Large Angular Aperture Research Articles

On some factors controlling high-accuracy measurements of X-ray quanta positions with multiwire proportional chambers

Multiwire proportional chambers (MWPC) may provide a useful tool for dealing with many problems concerned with the localization of X-ray quanta. The accuracy in the determination of the spatial coordinates of a single quantum in a chamber is controlled by several factors: • - the spatial extension of the secondary electrons produced by the absorption of the quantum; • - the mean free path of the secondary X-rays emitted by the absorbing atom; • - the chamber parameters: detection thickness and wire spacing; • - the read-out method. At LURE, we are trying to build large-size detectors, having a large angular aperture (≈ 90°), accuracies better than 1 mm (fwhm), high counting-rate capabilities (≈ 1 MHz), and high efficiencies (≈ 100% at 8 keV); in this paper we will discuss some problems encountered in this enterprise.

Absorption-Modulation of Light in ZnSe Crystals and Its Application to a Page Composer

The absorption edge of ZnSe and its dependence on temperature and electric field are experimentally investigated for undoped and Cr doped crystals. The undoped crystals are found to be superior to the Cr doped crystal to carry out an efficient absorption-modulation of light. Light modulation experiments based on the field-induced shift and the temperature-induced shift of the absorption edge are made for the undoped crystals. The contrast ratio is almost constant within the range of the light incident angle of ±20°. By utilizing the temperature-induced light modulation, a page composer having a large optical angular aperture is realized.

Acoustooptic TeO_2 tunable filter using far-off-axis anisotropic Bragg diffraction

Noncollinear acoustooptic tunable filters using TeO(2) crystals are described. An anisotropic Bragg diffraction is studied applying simple design expressions to a tilt type in which an acoustic wave is launched a little off the [110] axis. Tuning property, filter bandwidth, angular aperture, required acoustic power, deflection angle, divergenceless configuration, and rapid spectrum scanning have been discussed. These filter properties have been confirmed with measurements using three kinds of filter devices: a basic type, an acoustically enhanced type, and a large angular aperture type. A 5-A bandwidth at 4000 A was obtained by slow scanning. A 32-A separation at 4000 A was observed by a rapid scanning of 0.5 msec throughout the visible region with an extremely low electric drive power.

Infrared Circular Dichroism and Linear Dichroism Spectrophotometer

An instrument for the measurement of circular dichroism (CD) between 5000 cm(-1) and 750 cm(-1) is described. This instrument also can perform high precision linear dichroism measurements in the same wavelength range. A germanium photoelastic modulator having a large linear and angular aperture is used to vary the polarization of monochromatic ir light between left- and right-circularly polarized at 8 kHz. This polarization-modulated light traverses the sample and falls onto a cooled InSb or HgCdTe detector. Phase-sensitive demodulation yields the CD of the sample to a precision of 4 x 10(-6) o.d. units near 3000 cm(-1). The instrument has been used to study ir circular dichroism arising from molecular vibrations in cholesteric liquid crystals, crystals, and fluids. CD arising from low-lying electronic transitions of metal complexes and linear dichroism of polymer films have also been studied.

Erratum: Noncollinear acousto−optic filter with large angular aperture

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Contrast of Kossel Patterns in Electron Diffraction

Electron diffraction patterns from a Si crystal taken with a convergent beam of large angular aperture (Kossel pattern) are compared with the diffraction pattern taken with a hollow cone convergent electron beam. For thin crystals the patterns are complementary. This behaviour is discussed.

Noncollinear acousto-optic filter with large angular aperture

A new type of noncollinear acousto-optic filter with large angular aperture is described. By an appropriate choice of acoustic wave propagation direction, the momentum mismatch caused by the angular deviation of the incident light from the matching condition in a noncollinear interaction can be compensated by the angular change of optical birefringence. A TeO2 noncollinear acousto-optic filter was constructed that had a half-power bandwidth of 40 Å at an f/4 (±7°) aperture. Tuning from 700 to 450 nm is obtained by changing the acoustic frequency from about 100 to 180 MHz. About 95% maximum transmission is obtained with an electrical power density of 1 W/cm2 applied to the transducer.

