Monochromatic Patterns Research Articles

Colored diffraction catastrophes.

On fine scales, caustics produced with white light show vividly colored diffraction fringes. For caustics described by the elementary catastrophes of singularity theory, the colors are characteristic of the type of singularity. We study the diffraction colors of the fold and cusp catastrophes. The colors can be simulated computationally as the superposition of monochromatic patterns for different wavelengths. Far from the caustic, where the luminosity contrast is negligible, the fringe colors persist; an asymptotic theory explains why. Experiments with caustics produced by refraction through irregular bathroom-window glass show good agreement with theory. Colored fringes near the cusp reveal fine lines that are not present in any of the monochromatic components; these lines are explained in terms of partial decoherence between rays with widely differing path differences.

Laue Crystallography for Studying Rapid Reactions

Precise quantitation of static and time-resolved Laue diffraction patterns is undeniably more complex than for monochromatic patterns. Recent advances in integration and scaling algorithms demonstrate that, with suitable care in the conduct of the Laue experiment itself, Laue data sets can be obtained which rival the best monochromatic data sets in accuracy and completeness. These algorithms deal in an integrated fashion with the several main problems of Laue diffraction patterns: the elongated spots which arise from mosaic crystals, the spatial overlaps which occur in crowded diffraction patterns, the energy overlaps which arise from the mapping of a central line in reciprocal space onto a single spot in detector space, and wavelength normalization.

The role of monkey inferior parietal cortex in visual discrimination of identity and orientation of shapes

Three cynomolgus monkeys (Macaca fascicularis) were tested for their visual learning ability following bilateral lesions to the inferior parietal cortex. In discrimination learning with patterns differing from each other in form and colour their performance was normal. In discriminating monochromatic patterns their performance depended on the nature of the pattern discrimination; when the discriminanda were different shapes (e.g. x,v) they performed well, while with discriminanda which differed only in the orientation of the shape (e.g. p,d) they performed relatively badly. These results were compared to the previously reported results of normal animals and animals with inferior temporal lesions performing the same task. The ratio of errors in orientation discrimination to errors in shape discrimination was significantly higher than normal in the group with parietal lesions, and significantly lower than normal in the group with inferior temporal lesions. These results suggest that the roles of inferior temporal and inferior parietal cortex in the discrimination of shape and orientation are complementary.

Achromatic system for far-field diffraction with broadband illumination

An achromatic Fourier transformer working with spatially coherent but temporally incoherent light is described both theoretically and experimentally. The achromatic Fraunhofer diffraction pattern results from the incoherent superposition of the monochromatic diffraction patterns generated by every spectral component of light. Wavelength compensation is achieved by a specific configuration combining airspaced zone plates and lenses. Several parameters, such as the spectral resolution of the setup or the spectral broadness of the source, govern the accuracy of coincidence of the various monochromatic patterns. Experimental results demonstrate the broadband operation of the system; the error on the achromatic Fourier transformation is 2% over the visible spectrum. Applications in colored image processing are also reported.

Spectral sensitivity of the landing blowfly

Whether or not colour discrimination is involved in the process of eliciting the landing response of the fly was investigated with rotating linear spiral patterns of different spectral composition. With monochromatic patterns execution of the response depends on the contrast in the pattern: no response occurs within a restricted range of intensity ratios corresponding with low pattern contrast. With dichromatic patterns execution of the response depends on the intensities of the two spectral components (which each dominate half the pattern). No response occurs within a restricted range of intensity ratios; this range characteristically depends on the spectral wavelengths in the pattern. The mean values of these ranges are used to determine a spectral sensitivity distribution of the landing response.

