Showing papers in "Journal of the Optical Society of America in 1960"

Use of Annular Apertures to Increase Focal Depth

Abstract: A theoretical study is made of the possibilities of using an annular aperture to increase the focal depth of a photographic objective. It is shown that for a given gain in focal depth the loss in speed is the same for both annular apertures and conventional stopping down. For images of isolated point objects, the definition is improved by using an annular stop. The gain in focal depth is less for off-axis points, but it is found that, for example, a factor of 2.7 in focal depth gained by means of an annular aperture is barely affected at a field angle of 30°.

The Wire Grid as a Near-Infrared Polarizer

Abstract: Wire grids have been fabricated with microscopic elements and periodicity These wire grids behave as linear polarizers over large portions of the infrared spectrum, confirming predictions from electromagnetic theory The method of fabrication of grids with periods as small as 463 mμ is described Limits on the performance of a wire-grid polarizer are set by fineness of period, optical constants of the metal, spectral transmission of the supporting substrate, and by skin-depth effects Within these limits, excellent polarizer performance may be obtained over a very wide band of wavelengths, and these new wire-grid polarizers do, in fact, cover the range 2–15 μ and beyond Appreciable polarization is observed at near-infrared and visible wavelengths Wire grids may be used in convergent beams of radiation or in restricted geometrical arrangements, unlike many pile-of-plate or prism polarizers These wire-grid polarizers have proved useful in measuring the imperfections of other types of infrared polarizers

Axicons and Their Uses

Abstract: The most common axicon is a flat cone. A small source of light on the axis of the cone is imaged into a line along a portion of the axis. In lenses the spot diagram has been useful in evaluating image quality. In axicons a corresponding line diagram where lines take the place of dots is useful. In general, axicon instruments correspond to the usual optical instruments. For example, an axicon may be used as an objective to form a telescope. The resulting axicon telescope may be used in aligning machinery such as paper mills. Similarly, an axicon autocollimator may be used to precisely set mirrors perpendicular to a line. One form of axicon microscope has been tried out for the special purpose of locating the position of shiny surfaces without touching them. A most useful form of optical aligner is the reflection cone axicon. It is used as a straight edge. One example is a reflecting cone of 6 in. diam and maximum range of 40 ft with precision of 5 or 6 wavelengths over the entire range. Another example is a 5 in. diam cone with a range of 10 ft and precisions of about 1 wavelength. In this case the use of a suitable radius for the reflecting surface had the effect of making the image brightness substantially uniform over the 10 ft range. Photo cell pickup has been shown to be successful with very high precisions of setting. This opens the way for automatic machine guiding to very high precisions.

Memory colors of familiar objects.

Abstract: The memory colors of ten familiar, naturally occurring objects have been determined. Fifty observers chose their memory colors from an array of 931 Munsell color chips. The variability of the judgments is shown and their means are compared with the average chromaticities of the corresponding natural objects. The ten mean memory colors were all significantly different from the natural colors. Each memory color tended to be more characteristic of the dominant chromatic attribute of the object in question; grass was more green, bricks more red, etc. In most cases, saturation and lightness increased in memory.

Iridescent Colors of Hummingbird Feathers

Abstract: The reflectance spectra of iridescent hummingbird feathers were measured spectrophotometrically for many species, and the structure of the iridescent parts of such feathers investigated by electron microscopy. In the direction normal to the iridescent feather surfaces the structure is found to be periodic in optical dimensions. It consists of stacks of 7–15 elliptical platelets of a material with refractive index ~2, each containing a structurally reinforced air gap. To interpret the reflectance spectra in terms of the surface structures, a theory is given of reflection coefficients of stratified media with analytic refractive-index variations. The theory is applied to the iridescence of hummingbird feathers and accounts quantitatively for the iridescent colors as interference colors by reflection from the observed feather structures.

Analysis of Eye Movements during Monocular and Binocular Fixation

Abstract: Recordings of the horizontal component of movements of the eyes were made during monocular and binocular fixation. The variation in the vergence of the eyes over time was found to be of the same order of magnitude as the variation in position of the individual eyes, even though the lateral positions of the two eyes are somewhat correlated. The drift and tremor of the two eyes are not correlated; the over-all correlation is due to the saccadic movements. Saccades in one eye seem to be always accompanied by simultaneous saccades in the other eye which are almost always in the same direction and about the same in size. The maintenance of binocular fixation does not seem to be dependent on a direct response to or sensing of vergence error. Rather, it appears to be dependent on the saccadic responses of the two eyes to their own fixation errors.