PHASE-MATCHED UP CONVERSION OF 10.6-μ RADIATION IN ZnGeP2

Phase-matched sum mixing of 10.6- and 1.06-μ radiation has been observed at near normal to the optic axis in ZnGeP2. Its theoretical performance as a 10.6-μ detector via up conversion compares favorably with direct detection by a cooled Ge:Hg photoconductive detector. The sum-mixing efficiency of ZnGeP2 is 140 times superior to that of Ag3AsS3, the next best material. The large angular aperture resulting from θm ≃ 90° indicates that the material will be useful for image up conversion.

Video-frequency light modulator having a wide angular aperture

This paper discusses the 45° ycut ADP modulator. An investigation of the propagation of light through the crystal shows that there is an inherent limitation in the angular aperture of the simple 2-crystal modulator. The inclusion of a halfwave plate between the crystals to improve the temperature stability also enables large angular apertures to be achieved, provided the crystals are correctly aligned. Results of measurements carried out on experimental modulators are presented and a comparison made with the 45° zcut modulator.

On the Properties of Polarization Elements as Used in Optical Instruments III Angular Aperture Functions of a Positive Dichroic Film Polarizer*†‡

This paper discusses the important properties that affect the use of a polarizer in applications involving large angular aperture or oblique incidence. Experimentally determined values of these properties are reported for Polaroid HN22 polarizer, one of the best known types of high extinction, commercially available polarizers. It is shown that these properties may be rather simply described. Equations are given from which polarization direction and transmittance of positive dichroic polarizers can be calculated for oblique rays. The calculated values agree well with the experimentally obtained values given here as well as with those for very differently constructed positive dichroic polarizers studied by previous investigators.

Long Optical Paths of Large Aperture

THE measurement of the vapor phase spectra T of compounds having high boiling points presents an experimental problem that may be solved either by heating the absorption cells or by making them very long. In the infra-red region radiation from the hot gases in heated cells decreases the accuracy of absorption measurements. If only a small amount of sample is available, the only possibility is to use an optical system in which the radiation goes back and forth through the same volume a large number of times. Several designs for such systems have been published recently1' 2 but none of them permits the use of large angular apertures at points off the optic axis. In this paper an absorption cell is described in which the light traverses a small volume a large and arbitrarily variable number of times, and in which the angular aperture of the mirrors is not occulted either on or off the optical axis. The design gives very high light transmission and can be used for observing spectra that are very weak, or that belong to high boiling point compounds or to compounds obtainable only in very low concentrations. It can be used for any liquids or gases that do not injure the mirror surfaces, with which they are directly in contact. The essential parts of the equipment are three spherical, concave mirrors that all have the same radius of curvature. These are set up as shown in Fig. 1 with two mirrors A and A' close together at one end of the absorption cell, and the third mirror B at the other end. The centers of curvature of A and A' are on the front surface of B, and the center of curvature of B is halfway between A and A'. This arrangement establishes a system of conjugate foci on the reflecting surfaces of the mirrors, by which all the light leaving any point on A is brought to a focus by B at the corresponding point on A', and all the light leaving this point on A' is focused back again to the

THE LIMITATIONS AND ADAPTABILITY OF ULTRAMICROSCOPES TO THE STUDY OF COLLOID SYSTEMS

Ultramicroscopes are of two distinctly different types. The best known is the cardioid ultramicroscope in which the illuminating portion produces a hollow cone of large angular aperture with its apex at the object. The observing system is a microscope consisting of a high aperture objective of angular aperture slightly less than that of the hollow cone and a high power eyepiece. The other is the slit ultramicroscope. The illumination system in the slit ultramicroscope is perpendicular to the observing system. Both systems are of high numerical aperture, but no direct light can enter the observation system. Details of construction of both types of ultramicroscopes and their sensitiveness of adjustment are discussed. Expressions of their limitations in colloid microscopy are given.

Phenomena Developed by Heliostatic Star-Disks

A HELIOSTAT of the highest class is doubtless beyond the means of ordinary observers, but such an instrument as the one now described is readily obtainable. Three sets of achromatic lenses forming a focal power of forty at ten inches, or a miniaturing power of one-fortieth, are in general sufficient. If formed into a microscopic object-glass, the front is turned towards the sun. The glass then refracts a beautifully small star-disk, which, owing to the large angular aperture of the combination, remains steadily in view for several hours. The optical characters of this disk vary considerably with the quality of the lenses; practically a very fine one-quarter by Powell and Lealand produces disks of remarkable beauty and precision. In some cases a plane mirror is conveniently attached to reflect the oblique solar rays.