Thomas P. Vogl

Image Converter for Thermal Radiation

The development of the phothermionic image converter is described, a device which reproduces scenes by virtue of differences in their thermal radiation. The method uses the principle of projecting a temperature image on a free multilayer film about 0.05 μ thick which contains a photosurface with thermally sensitive yield. Image conversion is achieved by probing the temperature distribution with a flying light spot and displaying synchronously the thermally modulated photocurrent on a monitor. Section 1 discusses the limits of temperature imaging. Under idealized shielding conditions, the relative temperature distributions in the image assume a very simple and general relationship to that of the object, e.g., are equal in radiation equilibrium. Practical heat exchange processes will present other signal limitations which, in combination with the random noise inherent in any conversion process, determine the system performance. Section 2 deals with the experimental work of searching for, and improving on, the phenomena on which the operation of the image converter is based. This includes theory and experiments on temperature sensitive photoelectric yields and on the electrical and optical properties of thin films. Certain components such as sealed crystal windows, cooled radiation shields, and free thin films had to be developed for the image tube. Several models were built and accommodated by suitable optical and electronic systems. When completed, the device had television bandwidth and motion portrayal capabilities and the ability to detect, or image, temperature differences of 10° to 50°C.

Spline Functions: an Alternative Representation of Aspheric Surfaces

This paper offers an alternative to current practice in the specification of aspheric optical surfaces. Spline functions, a numerical analog of a flexible draftsmans curve, are used to represent a rotationally symmetric aspheric surface. We develop the interpolation formulas pertinent to ray tracing and apply an optimization procedure to design a simple Schmidt system as an illustration. We conclude by showing that the new formulation seems to be more compatible with the optimization process than the usual polynomial aspheric.

Asymmetric Lens Design Using Bicubic Splines: Application to the Color TV Lighthouse

Any two-dimensional interpolation scheme which has continuous derivatives may be used to represent an optical surface for ray-tracing purposes. We present bicubic splines in their application to the design of asymmetric surfaces. An as example of a problem requiring an asymmetric system, we analyze the design problems of a color TV lighthouse lens.

Photoconductive Time Constants and Related Characteristics of p-Type Gold-Doped Germanium

To measure the very short photoconductive time constants of p-type gold-doped germanium, two alternative methods were applied and compared. The first is indirect, using the relationship between the magnitude of generation-recombination noise and carrier lifetimes. The second method is direct, employing a high-speed light-pulsing technique. If no other noise sources are important, the results of the indirect method approach those of the direct method as a lower limit. A combination of such time-constant measurements was performed on a series of crystals in which impurity densities and carrier concentrations had been evaluated by Hall coefficient and conductivity measurements. From these data quantum yields of carrier generation, and cross sections for photon capture and carrier recombination were evaluated. The photon capture cross section of the 0.15 ev gold acceptor level at 5 μ is 1.3×10−16 cm2, averaging 0.9×10−16 cm2 for 2–9 μ. The hole capture cross section by the Au− ion in germanium was found to be 2.3×10−14 cm2.

Semiautomatic Generation of Optical Prototypes

This paper presents the theory and application of a computer approach to the problem of finding lens prototypes from given sets of design goals. The procedure is based on the yy method of Delano and has two independent segments: a strictly first-order phase and a combined first- and third-order phase. The former optimizes the first-order layout with respect to geometrical and optical constraints. In this phase the independent variables are the paraxial marginal and principal ray heights. The starting point is a set of yy coordinates which may be either generated by the computer program or supplied by the designer. The second phase combines third-order information with the constraints of the first phase. Here it is more convenient to use ray angles as independent variables. The optimization scheme for both phases is an orthogonal method which has been used successfully on a variety of optical problems.

Grey’s method for nonlinear optimization

The optimization technique first developed by Grey for lens design is described in terms of more recent work that has appeared in the mathematical programming literature. We present two modifications of the original algorithm that improve its performance on difficult problems. To confirm the confidence that the many users have placed in the method, the outline of a convergence proof is included. Finally, the theoretical work is supported by the results of numerical experiments.

Semiautomatic Design of Illuminating Systems

The design of illuminating systems, as contrasted to imaging systems, presents novel problems. A successful first approach to the solution of these problems is presented with emphasis on some of the more fundamental problems and their solution. Methods for handling such systems and the unusual surfaces encountered are described, and performance criteria suitable for system optimization with nonlinear programming algorithms are discussed.

An f/0.75 Diffraction Limited Triplet for the 8-12 Micron Atmospheric Window

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.

Signal Transfer Function of Optical Components

The effect of an optical component or subsystem on an electromagnetic wave is considered and a signal transfer function associated with the propagation is derived. A coordinate transformation is introduced which makes it possible to compute this transfer function. A lens can be characterized by the zeros of this transfer function in the complex plane and the effect of variations of the usual optical parameters on this transfer function is expressible in terms of root loci. These loci are simple functions of the lens parameters and one basic set of loci defines the properties of a large class of rotationally symmetric lenses. Their use in the preliminary design of optical systems is indicated and a simple case is carried to completion.