Objectives with numerical apertures of 0.1-0.4 have been developed and fabricated for focusing laser radiation. A feature of the objectives is that they use an aspheric surface of the same form regardless of the numerical aperture, while the objectives are capable of obtaining a diffraction-limited focusing spot at various wavelengths.
© 2010 Optical Society of America
N. I. Potapova and A. D. Tsvetkov, "Projection objective for focusing laser radiation," J. Opt. Technol. 77, 212-215 (2010)