GB1430193A - Illumination optical system of projection printing apparatus - Google Patents

Abstract

1430193 Illumination system for printing photomasks CANON K K 22 Feb 1973 [23 Feb 1972] 8615/73 Heading G2A In projection printing of photomasks with an illumination system comprising projection and condenser lenses (giving an illumination similar to Kohler's), an eccentric lens having two or more parallel optical axes is selectively interposable between the light source and the photomask to change the illumination to "critical" for aligning the photomask pattern on the object. Thus for printing there is high contrast with uniform illumination and numerically low aperture, while for alignment the image of the photomask is brighter but non-uniformly illuminated with a numerically higher aperture. Preferably, the insertion of the eccentric lens is coupled with the insertion, at a point where the luminous flux is narrow, of a filter which cuts out actinic light but passes light favourable for observing alignment. Diagrams are given Figs. 7-9 (not shown) of the light paths for printing and observation, using respectively (1) a single condenser lens and convergent eccentric lens, (2) two condenser lenses and convergent eccentric lens, and (3) two condenser lenses with a field lens and a divergent eccentric lens.