This paper will discuss the problems associated with excimer laser photo-lithography -the combination of a KrF narrow band width excimer laser (non-injection locked type) with a large field fused silica monochromatic reduction lens. An excimer laser with a KrF narrow bandwidth, in combination with a large field monochromatic lens which is appropriate for use with such laser, have been developed and tested. The system's resolution capability has been confirmed at 0.4 um L/S with MP2400 resist. The laser has been designed so as to be installed and maintained in a clean room environment as well as to have a very narrow spectrum line. A very narrow band-width beam, down to 0.003nm, has been attained through a stable resonator with more than 20mJ pulse energy. The ultra-compact laser head (300mm x 545mm x 1100mm) contains a small laser discharge unit (182mm x 156mm x 584mm), and no amplifier because the oscillator is highly efficient in spite of the narrow line emission. Maintenance is much easier in the clean room environment. Users can replace the discharge unit as easily as they would change Hg-lamp, only taking twenty minutes, and while they clean the window and check the electrodes of the removed unit, the laser can be operated with the easily installed replacement -already passivated discharge unit. The laser head unit is separated from a gas circulating unit and trigger pulse circuit - vibration, heat, EMI noise and particle generation. Therefore, it can be installed even in the thermal clean chamber of a stepper. The N.A. (numerical aperture) of the monochromatic lens is 0.36 and the field size is 15mm x 15mm. In fact, three kinds of lenses with N.A.s of 0.4, 0.35 and 0.3 respectively, were designed and individually evaluated for their OTF's and defocus's dependence on the light source's spectral width, and also their co-relationship. In parallel, simulations on the relationship between each lens' chromatic aberration and laser spectral width were completed and such results have been integrated with the laser performance data ie. output power v.s. spectral line width, and its interactions from speckle noises/background emmissions per line. The system is capable of achieving a resolution preformance of 0.4 microns over a field measuring 15mm x 15mm, and it is very reliable for practical applications since it employs the combination of 5-7/1000nm spectral bandwidth excimer laser with N.A. 0.36 lens, which has been optimized from the preceding simulations and experimental data. When the N.A. of a lens is being determined it is necessary to take into consideration the spectral width of the light source because in the case of a narrow bandwidth laser, it would be difficult to satisfy two contradictory factor6 - speckle noises/background emissions and narrow bandwidths. An extremely narrow bandwidth laser (less than 3/1000nm) would be much less reliable as it would demand sophisticated configuration and adjustments. A monochromatic lens with a high N.A. would struggle in its efforts to locate such a very narrow linewidth industrial laser source with easy maintenance and compact size in the clean room environment.