Over the past decade, three generations of scan lens systems have been developed, culminating in a 33x, 364 nm, 0.8 numerical aperture lens system for 0.25 mu m generation mask writing. The specifications and design performance of this latest lens from Tropel are compared with measurements made on a lens test bench and on a scanned laser writing system. A full-width at half-maximum spot size of 270 nm, and linearity performance under 20 nm have been measured. CD linearity data shows deviations less than 20 nm down to 0.3 mu m feature sizes. Corner rounding data shows marked improvement over previous generation imaging optics with a minimum radius of 253 nm. These lens distortion and resolution measurements confirm that the new lens system has the imaging performance required for 0.25 mu m generation masks.