Showing papers in "Journal of the Optical Society of America in 1954"
TL;DR: In this paper, a method was developed for interpreting the statistics of the sun's glitter on the sea surface in terms of the statistic of the slope distribution, which was applied to aerial photographs taken under carefully chosen conditions in the Hawaiian area.
Abstract: A method is developed for interpreting the statistics of the sun’s glitter on the sea surface in terms of the statistics of the slope distribution. The method consists of two principal phases: (1) of identifying, from geometric considerations, any point on the surface with the particular slope required for the reflection of the” sun’s rays toward the observer; and (2) of interpreting the average brightness of the sea surface in the vicinity of this point in terms of the frequency with which this particular slope occurs. The computation of the probability of large (and infrequent) slopes is limited by the disappearance of the glitter into a background consisting of (1) the sunlight scattered from particles beneath the sea surface, and (2) the skylight reflected by the sea surface.The method has been applied to aerial photographs taken under carefully chosen conditions in the Hawaiian area. Winds were measured from a vessel at the time and place of the aerial photographs, and cover a range from 1 to 14 m sec−1. The effect of surface slicks, laid by the vessel, are included in the study. A two-dimensional Gram-Charlier series is fitted to the data. As a first approximation the distribution is Gaussian and isotropic with respect to direction. The mean square slope (regardless of direction) increases linearly with the wind speed, reaching a value of (tan16°)2 for a wind speed of 14 m sec−1. The ratio of the up/ downwind to the crosswind component of mean square slope varies from 1.0 to 1.9. There is some up/downwind skewness which increases with increasing wind speed. As a result the most probable slope at high winds is not zero but a few degrees, with the azimuth of ascent pointing downwind. The measured peakedness which is barely above the limit of observational error, is such as to make the probability of very large and very small slopes greater than Gaussian. The effect of oil slicks covering an area of one-quarter square mile is to reduce the mean square slopes by a factor of two or three, to eliminate skewness, but to leave peakedness unchanged.
2,270 citations
TL;DR: The axicon autocollimator as discussed by the authors is a projector which projects a straight line of images into space, and it can be used to determine the perpendicularity of a mirror.
Abstract: A search for a universal-focus lens has led to a new class of optical elements. These are called axicons. There are many different kinds of axicons but probably the most important one is a glass cone. It may be either transmitting or reflecting. Axicons form a continuous straight line of images from small sources.One application is in a telescope. The usual spherical objective is replaced by a cone. This axicon telescope is in focus for targets from a foot or so to infinity without the necessity of moving any parts. It can be used to view simultaneously two or more small sources placed along the line of sight.If a source of light is suitably added to the telescope it becomes an autocollimator. Like ordinary autocollimators it can be used to determine the perpendicularity of a mirror. In addition, it can simultaneously act as a telescope for a point target which may be an illuminated pinhole in the mirror.The axicon autocollimator is also a projector which projects a straight line of images into space.
956 citations
TL;DR: It is shown by theory and experiment that reflectance and absorption of a nonhomogeneous specimen depend on the direction of illumination, whereas transmittance does not.
Abstract: The derived laws apply to layers whose scattering coefficient S and absorption coefficient K vary vertically to the surface of the layer. In the general case the differential equations of the preceding paper [ P. Kubelka , J. Opt. Soc. Am.38, 448 ( 1948)] must be used; the coefficients, however, hitherto constant, now are functions of the distance x from the surface. In the practically important case in which K/S is constant, one may introduce the variable p, such that p≡∫0x(x)dx. One reduces thereby the nonhomogeneous to the previously treated homogeneous case.Transmittance T1,2 and reflectance R1,2 of two nonhomogeneous sheets can be calculated by the following equations: T1,2=T1T21-R1R2, R1,2=R1+T12R21-R1R2,where T1, T2, R1, R2 are the transmittances and reflectances of the single sheets, and R1 represents the reflectance of the first sheet when illuminated in the inverse direction. Analogous formulas for more sheets and formulas relating transmittance, reflectance for specimens upon black, gray or white backing surfaces, and contrast ratio, are derived.It is shown by theory and experiment that reflectance and absorption of a nonhomogeneous specimen depend on the direction of illumination, whereas transmittance does not.
