This work applies digital holography to image stationary
micro-particles in color. The approach involves a Michelson
interferometer to mix reference light with the weak intensity light
backscattered from a distribution of particles. To enable color
images, three wavelengths are used, 430, 532, and 633 nm, as primary
light sources. Three separate backscattered holograms are recorded
simultaneously, one for each wavelength, which are resolved without
spectral cross talk using a three-CMOS prism sensor. Fresnel
diffraction theory is used to render monochrome images from each
hologram. The images are then combined via additive color mixing with
red, green, and blue as the primary colors. The result is a color
image similar in appearance to that obtained with a conventional
microscope in white-light epi-illumination mode. A variety of colored
polyethylene micro-spheres and nonspherical dust particles demonstrate
the feasibility of the approach and illustrate the effect of simple
speckle-noise suppression and white balance methods. Finally, a
chromaticity analysis is applied that is capable of differentiating
particles of different colors in a quantitative and objective
manner.
Digital in-line holography (DIH) is an established method to image small particles in a manner where image reconstruction is performed computationally post-measurement. This ability renders it ideal for aerosol characterization, where particle collection or confinement is often difficult, if not impossible. Conventional DIH provides a gray-scale image akin to a particle’s silhouette, and while it gives the particle size and shape, there is little information about the particle material. Based on the recognition that the spectral reflectance of a surface is partly determined by the material, we demonstrate a method to image free-flowing particles with DIH in color with the eventual aim to differentiate materials based on the observed color. Holograms formed by the weak backscattered light from individual particles illuminated by red, green, and blue lasers are recorded by a color sensor. Images are reconstructed from the holograms and then layered to form a color image, the color content of which is quantified by chromaticity analysis to establish a representative signature. A variety of mineral dust aerosols are studied where the different signatures suggest the possibility to differentiate particle material. The ability of the method to resolve the inhomogeneous composition within a single particle in some cases is shown as well.
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