Photon Density Wave Spectroscopy

We use intensity modulated laser sources throughout the visible and near infrared spectral range with modulation frequencies from 10 MHz well beyond 1 GHz to generate Photon Density Waves in strongly light scattering materials.

Detection and analysis of PDWs in terms of amplitude and phase with respect to modulation frequency and source-detector distance allow for the precise determination of fundamental optical properties of the material under investigation, namely the absorption and reduced scatttering coefficients. Whereas the first provides access to chemical concentrations, the latter allows for the determination of concentrations or particle and droplet sizes of the dispersed phase without any dilution or calibration.

The analysis is based on radiation transport theory, Mie theorie and several approaches concerning dependent scattering. Don't worry - we know about the details!
  • Intrinsically safe electronics-free, fibre-optics only probe for installations in critical environments (e.g. ATEX)
  • Laser power < 15 mW
  • Typical probe materials: stainless steel, glass, PTFE. Customer specific development possible
  • Probe diameter depends on application. Typical range is 15 - 25 mm
  • Time resolution: approx. 2 per minute
  • Size range: approx. 50 nm - 500 µm (diameter)
Highly turbid liquid dispersions can by analyzed (imagine a glass of milk). Typically we investigate liquid suspensions or emulsions with a content of particles or droplets in between 0.1 % to more then 50 %. In contrast to many other technologies, PDW Spectroscopy is not limited by too strong light scattering. In more than ten years academic research we never received a sample which needed dilution!

See our list of scientific contributions for more theoretical background and examples of investigated materials and processes (Literature).



Calibration Standards: Limitations of turbidity process probes and formazine as their calibration standard by M. Münzberg, R. Hass, N. Duc Khanh, O. Reich (open access available)

PNIPAM transition: Process analytical approaches for the coil-to-globule transition of poly(N-isopropylacrylamide) in a concentrated aqueous suspension by P. Werner, M. Münzberg, R. Hass, O. Reich (open access available)

Biogenic dispersions: Optical monitoring of chemical processes in turbid biogenic liquid dispersions by Photon Density Wave spectroscopy by R. Hass, D. Munzke, S. Vargas Ruiz, J. Tippmann, O. Reich

Particle sizing: Particle sizing in highly turbid dispersions by Photon Density Wave spectroscopy by L. Bressel, R. Hass, O. Reich

In-line particle sizing: Industrial applications of Photon Density Wave spectroscopy for in-line particle sizing by R. Hass, M. Münzberg, L. Bressel, O. Reich

PIT Emulsification:In-line Characterization of Phase Inversion Temperature Emulsification by Photon Density Wave Spectroscopy by M. Münzberg, R. Hass, O. Reich

Monitoring of Milk: Optical monitoring of milk fat phase transition within homogenized fresh milk by Photon Density Wave spectroscopy by S. Vargas Ruiz, R. Hass, O. Reich

Polymerization: Photon Density Wave Spectroscopy for Dilution-Free Sizing of Highly Concentrated Nanoparticles During Starved-Feed Polymerization by R. Hass, O. Reich

Emulsification: Sensing emulsification processes by Photon Density Wave spectroscopy by O. Reich, L. Bressel, R. Hass

Review: Inline-Partikelgrößenmesstechniken für Suspensionen und Emulsionen by R. Hass, D. Munzke, O. Reich


Bidispersity: Photon Density Wave spectroscopy: Bidisperse Systems by L. Bressel, J. Wolter, O. Reich

Monte-Carlo Simulations: Theoretical and experimental study of the diffuse transmission of light through highly concentrated absorbing and scattering materials. Part I: Monte-Carlo Simulations by L. Bressel, O. Reich

Photon Density Wave spectroscopy: Fiber-optical sensing in turbid media by Photon Density Wave Spectroscopy by R. Hass, S. Vargas Ruiz, O. Reich

Photon Density Wave spectroscopy: Optical sensing with photon density waves: Investigation of model media by O. Reich, H.-G. Löhmannsröben, F. Schael