We have applied our experience with digital holography to develop and instrument to image large aerosol particles from an unmanned aerial vehicle UAV, i.e., a drone. Because the drone used in this work is an inexpensive consumer-grade drone, there is a significant limitation of the weight of the instrument. Thus, we have designed the instrument's structural elements to be as compact as possible and fabricate them from polymer with a stereolithographic 3D printer. In this way, our initial prototype, called the "Holographic Aerosol Particle Imager (HAPI)," has a total weight of 3.3 kg. Laboratory and field tests have shown that HAPI is capable of imaging both single and multiple aerosol particles as they freely flow through HAPI's sensing region. Figure 1 below shows detail of the HAPI instrument along with a picture of it in-action near a pollinating tree at Kansas State University.
Figure 1: Diagram of the main components in the HAPI instrument (left), the actual instrument with its outer enclosure removed (center) and HAPI in-flight near a pollenating tree (right). Further details of the components and operation of the instrument are available in the [paper].
Consider Fig. 2 below where examples of particles observed in two field trials with the HAPI instrument are presented. These trials include flying the instrument in an agricultural setting with dirt roads and a pollinating spruce tree and the particle images (a)-(c) and (e)-(g) are derived from the holograms measured by HAPI. Road dust exhibits an irregular shape expected for MD while the spruce tree particles show a less complex, smooth shape. Samples of road dust and pollen from the tree are collected and imaged with a scanning electron microscope (SEM) and shown in Fig. 5 (d) and Fig. 5(h). Qualitatively, the particles in the SEM images appear very similar to those imaged by the HAPI instrument, which would support the identification of the particles in Fig. 5 (a) to Fig. 5(c) as MD and Fig. 5 (e) to Fig. 5(g) as spruce tree pollen. Indeed, the HAPI-imaged pollen particles in Fig. 5(f) show hints of the distinctive concavity, or dimple, as those seen in the SEM image and in the literature [59]. Moreover, their size is consistent with the SEM images.
Figure 2: Survey of a variety of suspended atmospheric aerosol particles imaged in situ with the HAPI instrument in the field. (a) to (c) Hologram-derived images of aerosol particles observed in the road dust field trial. (d) Scanning electron microscope (SEM) image of particles collected from the road dust site. (e) to (g) Same as (a) to (c) except for aerosol particles observed in the spruce tree trial. (h) SEM image of pollen particles collected from the spruce tree. Each image shows a different field of view and all scale bars are 100 micrometers.
Below are two videos showing the HAPI tests presented in Fig. 2 above. The first video is from the spruce tree trial and the second is from the road dust trial.