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.
