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11 Non-intrusive Measurement Techniques
In laser Doppler technique a laser beam is split into two beams and using a
focussing lens they are set to cross at a point to create a measurement volume within
the region of interest in the flow. This is shown schematically in Fig. 11.5. Due to
the high coherence of a laser beam, a fringe pattern is created at the crossing of the
two beams within the measurement or probe volume. As shown in Fig. 11.6, while
Fig. 11.5 Laser beams crossing forming the interference fringes of the emission part
(a) The fringe pattern created by the crossing of the laser beams
(b) The light scattered by a particle crossing a bright fringe
Fig. 11.6 The fringe pattern within the measurement volume (© ONERA)
11 Non-intrusive Measurement Techniques
In laser Doppler technique a laser beam is split into two beams and using a
focussing lens they are set to cross at a point to create a measurement volume within
the region of interest in the flow. This is shown schematically in Fig. 11.5. Due to
the high coherence of a laser beam, a fringe pattern is created at the crossing of the
two beams within the measurement or probe volume. As shown in Fig. 11.6, while
Fig. 11.5 Laser beams crossing forming the interference fringes of the emission part
(a) The fringe pattern created by the crossing of the laser beams
(b) The light scattered by a particle crossing a bright fringe
Fig. 11.6 The fringe pattern within the measurement volume (© ONERA)