Previous work has demonstrated the use of the LINNIK Point Diffraction Interferometer for the measurement of wavefront error in soft x-ray imaging systems. Operational wave-lengths from 632.8nm (He-Ne) down to 313.1nm (Hg) have been used. This paper describes our extension of the technique to 253.7nm together with a demonstration. of feasibility at 121.6 nm (Hydrogen. Lyman-α), we believe for the first time. Finally the possibility of working at soft X-Ray wavelengths is considered.