A Langmuir probe (LP) measures currents from incident charged particles as a function of the applied bias voltage. While onboard a spacecraft the particles are either originated from the surrounding plasma, or emitted (for example, through photoemission) from the spacecraft itself. The obtained current-voltage curve reflects the properties of the plasma in which the probe is immersed into, but also any photoemission due to illumination of the probe surface: as photoemission releases photoelectrons into space surrounding the probe, these can be recollected and measured as an additional plasma population. This complicates the estimation of the properties of the ambient plasma around the spacecraft. The photoemission current is sensitive to the EUV part of the spectrum, and it varies with the illumination from the Sun and the properties of the LP surface material, and any variation in the photoelectrons irradiance can be measured as a change in the current voltage curve. Cassini was eclipsed multiple times by Saturn and the main rings over its 14-year mission. During each eclipse the LP recorded dramatic changes in the current-voltage curve, which were especially variable when Cassini was in shadow behind the main rings. We interpret these variations as the effect of spatial variations in the optical depth of the rings and hence use the observations to estimate the optical depth of Saturn's main rings. Our estimates are comparable with UV optical depth measurements from Cassini's remote sensing instruments.