The object of the invention is to prevent interference fringes of images and allow precise measurement of the thickness of a layer by the optical interferometry by limiting the surface roughness of a conductive substrate. In an evaluation unit, all or at least some of the picture points of each individual image are examined with regard to at least one predefined feature. When the user begins entering characters of an input string, a predictive input engine is activated. In such case, the device has three or more stable states of transmissivity. The device also comprises a second vertical cavity surface-emitting laser of the monolithic vertical cavity surface-emitting laser array, the second vertical cavity surface-emitting laser being tunable to a second plurality of wavelengths, wherein at least one wavelength is in both the first plurality of wavelengths and the second plurality of wavelengths. This process may be continued on an iterative basis to acquire symbol timing.