Neural Inhibitory Units of the Eye and Skin. Quantitative Description of Contrast Phenomena

Abstract: Experiments on the skin show that it is convenient to use a “neural unit” for the description of some phenomena of sensation magnitude that have been observed. The neural unit consists of an area of sensation surrounded by a refractory area of inhibition. The paper tries to show how the numerical values for the unit can be determined for the eye and the skin and how they can be used in the description of Mach bands. In addition, step functions are described from which it is possible to deduce two simple formulas for the calculation of Mach bands. This procedure is very similar to the method used in telecommunication theory for the calculation of transients.

Emissivity of Dispersed Carbon Particles

Abstract: The emissivity of spherical carbon particles is calculated in both the infrared and visible regions of the spectrum. The scattering and absorption cross sections for individual particles are obtained from the Mie theory of scattering. A suitable dispersion equation is derived which represents the optical properties of carbon at flame temperatures. An expression is obtained for the radiation intensity emitted by a large number of dispersed particles which includes all higher order scattering processes. From these results the emissivity of carbon particles in flames is calculated for particle radii in the range from 50 to 800 A and for 109 to 1015 particles cm−2. In addition the emissivity is obtained for several different particle size distributions which are representative of actual flames. A quantitative explanation is given for the occurrence of the intensity maximum at shorter wavelengths than corresponds to the blackbody maximum at the same temperature.

Effects of Involuntary Eye Movements on Visual Acuity

Abstract: This research was conducted to evaluate the effect of involuntary eye movements on visual acuity. Three types of acuity target—vernier, fine line, and grating—were observed for varying exposure durations under two viewing conditions. One was the “stabilized image” condition where a mirror on the eye was used to reflect the target beam in such a way as to stop the motion of the retinal image that would otherwise accompany the eye movements. The other viewing condition was optically the same except that the eye movements produced normal motions of the retinal image. Acuity was defined in terms of a minimum angle of resolution, i.e., the threshold value of the angle subtended by the critical dimension of the target. Acuity was found to improve with increasing exposure time up to about 0.2 sec under both viewing conditions. The main conclusion is that acuity is neither enhanced nor impaired by the involuntary eye movements that are present during the inspection of a test object.

Precision Measurement of Absolute Specular Reflectance with Minimized Systematic Errors

Abstract: Instruments for making precision measurements of the specular reflectance at essentially normal incidence in the near ultraviolet, visible, and infrared regions of the spectrum are described. The square of the absolute reflectance is measured, with a resultant increase in measuring precision, and the major sources of systematic error in making reflectance measurements have been reduced or eliminated. The difficulty in making precision reflectance measurements in the infrared, where integrating spheres cannot be used, has been overcome by a unique compensating feature in the infrared reflectometer which prevents the image on the detector from changing size or position because of a slight tilt of the sample. The measurements made with these instruments on high reflectance samples are believed to be good to ±0.001.

Useful Representations for Measurements of Spectral Band Absorption

Abstract: Three different methods are discussed for the representation of laboratory measurements of spectral band absorption. Each method is valid over a considerable range of pressure and amount of absorbing gas; together they accurately represent the absorption for virtually all values of these variables by means of reasonably simple mathematical expressions. Since these approximations have a valid theoretical basis, they can be used to obtain reliable values of the absorption for pressures and amounts of absorbing gas outside of the original experimental range. The three methods are (1) strong line approximation; (2) weak line approximation; (3) nonoverlapping line approximation. Methods (1) and (2) are valid regardless of whether or not the spectral lines overlap. Method (3) is valid regardless of whether or not the absorption is weak or strong at the line centers. The regions of validity of each of these approximations are given for the Elsasser and statistical models. An analysis is made by these methods of the published data on the absorption of the 9.6-μ band of ozone.

