Optical Probes

The optical probe is the part of the measurement system installed directly at or close to the process. It shapes the illumination, defines the measurement region, and detects the light scattered by individual droplets or particles. The detected scattered light is converted into time-resolved optical signals that contain information about the particles passing through the measurement region.

Signal acquisition, calculation, and evaluation are performed in the connected control unit. The optical probe and the control unit are connected by optical fibers, allowing the sensitive electronics to be positioned separately from the measurement location.

Three standard optical probes are available: LSS1, LSS2, and LSS4. They differ primarily in the number of active detector channels and therefore in the measurement and evaluation concepts they support. Each probe is operated together with a corresponding control unit.

The product family — SprayQuantAI®, ParticleTensorAI®, or TSTOF — is not determined by fundamentally different optical probes. Instead, it is defined by the signal acquisition concept, firmware, calculation method, and evaluation software. This common optical platform makes it possible to use related hardware configurations for different measurement approaches.

All optical probes are manufactured and assembled in our laboratories. Because we develop both the optical hardware and the evaluation software, the probes can be customized for specific measurement tasks. Possible adaptations include the optical configuration, working distance, mechanical geometry, probe arrangement, and integration into the customer’s process.

For more complex applications, several optical probes or measurement volumes can also be combined. Such configurations have been investigated, for example, for rotary atomizers, high-concentration slurry sprays, and trajectory measurements [1–3].

In many industrial applications, adapting the optical probe to the atomizer, spray gun, nozzle, or production equipment is therefore the next step in developing a complete measurement and monitoring solution for the customer.

Probe	Active Detector Channels	Control Unit	Used By
LSS1	1	ZEON	SQA1, PTA1
LSS2	2	IMEA	SQA2, PTA2
LSS4	4	CLEON	PTA4, TSTOF

The number of detector channels determines how much independent optical information is recorded from each droplet or particle.

In classical TSTOF evaluation, the four detector channels of the LSS4 probe provide two independently calculated velocity values and four independently calculated particle-size values for each individual particle event. The differences between these independently determined values provide information about the uncertainty of the individual measurement event rather than only a statistical uncertainty calculated from a large number of particles.

This additional information can also be used to identify and reject implausible particle events before statistical distributions are calculated.

In ParticleTensorAI® operation, the same detector channels are used differently. Instead of calculating particle properties exclusively from predefined signal features, the time-resolved signals from the detector channels are combined into the input data structure used by the AI-based evaluation. With PTA4, the four detector signals therefore provide the optical information for the ParticleTensorAI® tensor representation.

An optical probe is selected according to the complete measurement task rather than according to particle size alone. Important parameters include the expected droplet or particle size, velocity, concentration, material properties, optical properties, atomizer geometry, available installation space, and required working distance.

Working distance, optical configuration, and laser power can be adapted to the application. The intensity and temporal structure of the scattered-light signals also depend on the optical properties of the measured material. Paint droplets, transparent liquids, suspensions, slurries, and solid particles can therefore require different configurations even when their particle sizes are similar.

For industrial applications, mechanical integration is often equally important. Optical probes can therefore be adapted to spray guns, nozzles, rotary atomizers, laboratory test rigs, and production equipment. The objective is to position the measurement region where representative information about the spray or particle flow can be obtained while keeping the probe suitable for the surrounding process environment.

Send us a description of your measurement task, material, atomizer or nozzle, and expected operating conditions. Based on this information, we can propose a suitable probe, control unit, and measurement configuration.

[1] Schäfer, W. Neues optisches Messverfahren für Rotationszerstäuber. J Oberfl Techn 66, 52–53 (2026). https://doi.org/10.1007/s35144-026-3895-3

[2] Schäfer, W. Verfahren und Vorrichtung zur Bestimmung von Trajektorien und Trajektorienverteilungen von Teilchen aus einem Rotationszerstäuber zur Charakterisierung von Zerstäuberparametern und Materialeigenschaften von Lackgemischen oder ähnlichen Flüssigkeiten. Deutsche Patentanmeldung, Aktenzeichen 10 2026 117 301.2.

[3] Schaefer, W., Fleck, S., Haas, M., & Jakobs, T. (2025). Optical measurement method for monitoring high-mass-concentration slurry sprays: An experimental study. Photonics, 12(7), 673. https://doi.org/10.3390/photonics12070673