Another method visualizes the internal gravity wave using the moíre method. This method utilizes the property of light that passes horizontally through a stratified fluid is refracted to the opposite direction of the density gradient. This method is much more inexpensive than the Shrearen method.

In principle, when a stripe image distorted by the density disturbance is overlaid with an undistorted image, moíre stripes appear showing the pattern of the internal gravity wave.⁶⁾  In this experiment, however, the curvature of the tank also distorts the image, and therefore, a simple application of this method produces moíre stripes even when the working fluid is at rest. For this reason, instead of using an undistorted image, a distorted image of the stripe pattern is overlaid with an image in a different phase. This overlaying process was performed with a computer after scanning the recorded film.

Figure 3 Top view of the photography system