A system and method for correcting for "wavelength drift" in an RES system. One advantage provided by the invention includes the ability to correct for offset amounts of "wavelength drift" that are not divisible by an integer number of pixels (e.g., fraction pixel offset amounts) in an optical detector used in the RES system. Correction of fractional pixel offset amounts may enhance spectral matching, enable substantially continuous calibration of an RES system, and/or other benefits.