A multiple-spark light source and camera utilizing the Cranz-Schardin optical method for schlieren and silhouette photography is described. Each light source is a spark in air, with an effective duration of 10-7 second and a peak of light of 24,000 candle power. The interval between successive sparks is independently adjustable from 10-5 to 10-7 second, permitting a variable picture rate to be used in a single sequence of photographs. The equipment contains ten spark light sources and camera lenses, permitting a ten picture sequence of high speed phenomena. Schlieren and silhouette photographs demonstrating the performance of the equipment are presented.

A visual optical T-bench method for the measurement of optical path difference nA in units of wavelength is described. The method employs an optical T-bench equipped with nodal slide, a movable double-slit system, and an angle measuring telescope. Determinations of longitudinal spherical aberration in millimeters and optical path difference in units of wavelength are based upon measurements of angular deviations in small selected regions of the collimated beam emergent from the lens under test. The theory of the method is presented together with a description of the apparatus used and the technique of measurements. Results of measurement on a single lens are included.

A triplet is described which consists solely of spherical surfaces in common IR transmitting materials. It has a speed of f/0.75 and is achro-matized over 8-12 microns. The resolution is diffraction limited over its 12° field of view. This lens was designed utilizing a fundamentally new computer optical design program written by David S. Grey at the MIT Lincoln Laboratory. The concepts underlying this program will be briefly described.