Convergent Light Beam Research Articles

Deformed boundaries determined by the method of caustics

The caustics formed by illuminating straight deformable boundaries by a parallel, convergent or divergent light beam were used to determine the load distribution along these boundaries. A universal method based on the characteristic geometric elements of the corresponding caustics (pseudocaustics) was developed. This method leads to a system of singular integral equations, which was solved numerically by a collocation method of successive approximations. The singular points of the integrals were removed by integrating along regular paths determined by the optically obtained pseudocaustics. This procedure constitutes a new combined numerical-experimental method for solving singular integral equations with singularities of the Cauchy type. Several examples are given to show the potentialities of the method.

Surface topography by caustics

The optical method of caustics, initially developed for recording abrupt plate slopes created by singularities in elastic stress fields, was extended to incorporate the study of the general case of any type of surface. A universal technique, based on the general theory of caustics developed in this paper, was formulated to study the topography of any surface from its corresponding caustics obtained by illuminating the surface by a parallel, convergent, or divergent light beam. The special case of an axisymmetric mirror with elliptical cross section, whose ellipticity varies from zero to infinity, was studied extensively to show the potentialities of the technique developed. It was shown that the caustics obtained are very sensitive to the particular form of the surface considered. From the procedure developed in this paper it was concluded that the method of caustics can be successfully used to record the topography of any surface with large or infinitesimal slopes.

Calibration of the Pollak Counter with Monodisperse Aerosols

Abstract The photoelectric condensation nucleus counter of Pollak with convergent light beam has been compared with an electrical aerosol detector using monodisperse aerosols with particle diameters between 0.025 and 0.15 μm, particle concentrations between 127 and 260,800 cm−3, and particles of two different chemical constituencies, e.g., NaCl and material volatilized from a heated nichrome wire. Very good agreement has been obtained. The discrepancy between these two methods was found to be less than 9% at concentration levels below 104 particles cm−2 and 17% at 2.5 × 105 particles cm−3. This discrepancy is well within the combined uncertainties in the two independent aerosol concentration measuring methods.

A Portable Aerosol Detector of High Sensitivity

A portable photoelectric nucleus counter, with similar sensitivity to the Pollak photoelectric nucleus counter with convergent light beam, has been developed and calibrated. This instrument has been incorporated into a packaged measurement system which allows the experimenter to determine the effective diffusion coefficient and fraction charged, of the natural aerosol, in uncontaminated areas. The photoelectric counter has comparable accuracy to the absolute (Aitken, Scholz) counters in the concentration range of interest, and is capable of determining the concentration once per minute.Field tests of the prototype instrument were conducted near sea level in Greenland. The concentration of natural aerosol in this area ranged from 150 to 200 particles cm−3. The instrumentation had sufficient sensitivity to detect a gradual increase in particle size at this low concentration.

The Optical Effects of a Double Calcite Plate on a Converging Beam of Light

The aberrations and variation in transmission of a beam of light converging to a focus and passing through plates of calcite have been investigated, in order to estimate possible photometric errors due to these effects when a crossed calcite plate is used to calibrate photographic plates. The aberrations are calculated in terms of ray deviations whose magnitudes are derived, and the transmission coefficient is determined by solution of electromagnetic boundary conditions. The resultant formulae are evaluated numerically, and set limits on the photometric accuracy of images formed by a Schmidt telescope used with a calcite plate calibrator.

Astigmatism in Light-Deflector Elements

Astigmatism introduced in a convergent light beam by isotropic parallel plates and prisms, birefringent parallel plates including fractional-wave plates and split-angle deflectors, and by Wollaston prisms is studied. Expressions for the astigmatic differences are obtained. The results are experimentally verified. This study is related to the influence on image quality of light deflectors.

Studies of Resolution in a Bragg Imaging System

This paper presents the results of experiments that were performed to investigate the resolution capabilities of a Bragg imaging system using a cylindrically convergent light beam. The usual images formed with such a system show up as silhouetts of the objects being imaged. These silhouettes are superimposed on a light background, which constitutes the visual representation of the incident sound field. The Rayleigh criterion governs the resolution for thin cylindrical objects oriented parallel to the direction of the convergence line of the light beam. For orientation at right angles to this direction, the resolution is of quite a different character. The experimental resolution capabilities for both directions are presented in this paper and are compared with theoretical predictions. Resolution data is also presented for the case of dark-field imaging, i.e., where the objects appear as bright images on a dark background. In addition, experiments were performed to determine the loss of resolution resulting from the introduction of optically opaque plates between the scattering object and the light-sound interaction region.

An automatic patient-contour measuring apparatus.

All optical-electronic system has been developed as a new approach to the task of measuring and recording patient contours. The system projects 2 slightly convergent light beams onto the patient. The resultant light spots are detected by a photomultiplier tube that is located behind a rotating slit wheel. The photomultiplier produces pulses that are analyzed to generate a voltage proportional to distance. An extremely stable system results from the use of time-dependent pulsed circuitry. The voltage generated by the system is used to drive the pen of a polar-coordinate recorder, while its table is synchronized to the rotation of the cobalt 6o unit in which the optical portion of the system is mounted.The polar recorder has 2 channels, one of which is used to plot patient contours, while the other is used in conjunction with a transmission ionization chamber to record transmission dose or the equivalent homogeneous contour. The recorder is mounted in a portable utility cabinet and can be used in the ther...