Eye Movements Recorded During Convergence and Divergence

Abstract: The purpose of the present experiments was to record horizontal binocular eye movements during normal amounts of convergence and divergence by a method having satisfactory sensitivity and accuracy. This is a method of direct photography, based upon collimated beams of light reflected from plane mirrors mounted on tightly fitting contact lenses. The records show that the eyes exhibit relatively rapid and accurate motions of vergence when fixating alternately a near and a far test object. There is no evidence for a systematic discrepancy between the extent of vergence and the geometrical location of the test object. It is concluded that when fusion is maintained, the images of a fixation object are brought to corresponding areas of the retina to an accuracy of about two minutes of arc. This finding is consistent with the reported sizes of Panum’s area and the region of optimal visual resolution.

Search in an unstructured visual field.

Abstract: Search in an unstructured visual field is an independent random process under the conditions described in this paper. The basic data for this report are 3072 search trials for each of four practiced observers. The four targets were circular and intercepted angles of 4.8′, 13′, 24′, and 46′. Four search areas which measured 0.011, 0.084, 0.26, and 0.48 sterad, and four values of background luminance 0.01, 0.1, 1.0, and 12.4 ft-L were employed. Four contrasts were used for each of the 16 target size and background luminance conditions. These contrasts were generally at least twice the 95% threshold contrast.The data for the four subjects were combined, and since a one-parameter distribution was an adequate description, the mean time to detection has been tabulated for the 64 different experimental conditions to summarize these findings.

Spectral Radiance of Sky and Terrain at Wavelengths between 1 and 20 Microns. II. Sky Measurements

Abstract: Quantitative data are presented to illustrate the infrared spectral radiance of the sky under a variety of situations. A qualitative discussion of the dominant spectral features is given to show the importance of the emission and scattering processes, the effect of angle of elevation, angle of azimuth, sun angle, and cloud cover.

Lamellar Grating Far-Infrared Interferomer

Abstract: The new variable-groove-depth grating-type interferometer that was used to obtain experimental interferograms and spectra used for short and long wavelengths (λ 4 mm), together with its construction and testing, is described.

Reflectance-Increasing Coatings for the Vacuum Ultraviolet and Their Applications*

Abstract: The extreme ultraviolet reflectance of aluminum prepared under optimized conditions is reviewed, including a study of the aging parameters. The theory of reflectance-increasing films for the vacuum ultraviolet is discussed. It is shown that single dielectric films, in addition to preventing the growth of an oxide film, can have a surprisingly strong reflectance-increasing effect on aluminum in this spectral region. The use of MgF2 as a reflectance-increasing coating for the extreme ultraviolet above 1100 A is considered in some detail. It is pointed out that two-layer reflectance-increasing coatings on aluminum have only a very small advantage over single-layer coatings in the extreme ultraviolet. The usefulness of single slightly absorbing films as reflectance-increasing coatings is treated. The problem of increasing reflectance in the spectral region below 800 A is discussed. A new apparatus is described which allows the preparation and measurement of film samples without exposure to air. The effect of the new high-reflectance coatings on the relative merit of vacuum ultraviolet monochromator designs is considered.

Movement Thresholds in Peripheral Vision

Abstract: The absolute velocity thresholds of movement were determined at 48 positions in peripheral vision. An aircraft-type instrument, with a standard altimeter hand, was located at random positions on the concave, black surface of an 80-in. Fiberglas hemisphere. Four types of movement were investigated (clockwise and counter-clockwise rotation, vertical and horizontal motion) under conditions of constant photopic lighting. While the subject fixated on the center point of the hemisphere, the absolute velocity threshold of each type of movement was determined for each position using the method of limits. Ten airline pilots served as subjects. The absolute threshold isograms on perimetric charts for both rotary and linear motion are elliptical in shape, with the horizontal axis approximately twice as long as the vertical axis. There is no difference between a subject’s ability to see clockwise or counter-clockwise rotation. An individual’s ability to perceive vertical motion is slightly better than his ability to perceive horizontal motion in the area adjacent to the horizontal axis. Velocity and area swept by the instrument hand are significant factors in the perception of movement, but they are not similarly correlated for all types of movement.

Reflection and Refraction at a Random Moving Surface. I. Pattern and Paths of Specular Points

Abstract: Light falling from a point source on a ruffled surface produces a pattern of images, which move about over the surface. The image points correspond to the maxima, minima, and saddle points of a certain function. It is shown that the images are generally created in pairs, a maximum with a saddle point or a minimum with a saddle point, and that the total numbers of maxima, minima, and saddle points satisfy the relation Nma+Nmi=Nsa+1.The process of creation or annihilation of images is studied in detail, and also the tracks of the image points, in certain special cases. It is shown that closed tracks may be common. This is confirmed by photography of the sea surface.

