Since the work and scope of Avco Everett Research Laboratory, Inc. (AERL) is not familiar to many optical engineers, I thought I would use the opportunity of this guest editorial to review the history of the Laboratory and its present activities as they relate to optics.
When a remote .volume is irradiated and the resulting isotropically scattered signal collected by an optical system, the calculation of the resulting signal is not easily performed. However, for rectangular apertures, the expressions are concise and lend themselves to ordinary numerical techniques. These expressions are presented with sample results.
The radiation at the output aperture of an unstable resonator generally consists of a circular or rectangular annular beam with a predictable intensity and phase distribution. This paper presents useful design curves, as a function of the intensity and phase distribution at the output aperture, of the normalized far-field on-axis intensity and the fraction of the total power within a prescribed spot size in the far-field focal plane.
A computer program which is called AIRCAP and calculates atmospheric path transmission and thermal radiation in the 8-25 micron spectral region is described. After a brief review of atmospheric absorption properties and atmospheric band model transmission calculations, AIRCAP's calculational procedures are outlined. Comparison of AIRCAP's predictions and several data samples are made. The agreement is considered quite good.
An airborne telescope-camera system is described which is used for high-speed, high spatial resolution photography of reentry vehicles during atmospheric reentry. The limits to spatial resolution in the field of the system are identified and the theoretical performance of the system is analyzed in terms of modulation transfer functions. The predicted resolution. of 2.0-2.5 arc sec agrees with a measured field performance of about 2.5 arc sec, and such performance is currently maintained in the field for a period of one year.
Exciting developments are underway in x-ray, radio-nuclide and optical imaging for medical diagnosis. Many of these developments were discussed this past November in Chicago at the Second SPIE Technology Utilization Seminar on Application of Optical Instrumentation in Medicine. As examples of these discussions, five papers have been selected for inclusion in this issue of Optical Engineering. All papers presented at the seminar are printed in the Seminar Proceedings.
A number of schemes employing coded apertures are currently under investigation for in vivo imaging of gamma-emitting tracers. These schemes are based on stationary aperture codes, which depend upon having a cleanly peaked spatial auto-correlation function to facilitate faithful recovery of the source distribution. The image is recovered by performing a spatial integration optically, digitally, or by some other suitable means. This paper describes an alternate method based on time modulated apertures with digital image reconstruction. In contrast to the stationary aperture, the time modulated aperture requires no spatial integration to achieve faithful reproduction of the source distribution. This solves the problem of false structure arising from out-of-focus sources. Good 3-dimensional point response functions free of side lobes have been obtained and it is expected that this approach will yield high quality clinical images.
The pressurized xenon-filled multi-wire proportional chamber (MWPC) is an imaging detector with high intrinsic spatial resolution and good uniformity of response. We describe the design and operation of a 20 x 20 cm2 MWPC with a mass of 0.14 g/cm2 of xenon, maintained at 60 psi absolute. Electromagnetic delay-lines are used to obtain position information. Imaging studies have been performed using a Nuclear Chicago low-energy high-resolution collimator. The intrinsic resolution of the system was determined for 60 keV and 140 keV photons using bar phantoms, and was found to be 1 mm and 2 mm respectively. The Modulation Transfer Function for 140 keV is down to 70 percent response at 1.4 cycles per cm. Stopping-power is about 65 percent for 1-125 and 10 percent for Tc-99m. The distribution of thyroid hormones in the rat was clearly demonstrated. Picker thyroid phantoms filled with 1-125 and TC-99m were imaged with good detail. Preliminary 1-125 thyroid studies in humans have shown a promising potential. Other studies in rats have included liver/spleen imaging with Tc-99m-sulfur colloid and skeletal imaging with Tc-99m-polyphosphate. The excellent spatial resolution of the detector, together with the addition of higher efficiency collimators and operation at higher pressures realize a system with characteristics that are adequate for clinical use. These components are now being incorporated in a 30 x 30 cm2 MWPC.
A clinically accepted xeroradiographic mammography system has been analyzed to determine its imaging properties. These properties will be compared with electronic image enhancement systems such as video soft focus subtraction and automatic dodging printing which can provide an MTF similar to that of xeroradiography. Other properties of all three imaging systems pertinent to a variety of clinical tasks will also be discussed.
