A design for inspecting specimens, such as photomasks, for unwanted
particles and features such as pattern defects is provided. The system
provides no central obscuration, an external pupil for aperturing and
Fourier filtering, and relatively relaxed manufacturing tolerances, and
is suited for both broad-band bright-field and laser dark field imaging
and inspection at wavelengths below 365 nm. In many instances, the lenses
used may be fashioned or fabricated using a single material. Multiple
embodiments of the objective lensing arrangement are disclosed, all
including at least one small fold mirror and a Mangin mirror. The system
is implemented off axis such that the returning second image is displaced
laterally from the first image so that the lateral separation permits
optical receipt and manipulation of each image separately. The objective
designs presented have the optical axis of the Mangin mirror image relay
at ninety degrees to the optical axis defined by the focusing lenses, or
an in-line or straight objective having one ninety degree bend of light
rays.