A wavelength-tunable detector for use in a label-independent optical reader for reading at least one resonant waveguide grating (RWG) biosensor is disclosed. The wavelength-tunable detector includes a tunable wavelength filter. Broadband light is made incident upon the RWG biosensor to produce resonantly reflected light, which is then passed through the tunable wavelength filter whose central wavelength is adjusted, such as by varying the filter angle of the tunable wavelength filter. This generates filtered resonantly reflected light, which is detected by a photodetector that generates an electrical signal in response thereto. The electrical signal is representative of the detected spectrum, which includes the center resonant wavelength of the RWG biosensor. A controller can be used to determine the center resonant wavelength from the electrical signal.