A device for monitoring radiation emitted by luminophores present in an analyte fluid, comprising: a biosensor having at least one non-transparent material arranged on a substrate of a transparent material and forming at least one aperture for being filled with an analyte fluid, wherein said aperture has a first in-plane dimension, which is smaller than half an effective wavelength of radiation in the biosensor and a second in-plane dimension which is larger than half an effective wavelength of the excitation radiation, and has a plane of transmission; luminophores immobilized adjacent the substrate side of the aperture that are able to generate a radiation to be detected and luminophores present at or outside the analyte side of the aperture that are able to generate an undesired background, an excitation source (10,41) configured to emit an excitation radiation which is non-polarized or circularly, elliptically or linearly polarized as a linear combination of evanescent and propagating excitation light inside the aperture for exciting the luminophores present in the analyte fluid of said biosensor; an imaging block comprising a detector for detecting radiation emitted by the luminophores upon excitation, and a lens for directing radiation emitted by the luminophores towards detector, said imaging block arranged at a substrate side of the biosensor; and a polarizing filter arranged in the optical path of the radiation emitted by said luminophores between the biosensor and the detector and outside the optical path of the excitation radiation, said polarizing filter being configured to substantially suppress the polarization component of the radiation emitted by said luminophores which is parallel to the transmission plane of the aperture; wherein the excitation radiation is directed toward the biosensor from the substrate side at an angle larger than the numerical aperture of lens, in order to be reflected in another direction than towards the detector. a biosensor having at least one non-transparent material arranged on a substrate of a transparent material and forming at least one aperture for being filled with an analyte fluid, wherein said aperture has a first in-plane dimension, which is smaller than half an effective wavelength of radiation in the biosensor and a second in-plane dimension which is larger than half an effective wavelength of the excitation radiation, and has a plane of transmission; luminophores immobilized adjacent the substrate side of the aperture that are able to generate a radiation to be detected and luminophores present at or outside the analyte side of the aperture that are able to generate an undesired background, an excitation source (10,41) configured to emit an excitation radiation which is non-polarized or circularly, elliptically or linearly polarized as a linear combination of evanescent and propagating excitation light inside the aperture for exciting the luminophores present in the analyte fluid of said biosensor; an imaging block comprising a detector for detecting radiation emitted by the luminophores upon excitation, and a lens for directing radiation emitted by the luminophores towards detector, said imaging block arranged at a substrate side of the biosensor; and a polarizing filter arranged in the optical path of the radiation emitted by said luminophores between the biosensor and the detector and outside the optical path of the excitation radiation, said polarizing filter being configured to substantially suppress the polarization component of the radiation emitted by said luminophores which is parallel to the transmission plane of the aperture; wherein the excitation radiation is directed toward the biosensor from the substrate side at an angle larger than the numerical aperture of lens, in order to be reflected in another direction than towards the detector.