The GLV is a diffractive device and is used in conjunction with monochromatic light sources. Single-mode laser illumination works best, but LEDs can also be used at a reduced efficiency. Because of the GLV’s linear aspect ratio, circular laser illumination is first transformed into a line. The figure shows a typical GLV illumination and imaging configuration. In the figure, the optical path has been unfolded around the GLV. Typically, the incident and reflected light paths are separated by imposing a small incidence angle to the illumination onto the GLV (typically 5-20° from surface normal). Alternatively, a polarization rotator and beam splitting optics can be used for normal incidence geometries.
Line illumination for the GLV can be generated from laser sources by a variety of methods including a Powell lens, a DOE (Diffractive Optical Element), or simply a cylindrical lens. In the latter case, the GLV itself can be used to attenuate or “flatten” the Gaussian light distribution along the GLV array axis. For illustration purposes, a Powell lens is shown in the figure. The circular beam from the laser is dispersed uniformly in angle in one axis. A “slow-axis” collimator is then used to collimate the rays into a “top-hat” profile of uniform intensity along the GLV array axis. Next, an orthogonal “fast-axis” cylindrical lens is used to focus the beam onto the GLV. Typical numerical apertures for the fast axis focus are between 0.01 and 0.1. The full width of the line illumination should be less than one third of the ribbon length (50-75um) for best contrast. As explained earlier the imaging system of the GLV comprises a conjugate lens pair with an aperture in the Fourier plane between the lenses. Image magnification is determined by the ratio of the focal lengths of the lens pair (i.e., M = f2/f1).
