An apparatus with a radiation source 100 which has means for irradiation 610 of electromagnetic radiation with a plurality of wavelengths onto a sensor surface 210 which can support a surface plasmon, and said radiation is reflected onto at least one photosensitive detector 300, and create reflectance values, characterized by that said wavelengths at said detector are separated by wavelength, forming a wavelength ensemble, in such way that a continuous response signal can be calculated using at least three of said wavelength reflectance values, finding the wavelength closest to the resonance condition and use of adjacent wavelengths to calculate said continuous response signal 510 being a function of an effective refractive index at said sensor surface 210, where, if the resonance wavelength is the first or last of said wavelengths, at least two adjacent wavelengths closest to said closest wavelength to said resonance condition are used to calculate said continuous response signal. The present invention also relates to a method.