Further computational analysis of the resulting image can be performed for detection of particle size, shape, surface roughness, refractive index, and absorption. One advantage of this technique is that there may be no further need to interpret or invert a complicated scattering pattern, as information can be retrieved directly from the image. Holography permits the reconstruction of phase, magnitude, or both, of the scattered wave pattern.

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