Friedrich-Alexander-Universität Erlangen-Nürnberg

in situ process diagnostics

Introduction: In situ optical process diagnostics

Optical measurement techniques feature some advantages over conventional measurement techniques.


Advantages of in situ optical diagnostics:

  • Non invasive probing à no alteration of the sample
  • Measurements with high repetition rate à transient mechanisms can be resolved
  • High temporal resolution à fast processes can be frozen temporally
  • High spatial resolution à gradients can be resolved, even during one measurement event, if line or chip detectors are used


On this website the fundamentals of measurement techniques are elucidated, which are frequently applied within the ARS-Laboratory to study chemical engineering tasks.


In the figure below the grey disc represents the process under investigation, while the light bulb represents any kind of a light source and the black bars represent detectors.

The incident light is used to excite the matter inside the process. Due to the interaction of light and matter, the characteristics of the incident light are changed with respect to:

  • Irradiance
  • Wavelength
  • Light pulse duration
  • And phase


The change of these characteristics can be probed by detecting either the transmitted or the scattered and emitted light. The characteristics of the matter inside the process can be extracted from the comparison of the characteristics of the incident and the probed light (transmitted, scattered, emitted). The information extractable is:

  • Density
  • Temperature
  • Composition
  • pH-value
  • Velocity
  • Distance
  • Pressure
  • Interfaces


Additional information:

  • Light emitted from the light source is send to the process (incident light).
  • That portion of the incident light, which passes through the process is termed “transmitted light”.
  • Another portion of the incident light might be blocked, deflected, scattered or absorbed on its way through the process.
  • The Schlieren-Technique can filter photons which were deflected from their original way and hence qualifies to visualize gradients of the refractive index (line in sight measurement technique).
  • Simple shadowgraphy visualizes shadows causative to either deflection, scattering or blocking of photons (line in sight measurement technique).
  • Elastic light scattering techniques detect the photons which were scattered from either molecules or phase boundaries. In this context “elastic” means, that the scattering process did not go along with an energy transfer between the photon and the scatterer. Hence the energy (“color”) of the incident the scattered photon are the same (Imaging technique with spatial resolution).
  • Inelastic light scattering techniques detect the photons which were scattered from molecules inelastically. In this context inelastic means, that during the scattering process an energy transfer took place between the involved photon and the scatterer. Hence the energy (“color”) of the incident and the scattered photon are different (Usually spectroscopic technique with 0-D, 1-D, or even 2-D spatial resolution).
  • The incident photons can be absorbed by the matter inside the process, if the energy of the incident photons corresponds to energy transition of the involved molecules (line in sight measurement technique).
  • If the absorption of the photon caused an electronic transition, the absorption event is sometimes followed by an emission event (spectroscopic technique with 0-D, 1-D, or even 2-D spatial resolution)
  • Other light matter interaction mechanisms (non linear mechanisms) are not considered here.