Conventional Digital In-Line Holography (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 in such silhouettes. Based on the recognition that the spectral reflectance of a surface is partly determined by the particle material, we have demonstrated a method, see Fig. 1 below, to image free-flowing particles with DIH in color with the eventual aim to differentiate materials based on the color observed. 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 then reconstructed from the holograms and layered to form a color image, the color content of which can then be quantified by chromaticity analysis to establish a material-representative signature. We call the method Multi-Wavelength Digital Holography (MWDH). The study here applied the method to a variety of mineral dust aerosols where the different chromaticity signatures suggest the possibility to differentiate particle material.
Figure 1: Optical design used to record color backscatter-holograms of free-flowing aerosol particles. Pulsed red (656 nm), green (527 nm), and blue (440 nm) laser beams are combined into a white beam, which is split by the BSC to form incident and reference beams. A Mach-Zehender configuration is used to interfere the light backscattered by an aerosol particle in the sensing volume with the reference beam at the sensor. The particles are delivered to the sensing volume via a sheath-flow nozzle fed by an aerosol generator, (not shown). An optical trigger system (dashed outline) senses when a particle is present in the volume and supplies a signal to activate emission of a ~300 ns pulse from each laser and begins the sensor exposure. This is achieved by illuminating the sensing volume by a CW trigger beam, which scatters from any aerosol particle present. The scattered light (B4) is separated from unscattered light by the mirror SFM and then sensed by the PMT to create the trigger signal. See [here] for further description.
Figure 2 below presents the chromaticity analysis of the backscatter MWDH images of mineral dust aerosols. First consider the olivine category. This mineral is generally light yellow-green in appearance, which can be seen in the framed inset image. The other insets in Fig. 2 are the backscatter MWDH images of individual aerosol particles that trigger hologram measurements at various times in the experiment. The chromaticity signatures of these images for each mineral category are also shown. For the olivine, note how the signature is clearly in the yellow-green portion of the color space. The signatures here also overlap much, which indicates that the images possess similar color content despite the variation in overall brightness of the images shown.
Figure 2: Chromaticity analysis of a variety of free-flowing mineral dust aerosol particles obtained with the arrangement in Fig. 1 above. Each panel shows the backscatter MWDH image of the particles associated with their chromaticity signatures. Each particle-image corresponds to a separate measurement. A 100 um scale bar is shown along with an optical microscope image, in frame, of a representative particle taken from the stock powder from which the aerosols are generated.