A beam-combining "cube "prism has been incorporated into the eyelens of a simple magnifier type eyepiece in order that a collimated virtual image of a cathode ray tube presentation may be seen superimposed upon the normal direct view of the observer's environment. This unique prism !eyepiece configuration has facilitated the design of high resolution head-mounted optical displays with apparent fields of view as large as 40° while minimizing the physical size of the projection optics. By use of moderately high index glass in the prism, a significant reduction in lens aperture as compared to that required with the equivalent beam-combining mirror arrangement can be achieved. The major features of this special eyepiece are discussed and some useful applications of the device are described.
In most cases, holography of high-speed phenomena necessitates the use of a ruby laser to record the hologram. Work at this Laboratory has led to the development of `holocameras' - optical systems which compensate for the limited spatial and temporal coherence of common Q-switched ruby laser illuminators. Holocameras for both transmission and reflection action holography have been built. One of the former types has been employed to holograph combustion phenomena in liquid rocket engines, including one of 18 inch diameter and 25,000 pounds thrust. It can also be used to record projectiles in flight, aerodynamic wakes (via holographic interferometry ), electric discharge plasmas, etc. A reflected light holocamera has been used to record holographic inteferograms of impulsively loaded plates. Ruby laser holograms made with these `holocameras' reconstruct as brightly as comparable holograms made with a helium-neon gas laser. They exhibit high contrast ratios and are free of extraneous fringes and mottling throughout the reconstruction volume. Pulsed laser holography is important because it bypasses the depth of field problem of conventional photography. Thus, it is applicable to the recording of either events of unpredictable position or distributed phenomena. The added feature of double exposure holography mades interferometric measurement applicable to hitherto impossible situations.
A "schliere", in German, is a local inhomogeneity. Schlieren systems, then, are optical systems designed to detect local inhomogeneities. Schlieren occur when hot water mixes with cold water, when shock waves are generated in a gas, when liquid mixtures are separated into components of different refractive index as in ultracentrifugation, and in many other phenomena involving the presence of temperature, pressure, or density gradients. However, there is no reason to restrict the term schlieren to disturbances that deflect light passing through a medium. If light is reflected off an irregular surface, such irregularities may be called 3 schlieren also. Indeed; schlieren systems, be cause they are easy to adjust, have found wide use for the testing of large mirrors.
The study of meteorology deals with measurements of such parameters as pressure, temperature, humidity, wind, cloud cover, cloud altitude, and precipitation. Until the advent of weather satellites adequate coverage on a global basis was not feasible. However, since the satellite moves in an orbit which is outside the atmosphere, spacecraft measurements of meteorological parameters must be limited to those that can be sensed by electro-magnetic radiation. In some cases as we shall see later, it is possible to infer some meteorological parameters by indirect methods.
The study of image evaluation is a complex subject requiring the simultaneous consideration of the object, optical system, detector, and mechanism for processing the detector's signals. Image quality for any given photo-optical system depends upon such diverse considerations as granularity, light scattering, numerical aperture, tone reproduction, the degree of correction in the image-forming optics, etc.