Logo Leibniz Universität Hannover
Institute of Turbomachinery and Fluid Dynamics
Logo Leibniz Universität Hannover
Institute of Turbomachinery and Fluid Dynamics
  • Zielgruppen
  • Suche

Laser-2-Focus Anemometry (L2F)

Field of Application 

  • Non-intrusive measurement of flow velocity and turbulent flow quantities
  • Qualitative measurement of particle sizes and particle concentrations
  • Well suited for flow channels with low optical accessibility as a result of the strong focusing of laser beams

Measured Quantities

  • Flow velocity
  • Particle concentration
  • Particle size distribution


The laser-2-focus (L2F) or laser transit method is based on the non-contact measurement of the flight time of small particles between two parallel and strongly focused laser beams. This method is based on the recording of the flight time of particles between two photoelectric sensors or light barriers. The traveling velocity of the particles and thus, indirectly, the flow velocity, are determined using the known distance between the focused laser beams and the measured flight time of the small particles crossing the laser beams. 

Technically, the L2F requires first two strongly focused laser beams in the measuring volume. The particles passing through the control volume scatter the light once they cross through the first and the second laser-focus points. These light pulses resulting from the particle’s light scattering represent in turn a start and a stop pulse, from which the time difference corresponding to the flight time can be determined. The back-scattered light from the particles is detected by the outer part of the lens system and projected at the image plane of an optical microscope. The light pulses are in turn increased by the microscope optical system and directed into to a double pinhole. A subsequent optical assembly directs the scattered light pulses of the start and stop pulses into a corresponding photodetector. The photodetectors produce a voltage signal proportional to the scattered light intensity, which is subsequently used for data processing and evaluation.

The L2F method enables the measurement of two velocity components of the velocity vector on a plane defined by the focused laser beams. The velocity measurement at a specific point within the flow field is carried out for a large number of particles. In this way, and by applying statistical analysis methods, the mean flow velocities as well as fluctuation velocity components can then be determined in turbulent flows.

Advantages of the L2F system over other optical measurement methods are:

  • Small optical access required
  • High signal-to-noise ratio
  • Small measurement volumes
  • Measurements in boundary-layer flows are possible

Independent extensions of the measurement systems carried out at TFD

  • Qualitative measurement of particle concentration
  • Qualitative measurement of particle size distribution


Dipl.-Ing. Philipp Gilge