Uses of light based sensing technologies all have some dependence on the color, clarity and gauge of the film.  The more clear and thin the film, the more difficult it is for infrared, laser and camera based sensors to detect the position of the bubble.  Ultrasonic based sensing technologies have no dependency on the color or thickness of the film.  When used with a temperature compensation (internal or with another sensor), ultrasonic sensors have proven to be the most appropriate non-contact sensing devices for Internal Bubble Cooling.