The mathematical function describing the transmission of x-rays through an object depends on many geometrical and physical variables. A Taylor series expansion of the transmission function about an arbitrary point in the multivariable space displays the variety of terms which may be emphasized or isolated in order to accentuate certain information concerning the object under study. Conventional radiographic or fluoroscopic images are represented by the zero order term. This term can be used to derive higher order terms which depend explicitly on the spatial coordinates x and y, but contains only averaged or incomplete information about variables such as z (depth), E (energy), or t (time). One class of image enhancement procedures involves accentuation of whatever information is present on conventional radiographs by selective filtration of spatial frequency components or isolation of the x and y derivative terms. A second class of images with significantly different information content can be obtained by a priori use of knowledge about other variables in order to isolate other derivative terms. These images can be formed from linear combinations of zero order images associated with different values of the variable of interest. Examples of this class of images include tomography, time dependent subtraction, and absorption edge imaging. Because evaluations of "enhanced" images containing one subset of the possible image terms cannot be assumed to pertain to images containing other terms, it is suggested that the term "image enhancement" is too general and must be used with care.
The development of a hypodermic fiberscope for the visualization of internal organs and tissue is described. It is based on a unique capability of preparation of microlenses (0.5 mm diameter) and the fabrication of high resolution imaging multifibers. The design of typical prototype instruments is described. Among these are a hypodermic structure of gauge 18 (1.25 mm diameter) which includes a 0.5 mm diameter imaging multifiber made of 11,000 individual fibers about 5 pm each with two piano-convex distal lenses, over two hundred illumination fibers of 50 pm diameter each, and an ancillary channel for a biopsy tool, aspiration or insufflation. Also, separate illumination and imaging probes were constructed with the latter being of size 22 gauge and having three distal lenses of 0.5 mm diameter. Although provisions for focusing are available, the depth of field of the described probes ranges from several millimeters to infinity with a field of view of about 700. The application of currently designed prototype hypodermic fiberscopes in several medical procedures is discussed.
Focused shadowgraph systems employing schlieren quality optics have been used successfully in AEDC Ranges G and K to study the location of transition from laminar to turbulent flow in the boundary layers of test models. A spark light source proved advantageous in flow visualization studies of relatively low velocity models, while a pulsed ruby laser with its very short exposure time (20 nanoseconds) allowed excellent stop-motion shadowgraph photography at higher speeds. Use of the pulsed laser also allowed the preliminary evaluation of a combination front light/focused shadowgraph system - a unique instrumentation system in ballistic range application, to the best of the author's knowledge.
A simple method for generating real time equal height fringes from two stereo transparencies is explained and demonstrated. The two transparencies are bleached converting them to phase transparencies. An image plane hologram is made of one of the transparencies, using a plane wave as the object wave. The second transparency is used to reconstruct the hologram. On those regions where the two transparencies are phase correlated the reconstructed wave is plane. A suitable filter is placed behind the hologram that allows only those regions of the reconstructed wave that are plane to pass. The light that is allowed to pass is used to illuminate an image plane giving a fringe of equal height.
The nitrogen-laser-pumped dye laser produces pulses of 2-8 nanoseconds in duration at repetition rates currently up to 500 pulses per second. With such a low duty cycle system, box-car analyzers are often used to amplify the signal from the photodetector. Except for some of the newer, ultra high-speed models, the box-car does not yield information as to pulse shape The technique to be described utilizes the more commonly available, and less expensive lock-in amplifier to measure light pulses. It will be shown that the signal derived with the lock-in is proportional to the total energy per pulse of the incident light. The technology employed to record oscillograms of these short duration pulses using regular side-on and end-on photomultiplier tubes is also discussed.
A technique is described for use in the fabrication of glancing incidence telescopes which operate at large grazing angles (i.e. 8 to 15 degrees). Precision conic section plunge laps are used in a controlled grinding procedure to initially generate imaging surfaces which have a minimum of subsurface damage. A numerically controlled Moore Number 3 Measuring Machine is used throughout the fabrication procedure. Surface geometry accuracies on the order of one-tenth micron have been achieved.
A draft Standard for Passive, Hand-Held Night Vision devices has been developed for the Law Enforcement Standards Laboratory of the National Bureau of Standards. This Standard is now being circulated for comment prior to adoption as a Standard of the National Institute of Law Enforcement and Criminal Justice of the Law Enforcement Assistance Administration of the Department of Justice. The paper mentions some of the philosophy behind the standard, lists the performance requirements and describes briefly the test procedures for (A) focus adjustment, curvature of field and distortion of the eyepiece lens, (B) optical gain, optical gain stability, light equivalent background, light induced background, luminance of output screen, luminance uniformity, cathode and screen quality, contrast transfer function, distortion and flare of a night vision device complete with objective lens, but with the eyepiece removed, and (C) for resistance to vibration, high and low temperature storage, operation and thermal shock and humidity of night vision devices complete with both objective and eyepiece lenses, and (D) boresight adjustment, click movement and resistance to mechanical shock of night vision devices intended for use as rifle sights.