Determination of the Radial Distribution of Brightness in a Cylindrical Luminous Medium with Self-Absorption

Abstract: The two-path method (utilizing a mirror) is shown to provide sufficient information to solve for the true radial brightness distribution of a luminous medium in the presence of self-absorption. In the case where self-absorption is not large, an approximate solution to the problem is shown to be no more difficult than a solution for the “optically thin” approximation.

Thermodynamic Limitation on the Conversion of Heat into Light

Abstract: The problem of converting ambient heat into luminescence radiation is analyzed in terms of the thermodynamics of the electromagnetic field. The process is described in terms of the technical efficiency of a light source η, defined as the ratio of the power leaving the source in the form of luminescence radiation to the power supplied to the source in the form of work. For a source at the ambient temperature T, it is shown that the limitation imposed by thermodynamics is, in the steady state, η≤1+T/(Tf−T), where Tf is the ratio of the net rate at which the field carries energy away from the source to the net rate at which the field carries entropy away from the source as a result of the luminescence emission. Thus, T/(Tf−T) is the maximum possible contribution of ambient heat to the technical efficiency of a light source.An explicit expression for Tf in terms of the ambient temperature and the spectral distribution of the luminescence emission is obtained. It is shown that Tf≥T, and that Tf is a monotonic increasing function of the ratio of the integrated intensity of the luminescence emission to the bandwidth of the emission spectrum. For moderate integrated intensities and finite (but narrow) bandwidths, it is shown that Tf is approximately equal to the brightness temperature of the light source, and it is concluded that thermodynamics forbids technical efficiencies greater than about 160% for room-temperature light sources of practical brightness. As an example, Tf is calculated for the (green) emission band of a typical ZnS phosphor.

Relation between directional sensitivity and spectral response curves in human cone vision

Abstract: In human color vision each incident spectral energy distribution is coded in three values, corresponding to the responses of the red, green, and blue receptor systems. The Stiles-Crawford effect and the color phenomena associated with it indicate that these three values are in a different degree for each wavelength dependent on eccentricity of the point of entry of the light in the pupil.A rather quantitative description of the facts is given by a theory in which leak factor of the light to the surrounding tissue in the outer and the inner segment is involved. The leak factor implies that for oblique light the effective pathway in the outer segment is shortened. This results in narrowing of the fundamental response curve of the particular receptor.Theory strongly points to rather high densities of the photopigment in the receptor, but not improbably high. On the contrary, there is agreement with other sound suggestions and estimates of the density value. As absorption curves for the red, green, and blue pigments we used Pitt’s curves. The agreement of different experimental data with this theory strongly supports the Young-Helmholtz hypothesis, as far as the receptor layer is concerned, and also indicates that the curves derived by Pitt are fairly good approximations of the absorption curves of the photopigments after correction for absorption of light by the ocular media.

Precise Measurement by Image-Splitting

Abstract: The image-splitting principle offers important advantages over the use of filar micrometers or graticules for the measurement of an image transmitted through an optical system. A method for applying this principle to the microscope is described.Theoretical considerations indicate that a setting accuracy of one-tenth of the radius of the Airy disk should be attainable, and it is shown that under certain conditions no systematic error is introduced. Experiments are described which substantiate these conclusions and illustrate some applications of the instrument.

Two Instruments for Atmospheric Optics Measurements

Abstract: A description is given of the design and construction of two instruments for studying atmospheric optics One device, the Recording Polar Nephelometer, measures the volume scattering index of light passing through a sample of natural atmosphere, with scattering angle, polarization, and wavelength as variables A new calibration procedure has been developed which employs a diffusing screen of known reflectance and transmittance as the standard The second device, the Portable Transmissometer, measures the extinction coefficient with an accuracy of 5% under all conditions These instruments are transported by a specially-equipped station wagon to form a mobile research unit Samples of results obtained in fog and clear air are included

Appraisal of Land's work on two-primary color projections.

