2T21. Speckle Interferometry 405 / -- -- - - - -- - - - - - - -- - - - - REDUCTION OF PSELIDO VIBRATIONS IN ROTATIONAL MEASUREMENTS, USING SENSOR ARRAYS M.L. Jakobsen and S.G. Hanson Department of Optics and Plasma Research. Risoe National Laboratory, DK-4000 Roskilde, Denmark. michaeI.Iinde.jakobscn@risoe.dk and steen.hanson@risoe.dk Angular velocity measurements obtained with optical technologies, using coherent sources, such as laser Doppler velocimetry (LDV) (Halliwell et a[. [I]) and speckle shearing velocimetry (SSS) (Takai ef al. [I]), are often contaminated by a noise phenomenon called pseudo vibrations (~~thberg et a/. [2]). Pseudo vibrations are minor fluctuations appearing in the angular velocity measurements, and caused by laser speckles and their influence through the measurement algorithm. In essence, they appear as the speckle pattern repets itself for each revolution, and the processing algorithm will repeat the same "errors" not connected to a real variation in the rotational speed but not being distinguishable from a real torsional vibration. Therefore, they appear as a noise contribution, which strongly correlate with the angular position of a rotating target, through the fine structure of its surface. It is often observed in e.g. the power spectrum of angular velocity time series, where it will appear as a comb function with harmonics of the fundamental frequency of the rotating target. Therefore, pseudo vibrations will interfere particularly with any measurements of fluctuations in angular velocity, which correlate to the cycle of the target rotation. In the light of the option of using compact, high-performance sensors at low- cost prices, the prospect of implementing sensor with multiple laser sources or even sensor arrays appears. Therefore, this paper will study these new practical opportunities to reduce speckle- related noise. A compact, high-performance optical sensor could be based on optical spatial-filtering velocimetry (Jakobsen and Hanson [I]) of the dynamical speckles arising from scattering off a rotating solid object with a non-specular surface. A schematic drawing of the sensor is illustrated in Fig. 1. The speckles are produced when illuminating a non-specular target surface with coherent light. The scattered Iight propagates through a Fourier-transforming optical arrangement to the sensor inlet (Rose ef 01.' [I). The sensor for measuring speckle velocity is here based on a lenticular array and implements narrow spatial band-pass filtering of thc intensity distribution of the translating speckle patterns. The spatial filter extracts a given spatial frequency in terms o f a quasi-sinusoidal intensity distribution. The quasi-sinusoidal spatial intensity distribution is monitored at various phase steps by an arrangement of a couple of photodetector pairs. The photodetector signals have a similar quasi-sinusoidal response as a function o f the target angle and allow for simple processing schemes based on e.g. zero-crossing detection. Zero-crossing detection of the frequency of the signals provides real-time measurements of the angular velocity of a rotating target. Fig. 2. demonstrates the pseudo vibrations. The power spectra of two angular velocity timc series, obtained over several cycles of target rotation, are plotted while probing a surface when it is dry and when it is intentionally coated with a thin film of oil. In the first case, the surface will show a clear "finger print" for each cycle in the time signal, and this appears as a detrimental comb function in the power spectra. When coating the surface with a thin film of oil, the speckle patterns do not repeat themselves for each cycle, and the "finger print" of the surface is dramatically reduced. The presentation will discuss other ways to reduce the pseudo vibrations, and quantitative results will be presented. I