The mica two-color level meter is utilizing the different  reflective index and reflection characteristics of the light presenting in different medium, by means of the color filter to make liquid phase in green color and make the gas phase in red color to display the steam drum water level. The light from the light source goes through the red and green color filter and towards the fluid chamber of water level gauge proper. In gas phase section inside the chamber, the red light radiates to the  right ahead and the green light slants to the chamber wall and  is absorbed; In liquid phase section of the chamber, due to the refraction of water, the green light radiates at the right ahead, and the red light slants to the chamber and is absorbed. Therefore from observation of the right front, you can see the  water level gauge in gas-red and liquid-green, it is clear and transparent.