The detector of the IMST/SA‐CNRS improved Lyman‐alpha hygrometer is a solar‐blind photomultiplier tube that ensures large signal‐to‐noise ratio and allows a miniaturization of the sampling volume. This PM tube has a relatively large spectral response and can detect, in addition to the Lyman‐alpha line, hydrogen molecular transitions emitted by the discharge source between 120 and 180 nm. In order to avoid a spurious sensitivity to oxygen density fluctuations, an active filtering cell is added to absorb the undesired lines. The instrument sensitivity is calculated with the help of a two‐gas, three‐spectral‐line UV‐beam absorption model, fitted to calibration measurements. The technique of the active filtering associated with the three‐line absorption model produces the following advantages: (1) reduced sensitivity to oxygen fluctuations, (2) a clear physical meaning to the model terms allowing the sensitivity computation in most conditions, (3) increased signal‐to‐noise ratio, and (4) in most conditions, improved sensitivity to water vapor compared to the monochromatic Lyman‐alpha line.