A board with a stripe pattern is inserted in the innermost part of the doughnut-like tank as shown in Figure 3. A 22.5-lines/inch, 50% screen tone was pasted onto the board. When this board is seen through the tank while the fluid is in motion, the pattern is distorted.

Red and blue strobes are positioned behind the tank. The lights in the laboratory are put out, and the film is exposed. The red and blue strobes are flashed at intervals of half of the wave period, and the red and blue images are recorded on the same frame. In this way, the position of the two images can be synchronized.

Then, a film scanner digitizes the film. Two separate images are obtained by dividing the film into blue and red images. During this procedure, a highpass filter is applied to remove the unevenness of the flash.

The moíre stripes are created by taking the absolute difference in the values of these two images. The image of the internal gravity wave can be thus obtained.