Ellipticity in an illumination beam may be corrected by measuring an ellipticity of the illumination beam and substantially eliminating the ellipticity using a light attenuating filter at a defocus position of the illumination beam, wherein the light attenuating filter has a two-dimensional pattern that compensates for ellipticity variations in the illumination beam. A periphery of the aperture forms a fluid-tight seal around the element. The mechanism collects trailing record statistics for the frequently accessed non-sequential cache records. In one embodiment of the invention, the multiplex circuits initially allocate alternate bands within each channel and complementary bands between the channels, thereby emulating a complementary comb filter. The detector determines the position of the touch on the touch surface based on the light from the light enhancement layer. The bottommost horizontal portion of the dielectric flash layer is removed to expose a second portion of the substrate.