Light is reflected back to the sensor by the surface of the fluid. Optical fibers transmit reflected light from the sensor to the detector. This system responds quickly to changes in thickness because it is based upon light and a fast-acting detector. Application of the film thickness sensor. Principle of Operation Figure 2 shows the configuration of the film thickness sensor.
Light enters through the transmitting fibers which are placed around the perimeter of the sensor.
A reflector forms the light into a cone above the sensor. Light is reflected downward by the liquid surface because of the difference in the index of refraction between the liquid and air. Air bubbles in the fluid reflect the light. Immersed droplets of another fluid with a different index of refraction also reflect the light as do solid particles. The change in the index of refraction of a fluid passing over the sensor can be detected.