A trap bowl is provided to accumulate liquid droplets from a filter, as a liquid content. The trap bowl includes a transparent vertical prism. The transparent vertical prism includes a face that forms a vertical transparent surface facing against a content of the section. The face can provide a first angle of total reflection when content of the section is a type of gas, and a second angle of total reflection when the content of the section is the liquid content. A light source may emit a light beam incident on the face at an angle of incidence. The angle of incidence results in reflection of the light beam, striking the light receiver, when the face has the first angle of total reflection, and results in refraction of the light beam, missing the light receiver, when the face has the second angle of total reflection.