Real-time grey level display of velocity distributions on a surface

Takuso Sato, Tadao Kishimoto, and Yutaka Nakatani Faculty of Science & Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo, Japan. Received 22 April 1975.

In many fields, such as in the analysis of transient states of a moving object, there are ardent requirements to ob­serve whole velocity distributions on a surface in real time. In a previous Letter,1 a method of real-time display of a constant velocity region on a surface as bright parts of its image on a TV monitor was demonstrated. In this Letter, the previous method is developed by adding suitable elec­tronic devices. A system is presented that has the ability of displaying the whole velocity distribution on a surface as grey levels of a image real-time. The construction of the system and some experimental results are shown.

The basic idea of the system is to switch sequentially the amount of the Doppler shift to be detected and simulta­neously the brightness of the image on the TV monitor in synchronism to the vertical scan of the video camera.

A schematic diagram of the system is shown in Fig. 1. The operations of the system are as follows. Two ultrason­ic frequency shifters are used to give the frequency offset between the object illuminating and the reference light. Then on the image plane of the moving object there appear stationary interference fringes, only at those regions where the previous offset is just compensated by the Doppler shift. Since only the stationary fringes are picked up by the video camera, electrical circuits then given constant ve­locity regions can be discriminated from other regions. Now the amount of the frequency offset and the brightness of the image on the TV monitor are sequentially switched in synchronism to the scanning of the video camera, and the results are displayed. Then the grey level display of the whole velocity distribution on a surface is realized at every certain number of the video scanning time.

The amount of the frequency offset is controlled by ad­justing the driving frequency of one of the ultrasonic fre­quency shifters, and the brightness is adjusted by the vari­able gain amplifier inserted just before the TV monitor. The amounts of the offset are adjusted according to the ve­locities to be observed, and the brightness levels are chosen beforehand so the discrimination between neighboring lev­

els can be done most clearly. The saturated amplifier is also used at the output of the bandpass filter to give con­stant grey level for the regions close to the given constant velocity regions.

The details of the control circuits are shown in Fig. 2. The frequency is controlled by adjusting the bias of the crystal oscillator, the available maximum frequency offset is ± lkHz, the maximum number of levels is 6, and any fre­quency shift in this region can be set for each level. Some experimental results are shown in Figs. 3 and 4. The objects are oiled rotating ground glass disks. The observed regions and the direction of the observed velocities are shown in the left illustrations of the figures. In the middle of the figures the theoretically expected grey level distribu­tions determined from the given frequency offset and the optical arrangements are shown, and on the right of those figures the pictures taken from the TV monitors are shown. In these experiments the number of grey levels was 4. These results show the effectiveness and the usefulness of the system. For example, in Fig. 4(f) we can see clearly the difference of the intervals of constant velocity regions be­tween the upper disk and lower disk due to the difference of the rotating speed, as well as the opposite direction of the changes of the shades of the grey levels.

Fig. 1. Optical and electronic system of real-time grey level dis­play of velocity distributions. AMP, amplifier; Volt. Cont. Osc,

voltage controlled variable oscillator; BP Filter, bandpass filter.

Fig. 2. Control circuits for the amount of the frequency shift and the intensity of the image.

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Fig. 3. Experimental results (I). Objects and the observed re­gions with the direction of the observed velocity are shown on the left, and the theoretically expected grey levels are given in the middle. Pictures taken from the monitor are shown on the right.

Fig. 4. Experimental results (II). The explanations of the figures are the same as for Fig. 3.

In our system the frequency offset is changed in synchro­nism to every scan of the video camera, and the speed of the scan is 50 frames/sec. Thus when η{η < 6) levels are adopted, the whole velocity distribution can be displayed at every η/50 sec.

The display of velocity components of different direc­tions by distinct colors may give more interesting results. And the method presented in this Letter may also be ap­plied to the general Doppler imaging system recently dis­cussed.2

References 1. T. Sato, Y. Nakatani, and M. Ueda, Appl. Opt. 13, 2759 (1974). 2. C. C. Aleksoff and C. R. Christensen, Appl. Opt. 14, 134 (1975).

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