Wide-Band Modulation of a Laser Beam, Using Bragg-Angle Diffraction by Amplitude-Modulated Ultrasonic Waves

Light valves based on the ultrasonic-diffraction effect are capable of producing high-index intensity modulation of a light beam, but they are subject to a fundamental bandwith limitation associated with the finite time of transit of an ultrasonic wave across the light beam. It is shown theoretically that modulation bandwidth can be maximized by converging the incident light beam to a focus to minimize beam diameter and by confining the ultrasonic beam to the vicinity of the focal plane. Under properly chosen conditions, modulation bandwidth can be as large as 1/7.7 times the ultrasonic carrier frequency. An experimental investigation was carried out using water as the ultrasonic medium to verify the theoretical dependence of modulation bandwidth on ultrasonic- and optical-beam parameters and to evaluate constants of the theory. The experiments also showed that the wavefronts near the focus of a convergent light beam are sufficiently plane to produce Bragg-type diffraction when conditions are otherwise favorable. Bragg diffraction was used in the modulation bandwidth measurements. The dependence of diffracted-beam intensity on ultrasonic amplitude was investigated experimentally and, in accordance with theory, it was found that nearly all of the incident (zero-order) light can be transferred to a selected first-order diffracted beam. This fact permits either the zero-order or a first-order beam to be chosen as the useful output of the modulator.

Large aperture interferometers with small beam dividers

This paper shows some practical intcrferometers for testing very large and very small specimens. This is accomplished by using a particular type of beam divider that operates near the vertex of a convergent or divergent beam of light that permits a lmost un limi ted size fields of view. Thus very large or ver.v s mall specimens ean be tested. Also, the wave front reversing propert ies of this beam divider permit tests to be made without the necessity of using standards of reference, thus eliminating the requirement for large standards that a re usually expens ive and require large working a reas . The principl es of these interferometers are so closely related to previously described instruments t hat very li tt le addit ion al explanation is required for an understanding of their operation. Several variations of the I\:oste rs pris m type of interferom eter a re s hown schcmaticall y J

Measurement of the spectral intensity distribution of a convergent high intensity light beam with a magnesium-oxide-smoked plate

The absolute spectral intensity distribution at the center of the focused image of a carbon arc was determined in the region of 3000–9000 A. The basic optical system (used in studies of thermal radiation damage) consisted of a high intensity carbon arc at the focus of a 36-in. paraboloidal searchlight mirror, and a similar condensing mirror which focused the reflected beam in a cone of vertex angle 120°. The intensity at the focus of the condensing mirror was approximately 100 cal/cm2/sec. The high intensity and convergence of the beam at the focus led to the adoption of a new technique for the spectral analysis. A magnesium-oxide-smoked plate was inserted into the beam at the focus, and the light reflected normally from the plate was focused upon the slit of a spectograph. Absolute intensities were obtained by comparison with a standard pyrometric arc. The results are compared with absolute black-body measurements of the total intensity at the focus. The observed differences in spectrum between arc and image are discussed.

A High-Speed Air-Driven Shutter for Controlling Exposures to a Convergent Beam of High-Intensity Thermal Radiation

A high-speed two-bladed shutter was built to deliver square-wave pulses of energy from a converging light beam with an average intensity of about 5 cal/cm2/sec, and a diameter of 3 in., in the plane of the shutter. The blades open or close in less than 0.01 sec. They move in the same direction, so that all parts of the target are exposed for very nearly the same length of time. The shutter makes use of a new type of air cylinder which is suitable for driving high-speed mechanisms, and has a dynamic shock absorbing system to stop the moving parts in a short distance with little rebound. It is electrically controlled and fully automatic. Exposure times may be varied from 0.07 to 5 sec by setting an electronic interval timer. Longer exposures are controlled manually. To date it has been used more than 15 000 times and has required only two overhauls.

APPLICATION OF QUARTZ CRYSTALS TO THE MODULATION OF LIGHT

The resonating X-cut quartz crystal has been used as a self-compensator for white light modulation.The properties of the 49° cut quartz crystal are described. This crystal combines low power input and stable high frequency with a wide light aperture and improved light output. For many purposes a convergent beam of light may be used, without the need of a compensating device.

SCIENTIFIC SERIALS

American Journal of Science and Arts, April—In this number Prof. Hastings records observations which prove that the variation in dispersive power of glass, attending variation in temperature, is relatively enormously greater than that in the refractive power. This could hardly have before escaped notice, but for a singular relation in the co-efficients, in virtue of which, probably, an achromatic combination for one temperature is good for all others within moderate limits.—Prof. Rowland has made a new determination of the absolute unit of electrical resistance, his method being to induce a current in a closed circuit by reversal of the main current. He finds the B.A. unit too great by about *88 per cent. A difference of nearly 3 per cent, between his result and that of Kohlrausch he endeavours to explain from a criticism of the latter's method, pointing out what he thinks its defects. Prof. Langley differs from M. Janssen as to the ultimate form of the “grains”in the solar photosphere, regarding them as the ends of filaments (a simile he employs is that of a bird's-eye view of a field of grain acted on by wind), whilst M. Janssen thinks them literal spheres.—In the projection of microscope photographs, Prof. Draper increases the brilliancy of the result by removing the supporting stage of the slide further from the condenser so that a convergent beam of light may fall on the object.—Several papers in this number deal with points in American geology and physiography; the surface geology of South-West Pennsylvania, the driftless interior of North America, the ancient outlet of the Great Salt Lake, Lower Silurian fossils in Pennsylvanian limestone, intrusive nature of the triassic trap-sheets of New Jersey,&c. A tree-like fossil plant, Glypto-dendron, lately found in the Upper Silurian rocks of Ohio is described by Prof. Claypole as (from its position) possessing a peculiar interest.