In this project, a method to obtain contact-free images of aerosol particles with digital holography from three orthogonal directions is described and demonstrated. Diode lasers of different wavelengths simultaneously illuminate free flowing particles to form holograms on three sensors. Images of the particles are reconstructed from the holograms and used to infer the three-dimensional structure of single spherical particles or clusters of sphere-like particles. The apparatus employs inexpensive components and requires no lenses to achieve the imaging, which gives it a large sensing volume and simple design.
The apparatus is shown in Fig. 1 and achieves the imaging using three diode lasers of different wavelength. A hollow mounting cube with 25.4 mm diameter threaded holes through each face is shown in Fig. 1(a). In three of the holes that share a common cube-corner are 25.4 mm diameter lens tubes (although no lenses are used). Mounted to the ends of each tube is a diode laser (DL), each emitting at a different wavelength: 660 nm or “red” for short, 520 nm, or “green,” and 450 nm, “blue.” The beam path for the blue and green DLs can be seen in the side view in Fig. 1(b). Opposite each DL at the far cube-face is a bandpass filter, corresponding to the DL’s wavelength, followed by a board-level monochrome CMOS sensor. A hole ~1 mm in diameter is drilled through the cube along its main diagonal from one apex to another as shown in Fig. 1(a) and 1(b) by feature H. The hole allows an aerosol stream to be passed from top to bottom through the central region of the cube where the particles are simultaneously illuminated by the three orthogonal DLs and the resulting holograms are recorded.
Figure 1: Sketch of the orthographic imaging apparatus and coordinate systems. In (a), is a top view of the instrument showing the major components. Sketch (b) also shows the unscattered and scattered light from the particles that forms the holograms. Finally, (c) shows the three sensor surfaces defining the SCS, the particles, and the physical and reconstruction wave vectors.
Board-level sensors are used to allow the hologram recording to be as close to the aerosol stream as practical, which improves the eventual image resolution and gives the apparatus a small form-factor of approximately 10 x 10 x 10 cm. These sensors (FLIR, BFS-U3-50S5-BD) have an array size of 2448 x 2048 pixels, with a pixel size of 3.45 um, and a global shutter readout. Clear hologram fringes are recorded by pulsing the DLs simultaneously to emit for a 200 ns period after the electronic-shutter activation, having the effect of freezing-out the particle motion.
The purpose of the bandpass filters is to ensure that only light from the DL across the cube from a sensor reaches that sensor’s surface. In other words, the filters prevent optical “cross talk” between the sensors. This is preferable to pulsing the DLs sequentially because the likelihood that the particles would change orientation between each pulse.
Because each hologram is formed by illuminating the same group of particles from a different orthogonal direction, the 3D form of a given particle can be inferred from the 2D images, or “views,” that are reconstructed from each hologram. As will be seen below, this inference is incomplete and only approximate, yet is vastly better than what is typically achieved from a singleviewing direction in lensless DH.
Particle-image reconstruction begins with a background measurement, which is simply an exposure of the sensors to the DL pulses when no particles are present. This background is then subtracted from the same measurement when the particles are present. The result is a contrast hologram of which there are three corresponding to the orthogonal views. Both the particle-free and particle-present exposures are obtained with synchronized DL pulses ~200 ns in duration. Each contrast hologram is then used in the Fresnel-Kirchhoff integral with the Fresnel approximation to render images of the particles from the three viewing directions.
Figure 2: Orthographic imaging of a ragweed pollen aerosol. Image (a) shows the contrast holograms and (b) shows the particle silhouettes generated from those holograms for a single flowing particle-cluster. To provide more detail, (b) also shows different perspectives of the blue and green views to highlight different aspects of the cluster's structure.
An example is presented in Fig. 2 where a powder of dried, dead ragweed pollen grains is used as the particle source. These are sphere-like particles approximately 20-30 um in diameter with an echinate surface that promotes clustering of individual grains. The contrast holograms are presented in Fig. 2(a) and the resulting views are shown in Fig. 2(b). A circle enclosing the silhouette of a single cluster is drawn in each view and the views are then positioned in 3D space via the circles’ centers. Once positioned, 22 um diameter spheres are placed in 3D such that their projections onto the image planes approximately agree with the silhouettes. This sphere size is determined from the blue and green views where outlines of individual grains in most of the cluster can be discerned.
The utility of this imaging approach can be appreciated from Fig. 2(b) in that if one were provided a single view only, it would be unlikely to generate a reasonable 3D rendering of the full particle-cluster. Consider the red view in Fig. 2(b) as an example. While this view gives an approximate sense for the overall size of the cluster, is does not clearly reveal that the particle is a cluster of spherical grains like the other views do. Indeed, from the red view alone, one may not realize the comparatively long extent of the cluster along the z-axis (axial direction for this view). For further detail about this project, see the [paper].