Abstract: An analysis of the results of Land’s experiments with two-primary color projections has been carried out in terms of the known phenomena of object-color perception. It is shown that no new theory is required for the prediction of Land’s result that two-primary color projections can produce object-color perceptions of all hues; nor for his result that many choices of pairs of primaries yield substantially the same object-color perceptions. Land’s hypothesis that when the colors of the patches of light making up a scene are restricted to a one-dimensional variation of any sort, the observer usually perceives the objects in that scene as essentially without hue, is new; several special cases of it are supported by previous work as well as Land’s. This hypothesis deserves the serious attention of research workers in object-color perception.

Ultraviolet Spectral Energy Distributions of Natural Sunlight and Accelerated Test Light Sources

Abstract: A contributory factor to the failure of correlation between outdoor and indoor photodecomposition tests is the difference in the ultraviolet spectral energy distributions of natural sunlight and the various indoor test sources which do not duplicate sunlight and differ widely among themselves.The ultraviolet spectral energy distributions of natural sunlight and of a variety of indoor exposure test sources have been measured. By use of a ferrioxalate actinometer, the measurements have been put on an absolute basis permitting the intercomparison of the sources. The xenon arc was found to be the best approximation of sunlight in the ultraviolet. For shorter-wavelength ultraviolet (below 3500 A), the combination of fluorescent Sunlamp and Blacklight lamps also approximated sunlight.

Absorption by Infrared Bands of Carbon Dioxide Gas at Elevated Pressures and Temperatures

Abstract: Experimental data and empirical correlations for the total absorption of band groups at 15, 10.4, 9.4, 7.5, 5.2, 4.8, 4.3, 2.7, 1.6, and 1.4 μ are presented for carbon dioxide gas in nitrogen at total pressures from 0.5 to 10 atm, temperatures from 294° to 1390°K (530° to 2500°R) and mole fractions from 0.05 to 1.00. The data were obtained from low-resolution measurements of the spectral absorptivity. Use of optical paths of 129 cm at 294°K and 38.8 cm at that and higher temperatures resulted in a range of mass path lengths from 10 to 24 500 g/m2.

Turbid Media with Plane-Parallel Surfaces*

Abstract: In the case of uniformly diffuse internal incidence on a plane surface, formulas for the internal reflectance based on Fresnel’s laws have been given by Walsh and Judd. Several investigators of turbid media such as opal glasses have found, however, experimental values smaller than these. A new explanation is given in the present paper based on refraction instead of imperfect diffusion as suggested in the literature. It is verified by a series of observations with two kinds of opal glasses. The only formulas used in this paper are those given long ago by Stokes, adapted for diffuse incidence. A unique solution of the Stokes formulas is given here. The underlying theory is the two-constant Schuster theory, whose shortcomings are also discussed.

Vibration-Rotation Spectra of HCN†

Abstract: An analysis of the spectrum of the linear unsymmetric molecule HC12N has been made permitting the determination of the 21 constants necessary for predicting the vibrational frequencies and the 10 constants necessary for predicting the B value for the various vibrational states. To determine these constants new measurements were made on numerous bands in the region of 1–3 μ employing a 5-m vacuum spectrograph. Several instances of Fermi resonance were detected and analyzed. Except for a few bands where additional resonances may be present, the vibrational constants predict the measured values for the band origins of 44 bands within an amount not much greater than the expected experimental error. The rotational constants also predict the B value within the experimental error for 24 bands where data are available.Bands of HC13N and DC12N were also measured to determine the α values for calculation of the equilibrium moment of inertia. The 101–000 and 1111–0110 bands were used for all three isotopic forms of HCN to determine the Be values in a parallel fashion. From these values the bond length C–H=1.06593±0.00010 A and C–N=1.15313±0.00002 A were determined.In five different cases in HC12N it was possible to apply the Ritz combination principle to determine the frequency of the 0110 state. By using this value and the rotational constants it was possible to calculate the frequencies of lines in the 0110–000 band. The principle is also applied to HC13N and DC12N.

Vibration-Rotation Bands of N 2 O†*

Abstract: The spectrum of nitrous oxide has been measured with a high-resolution grating spectrometer in the region from 2395 to 3510 cm−1. Long absorbing paths were used in a heated cell, so that it was possible to observe many of the weaker bands, including those with lower vibrational levels ν2, 2ν20, 2ν22, and ν1. Accurate values of the rotational constants have been obtained, including the l-type doubling and variations with v and l of the centrifugal stretching constant. These have been correlated with other data to obtain improved values of the molecular constants, with particular attention to the Fermi interaction. Other weak interactions are observed and discussed.