Dislocations in diffraction patterns: continuous waves and pulses

When a monochromatic wave is scattered by fixed objects it produces a stationary three-dimensional diffraction pattern, but if the driving oscillation is a quasimonochromatic pulse the diffraction pattern naturally changes with time. Both the fixed and the moving patterns may be usefully characterized by their dislocation lines, where the amplitude is zero and the phase is indeterminate. The paper studies the relation between the fixed dislocation lines (continuous wave (c.w.) null lines, interference fringes) of the monochromatic pattern and the moving lines of the pulse pattern. The latter sweep out surfaces, called the dislocation trajectories. A simple model system with two interfering beams illustrates how dislocation lines can appear and disappear from the head or tail of the pulse and how pairs of dislocations can be created in two different ways. It also shows how the shape and bandwidth of the pulse affect not so much the positions of the trajectories, but their lengths and the way they are traversed in time by the dislocations. In a general theory the pulse is regarded as a slightly perturbed continuous wave and its behaviour is deduced from the continuous wave response of the (linear) system within the bandwidth of the pulse. The method is based on obtaining the fastest initial convergence of a functional series. For very small bandwidth it leads to a generalization of the concept of group velocity. In this case dislocations by interference appear only very close to c.w. nulls. For larger, but still small, bandwidth, moving dislocations appear whose trajectories are approximately parts of frequency minimum surfaces , that is, surfaces in the continuous-wave diffraction pattern where the amplitude is a minimum with respect to changes in frequency. These surfaces contain the c.w. null lines. In the next approximation it is found that the trajectory surfaces nearly, but not quite, contain these lines. In certain two-dimensional cases the trajectories follow valleys in the landscape defined by the continuous-wave amplitude pattern. The prediction of the arrival times of the dislocations is delicate. General formulae, suitable for numerical computation, are given in terms of frequency derivatives of the continuous-wave diffraction pattern. They give successful results in three applications to pulsed wave fields: the diffraction pattern near a cusped caustic, the two-beam model, and a piston radiator.

Two-dimensional spectroscopy of electrophoretic gels

Optical techniques are described which permit one to analyze two-dimensional electrophoretic gels in a fashion which is analogous to the one-dimensional spectroscopy of solutions. In the methods described, an electrophoretic gel is irradiated with monochromatic light and isozyme patterns are detected by the absorption of light or the fluorescent emission of light. The system described can both generate and detect monochromatic light in a range from 200 to 1100 nm. Without the use of histochemical stains, several isozymes have been visualized by purely optical means. Five methods for the visualization of lactate dehydrogenase and five methods for the demonstration of trypsin isozymes are described. In addition, general methods have been formulated for hydrolases and oxidases. Gel spectroscopy should permit the investigation of a wide range of new isozymes.

Evoked potentials specific to spatial patterns of luminance and colour

The human brain generated electrical responses (EPs) to changes in the chromatic contrast of an equiluminant spatially-patterned stimulus even when the eye's longitudinal and transverse chromatic aberrations were simultaneously cancelled. Pattern EPs were strongly affected by ocular chromatic aberration. EP and subjective estimates of longitudinal chromatic aberration agreed closely. Psychophysical contrast threshold correlated with the amplitudes of EPs elicited both by monochromatic patterns of checks and by equiluminant patterns of alternately-coloured checks or bars. Restricting stimulation to the fovea may enhance the effect of intersubject variations of neuroanatomy upon EPs. Physiological signals seem to be segregated into colour channels until the first responses to contrast. The EP data suggest that there is an antagonistically-organized chromati-contrast channel in parallel with colour-coded luminance-contrast channels. EP results suggest that the human visual system handles colour information differently when colour is linked with spatial form than when colour is not linked with form.

Antenna radiation pattern measurement using time-to-frequency transformation (TFT) techniques

A method of inferring microwave antenna radiation patterns over wide bandwidths from a series of subnanosecond pulse measurements is described. Monochromatic patterns can be inferred from the time-domain measurements with simple apparatus. Results are presented which show that the method is inherently insensitive to errors caused by reflections from the environment.

Splitting of the Sodium Doublet Lines

WHILE demonstrating the interference patterns of various lines by means of a Hilger C.D. instrument in conjunction with a Lummer-Gehrcke plate, a Philips 'Philora' sodium vapour lamp was used to produce the patterns of the sodium, doublet λ 5890/96. This lamp is in the form of a U-tube enclosed in an outer vacuum jacket and is excited by 230 V. A.C. mains through the usual electric control gear. The lamp usually takes about 10–12 minutes to reach its maximum intensity. In the first few minutes, each, of the two lines showed its normal monochromatic interference pattern as sharp fringes; but as the lamp got heated and acquired its steady state of illumination, the fringe system of both the lines presented a complex appearance. To follow this development of complexity, the lamp was switched off for some time to cool down and then restarted. The fringe pattern was viewed through the telescope from the time it was switched on. As the lamp gained its full wattage (65 W.), each of the fringes in the two wave-lengths began to broaden out and ultimately developed almost symmetrical splitting into two defined components. The magnitude of the actual splitting computed from measurement is, within the limits of experimental errors, 0·064 A. for λ 5890 and 0·053 A. for λ 5896.

Distortion of the Photoelastic Fringe Pattern in an Optically Unbalanced Polariscope

Abstract This paper describes the results of an investigation of the effect of inaccuracies in the two quarter-wave plates of a photoelastic polariscope on the configuration of the monochromatic fringe pattern. It is shown that the fringe pattern is distorted to an extent which depends upon the orientations of the axes of principal stress and the difference between the retardations of the two plates.