Digital Holography for Particle Diagnostics

MetroLaser has developed Digital Holographic Technology (DHT) for characterizing the size and shape of particles from about 10 μm to over 1,000 μm. DHT uses a compact detector and laser with low power requirement. This makes it suitable for deployment on a variety of unmanned airborne vehicles (UAVs).

DHT is based on a digital scanning holographic camera. That is, the holographic system moves across the particle field and records a hologram that may be tens of meters long. This allows the object field to sweep the diffraction pattern over a linear detector array, a principle that is already proven in digital holography. The linear detector array samples and digitizes the diffraction pattern of the particles as they move over the array, producing the same information that can be captured on a two-dimensional array, but doing it continuously. Linear arrays can be operated much faster than two-dimensional arrays, are easier to protect using smaller windows, and require less weight and power to operate. Data from extremely large sample volumes can be stored on a small memory card and processed by computer later.

The DHT may also be employed in a snapshot mode using a conventional CCD sensor. In this configuration that laser/sensor takes pictures of the particle field, which are recorded as holograms on the CCD. The underlying principle of DHT is in-line holography.

Some of the applications for the DHT are meteorology experiments from an airborne platform, numerous commercial applications associated with sprays such as combustion engine injector design, agricultural spray characterization, and the characterization and design of spray fire extinguishers.

The figure below shows a schematic of the DHT.

Digital holography particle sizing system
Digital holography particle sizing system

The next figure shows some sample data collected and analyzed with the digital holography system.

Holographic reconstruction of 40Ám (left), 20Ám (middle) and 15Ám (right) particle images.
Holographic reconstruction of 40μm (left), 20μm (middle) and 15μm (right) particle images.