Here is just a little bit more about the working principle of what is happening.

On the right side we have our Moiré pattern and a light source. The light source is not explicit necessary, if you ambient light is bright enough.

On the left side is our camera.

As the Light now travels from our light source to the Moiré pattern it will be reflected. The dense air between the pattern and the camera will now "disturb" the light.

That is what we want to record, this disorder/interference of the light.

To do so we have captured a reference image of the pattern before and calculate now the difference of these two images.