Reflex Level Gauge
Reflex level gauges use glasses having the face fitted towards the chamber shaped to have prismatic grooves with section angle of 90°. When in operation, the chamber is filled with liquid in the lower part and gases or vapors in the upper part.
The liquid level is well-known by different brightness of the glass in the liquid and in the gas/vapor zone. The reflex level gauges do not need a specific illumination. The day light is enough for seeing the level.