A tunable light source for interrogating at least one resonant waveguide grating (RWG) biosensor having a resonance linewidth. The tunable light source includes a broadband light source that emits a light beam having a first spectral bandwidth greater than the RWG biosensor resonance linewidth. The broadband light source may be substantially spatially incoherent. A tunable optical filter having a tunable spectral linewidth is arranged to receive and filter the light beam to cause the light beam to have a second spectral bandwidth substantially the same as the RWG biosensor resonance linewidth. Label-independent optical readers that employ the tunable light source are also disclosed.