I worked for an optical company building complete objective lenses and illuminator optics for steppers using G-line, I-line, and ArF and KrF laser sources. "So

I worked for an optical company building complete objective lenses and illuminator optics for steppers using G-line, I-line, and ArF and KrF laser sources. "Solarization" and ablation of the lens coatings was an ongoing problem, especially for UV sources. A stepper objective has 20+ lens elements. Lenses were refurbished by replacing elements as their transmission fell.