Abstract: This paper describes the optical design, fabrication and use of single element null lenses for the interferometric testing of generalized aspheric surfaces. The aspheric singlet is designed to work in collimated light, without a field lens, and allows for measurement of surface figure error over the entire clear aperture. The null lens is a single element of multi-spectral zinc sulfide (ZnS). This material is durable, easily diamond machined and provides good transmission at the test wavelength of 0.6328 microns. All reference surfaces needed for alignment are diamond machined into simple collars that hold the null lens and surface under test. This allows easy and accurate optical alignment and minimizes setup time.