A projection optical system, which is telecentric with respect to an object side
and an image side, projects a pattern onto an image plane, which is substantially
parallel with the pattern, at a substantial equi-magnification. The projecting
optical system is provided with first and second condenser lenses having a common
(first) optical axis which is perpendicular to the pattern and the image plane.
The projection optical system further includes an imaging lens having a second
optical axis that intersects with the first optical axis at an intermediate position
between the first and second condenser lenses. A first mirror reflects a beam from
the pattern and passed through the first condenser lens toward the imaging lens.
A second mirror reflects the beam passed through the imaging lens to the imaging
lens, and a third mirror reflects the beam passed through the imaging lens toward
the second condenser lens.
