The authors performed a polarimetric prototype for wavelength measurements. In the design both Faraday rotators acting for modulation of the incoming laser light and also as compensator, allow together with a quartz plate, a precise measurement of laser wavelength from 400 to 900 nm, with a resolution better than 0.1 nm. Equations and graphics current versus lambda for a quartz tungsten halogen lamp are presented for sensor adjustments. It is also presented a signal to noise analysis and a comparison with a conventional method.