How does Fraunhofer holography help measure small particles?

Fraunhofer holography is a specialized technique used to measure the size distribution of small particles, such as dust and liquid droplets. It functions by recording the interference pattern created between light diffracted by the particles and the coherent background light that passes directly through the sample.1

Because these particles are often in motion, the hologram must be captured instantaneously, typically using a pulsed-ruby laser. This process effectively transforms a transient event into a stationary image, allowing the particles to be examined at leisure during reconstruction.1

The technique is particularly effective because it utilizes the far-field diffraction pattern of the object. By operating in the far field, the deleterious effects of the twin images—a common issue in in-line Fresnel holography—are minimized, providing clearer data for particle-size analysis. In this sense, the application functions as a form of microscopy, enabling precise evaluation of objects that would otherwise be difficult to measure.21