Fabry-perot spectral image measurement

A method of relative spectral measurement, comprising:

• providing one or more broadband sources to illuminate a rugate tag, wherein the one or more broadband sources have a short broadband source coherence length, wherein the light from the one or more broadband sources illuminates the rugate tag at a first numerical aperture and is collected at a second numerical aperture that is different from the first numerical aperture;
• providing an adjustable Fabry-Perot etalon to optically process the reflected light to extract spectral information with fine spectral resolution;
• and
• providing a detector to detect reflected light from the rugate tag, wherein the reflected light is comprised of multiple narrow-band subsets of the illumination light having long coherence lengths and is optically processed using a plurality of settings for the adjustable Fabry-Perot etalon, and wherein the plurality of settings includes a separation of the Fabry-Perot etalon plates that is greater than the broadband source coherence length.