A surface refractive index image acquiring system for characterization of a sample comprising: - a grating device for receiving the sample, the device comprising at least a first grating region having a first grating width along a transverse direction, and a second grating region having a second grating width in the transverse direction, the first grating region and the second grating region being adjacent in the transverse direction, wherein the first grating region has a grating period Λ 1 in a longitudinal direction, the longitudinal direction being orthogonal to the transverse direction, and the second grating region has a grating period Λ 2 in the longitudinal direction, a grating period spacing ΔΛ = Λ 1 -Λ 2 being finite, the first and second grating periods are chosen to provide optical resonances for light respectively in a first wavelength band and a second wavelength band, light is being emitted, transmitted, or reflected in an out-of-plane direction, with respect to a plane defined by the longitudinal direction and the transverse direction, wherein the first wavelength band and the second wavelength band are at least partially non-overlapping in wavelength, - a light source for illuminating at least a part of the first grating region and a part of the second grating region simultaneously with light at an illumination wavelength band, - an imaging system for imaging the emitted, transmitted or reflected light from the part of the first grating region and the part of the second grating region, the imaging system comprising an optical element, configured for focusing light in a first transverse direction and for being invariant in an orthogonal transverse direction, the optical element being oriented such that the longitudinal direction of the grating device is oriented to coincide with an invariant direction of the optical element, and an imaging spectrometer comprising an entrance slit having a longitudinal direction oriented to coincide with the invariant direction of the optical element, the imaging spectrometer further comprising a 2-dimensional image sensor. - a grating device for receiving the sample, the device comprising at least a first grating region having a first grating width along a transverse direction, and a second grating region having a second grating width in the transverse direction, the first grating region and the second grating region being adjacent in the transverse direction, wherein the first grating region has a grating period Λ 1 in a longitudinal direction, the longitudinal direction being orthogonal to the transverse direction, and the second grating region has a grating period Λ 2 in the longitudinal direction, a grating period spacing ΔΛ = Λ 1 -Λ 2 being finite, the first and second grating periods are chosen to provide optical resonances for light respectively in a first wavelength band and a second wavelength band, light is being emitted, transmitted, or reflected in an out-of-plane direction, with respect to a plane defined by the longitudinal direction and the transverse direction, wherein the first wavelength band and the second wavelength band are at least partially non-overlapping in wavelength, - a light source for illuminating at least a part of the first grating region and a part of the second grating region simultaneously with light at an illumination wavelength band, - an imaging system for imaging the emitted, transmitted or reflected light from the part of the first grating region and the part of the second grating region, the imaging system comprising an optical element, configured for focusing light in a first transverse direction and for being invariant in an orthogonal transverse direction, the optical element being oriented such that the longitudinal direction of the grating device is oriented to coincide with an invariant direction of the optical element, and an imaging spectrometer comprising an entrance slit having a longitudinal direction oriented to coincide with the invariant direction of the optical element, the imaging spectrometer further comprising a 2-dimensional image sensor.