A sensor device (340, 440) for determining a flow characteristic of an object being movable in an element, the sensor device (340, 440) comprising: - a light emitting unit (344, 480) configured for emitting light towards the element, - a light detecting unit (344, 482) configured for detecting light scattered back from the element, - an optical unit configured for spatially separating a light incidence element portion of the element and a light detection element portion of the element from one another, wherein the light incidence element portion is associated with the emitted light inciding on the element and the light detection element portion is associated with the back-scattered light scattered back from the element for detection, wherein the optical unit comprises a light path separation element configured for separating a light path of the emitted light towards the light incidence element portion from a light path of the back-scattered light scattered back from the light detection element portion, wherein the light path separation element is configured as a polarizing beam splitter, wherein a polarization direction of the emitted light emitted by the light emitting unit (344, 480) is oriented under an angle of 45 degrees with respect to a polarization axis of the polarizing beam splitter, - a determining unit (358, 458) configured for determining the flow characteristic of the object being movable in the element based on light indicative of the emitted light and the detected back-scattered light, and characterised in that it further comprises: - a polarization alternation element arranged between the optical unit and the element, wherein the polarization alternation element is configured for altering a polarization of the emitted light received from the optical unit and a polarization of the back-scattered light scattered back from the element, wherein the optical unit and the polarization alternation element are configured and are arranged relative to one another such that a polarization of the back-scattered light detected by the light detecting unit (344, 482) and a polarization of the light indicative of the emitted light are orthogonal to one another. - a light emitting unit (344, 480) configured for emitting light towards the element, - a light detecting unit (344, 482) configured for detecting light scattered back from the element, - an optical unit configured for spatially separating a light incidence element portion of the element and a light detection element portion of the element from one another, wherein the light incidence element portion is associated with the emitted light inciding on the element and the light detection element portion is associated with the back-scattered light scattered back from the element for detection, wherein the optical unit comprises a light path separation element configured for separating a light path of the emitted light towards the light incidence element portion from a light path of the back-scattered light scattered back from the light detection element portion, wherein the light path separation element is configured as a polarizing beam splitter, wherein a polarization direction of the emitted light emitted by the light emitting unit (344, 480) is oriented under an angle of 45 degrees with respect to a polarization axis of the polarizing beam splitter, - a determining unit (358, 458) configured for determining the flow characteristic of the object being movable in the element based on light indicative of the emitted light and the detected back-scattered light, and characterised in that it further comprises: - a polarization alternation element arranged between the optical unit and the element, wherein the polarization alternation element is configured for altering a polarization of the emitted light received from the optical unit and a polarization of the back-scattered light scattered back from the element, wherein the optical unit and the polarization alternation element are configured and are arranged relative to one another such that a polarization of the back-scattered light detected by the light detecting unit (344, 482) and a polarization of the light indicative of the emitted light are orthogonal to one another. - a polarization alternation element arranged between the optical unit and the element, wherein the polarization alternation element is configured for altering a polarization of the emitted light received from the optical unit and a polarization of the back-scattered light scattered back from the element, wherein the optical unit and the polarization alternation element are configured and are arranged relative to one another such that a polarization of the back-scattered light detected by the light detecting unit (344, 482) and a polarization of the light indicative of the emitted light are orthogonal to one another.