A rotation angle encoder apparatus is disclosed. The rotation angle encoder apparatus may comprise a plurality of light sources, a plurality of light detectors, and a plurality of pinions rotatable about a shaft. Each of the pinions may comprises a plurality of teeth and a plurality of gaps. The rotation angle encoder apparatus may comprise a plurality of stacked configurations such that, when the shaft is rotated, transmissivity may be measured to determine or calculate at least one measurement, such as an angle of rotation, position, movement, distance, or other discernable measurement. The rotation angle encoder apparatus with a plurality pinions and associated measurable transmissivities may enable an optical spectrum analyzer to increase wavelength accuracy and improve resolution in optical measurements.