Device for determining the degree of soiling of a surface of a solar module, in particular an opaque solar module, and in particular for determining dust and dirt deposits on the surface of the solar module, comprising: a radiation means, configured to emit an emitted radiation of at least one defined wavelength onto at least a partial area of the surface of the solar module, a sensor means to detect radiation reflected by this partial area, and an evaluation means for determining the degree of soiling of the partial area from a comparison between the detected reflected radiation and a reflected standard radiation, optionally detected beforehand, wherein the reflected standard radiation is determined by the emission of a standard radiation identical to the above emitted radiation onto the partial area in at least one standard state of the partial area, in particular a state with 1% lower attainable output of the solar module and/or a non-soiled state, optionally with 100% attainable output of the solar module, and the detection of the reflected radiation resulting thereby, wherein the radiation means and the sensor means are configured so that a substantial portion of the emitted radiation is directed by the sensor means away onto the partial area of the solar module surface, while the radiation reflected at the particles deposited on the partial area of the solar module that substantially cause the dust and dirt deposits is reflected back onto the sensor means, characterised in that the reflected radiation to be detected by the sensor means is Mie radiation, and the sensor means is configured to detect the same, wherein the radiation means is configured and arranged so that the emission axis A E of the emitted radiation is inclined at an angle β 1 of 2° to 15°, optionally of 5° to 10°, and further optionally of 6° to 8°, to the surface of the solar module, and that the sensor means is configured and arranged so that its detection axis A D is inclined at an angle β 2 of 2° to 15°, optionally of 5° to 10°, and further optionally of 6° to 8°, to the surface of the solar module. a radiation means, configured to emit an emitted radiation of at least one defined wavelength onto at least a partial area of the surface of the solar module, a sensor means to detect radiation reflected by this partial area, and an evaluation means for determining the degree of soiling of the partial area from a comparison between the detected reflected radiation and a reflected standard radiation, optionally detected beforehand, wherein the reflected standard radiation is determined by the emission of a standard radiation identical to the above emitted radiation onto the partial area in at least one standard state of the partial area, in particular a state with 1% lower attainable output of the solar module and/or a non-soiled state, optionally with 100% attainable output of the solar module, and the detection of the reflected radiation resulting thereby, wherein the radiation means and the sensor means are configured so that a substantial portion of the emitted radiation is directed by the sensor means away onto the partial area of the solar module surface, while the radiation reflected at the particles deposited on the partial area of the solar module that substantially cause the dust and dirt deposits is reflected back onto the sensor means, characterised in that the reflected radiation to be detected by the sensor means is Mie radiation, and the sensor means is configured to detect the same, wherein the radiation means is configured and arranged so that the emission axis A E of the emitted radiation is inclined at an angle β 1 of 2° to 15°, optionally of 5° to 10°, and further optionally of 6° to 8°, to the surface of the solar module, and that the sensor means is configured and arranged so that its detection axis A D is inclined at an angle β 2 of 2° to 15°, optionally of 5° to 10°, and further optionally of 6° to 8°, to the surface of the solar module.