The laser beam can be made to miss the central obscuration of a reflective objective by using a pyramid prism to split the beam into four pieces. A homogenous beam profile across the mask can be obtained by placing the mask at the point where the four beamlets overlap. After the mask, the beam separates into four beamlets, each entering the objective off-axis. Splitting the beam with this technique increases the total amount of light that can hit the primary reflector without causing optical damage. Careful selection of the prism angle yields optimum results.
Pyramid prism illuminator for Schwarzschild objective
Selection of prism angle optimized off-axis exposure
The divergence angle of the beamlets passing from the prism must be matched to the back numerical aperture of the reflective objective. This angle is indicated by the following equation:
Where NA is the published forward numerical aperture and d is the design demagnification factor (i.e., 30X).
As you can see in the illustration above, the mask size is limited by the largest beamlet that can fit in the annular region between the edge of the centrally obscured zone and the outer edge of the primary reflector. If a larger mask size is used, the transmission efficiency of the objective is reduced. The major drawback to this technique is that if the part is slightly out of focus, not only will the image be blurred, but it will also project four diagonally misaligned features. For this reason, such a system is best used for thin film materials on flat workpieces.