777 citations
TL;DR: In this article, a comparison method for determining the values of the absorption coefficient k of a metal relative to previously determined values for Ag was given. But the method was only applied to the wavelength region of 0.65μ to 0.95μ for metals with high reflectivities.
Abstract: A description is given of a comparison method for determining the values of the absorption coefficient k of a metal relative to previously determined values for Ag. In this method transmission interference filters are constructed which have mica as the dielectric and have reflecting layers of Ag and of the metal x being studied. On adjacent areas of a single sheet of mica, pairs of filters are formed, one with a Ag-mica-Ag combination and the other with a Ag-mica-x combination. Components of a filter pair will have identical mica thicknesses. For a given filter pair the wavelengths transmitted by one combination are compared with those transmitted by the other. Values of k are then calculated from the change in phase accompanying the reflection of light at normal incidence at mica-metal interfaces. The method was found to have special advantages in the wavelength region of 0.65μ to 0.95μ for metals with high reflectivities. Results are given for Au, Cu, and Al in the range of 0.45μ to 0.95μ. The new values of k tend to be higher than older values obtained by other methods. A discussion is included concerning the validity of surface measurements for determining the bulk properties of metals.
348 citations
TL;DR: In this article, the authors compare the luminosities of the three types of instruments with equal resolving power and equal dimension, and reveal a great superiority of the grating over the prism for all regions of wavelengths.
Abstract: Formulas have been established which express the flux given by a spectrometer as a function of the effective resolving power and of the dimension of the dispersive system (area of the base of the prism, or area of the grating, or area of the plates of the etalon). It is thus possible to compare the luminosities of the three types of instruments with, in each case, equal resolving power and equal dimension. This comparison reveals a great superiority of the grating over the prism for all regions of wavelengths, and a great superiority of the etalon over the grating. In the case of the etalon, the overlapping may be overcome by the use of several etalons in series.
347 citations
TL;DR: In this article, the sine wave response factor is calculated by measuring the response to a square wave (bar pattern) and calculating by a simple formula the corresponding SINR response factor.
Abstract: The imaging properties of an optical system can be in many cases completely specified by a function of a single variable. A convenient function is the response to a sine wave test pattern as a function of “frequency,” i.e., lines/mm. The difficulty of experimentally providing such a test pattern can be avoided by measuring the response to a square wave (bar pattern) and calculating by a simple formula the corresponding sine wave response factor. The convenience of the sine wave response factor in calculating system performance is illustrated by application to a fluoroscopic imaging system.
296 citations
TL;DR: In this paper, the reflectivities of Ag, Au, Cu, and Al were measured at an angle of incidence of 45°, in the wavelength range of 0.40μ to 0.95μ at glass-metal and air-metal interfaces.
Abstract: The reflectivities of Ag, Au, Cu, and Al were measured at an angle of incidence of 45°, in the wavelength range of 0.40μ to 0.95μ at glass-metal and air-metal interfaces. These reflectivities, together with previously determined values of the absorption coefficient k, were used to calculate the index of refraction n. Samples were prepared by evaporation and deposition from the vapor. The important experimental results are: (1) The ratios of the reflectivities in the s plane to those in the p plane agreed with the theoretical values. This result indicates that the boundary conditions required for the application of the equations of electromagnetic theory have been satisfied. (2) Ageing and annealing resulted in increased reflectivity at both air-metal and glass-metal interfaces. (3) The values of n obtained from aged or annealed samples were in many cases considerably lower than previously published values.
266 citations
TL;DR: The results of measurements of the critical flicker frequency of the eye depending on both the average luminance of the test field and the time function of this luminance were recorded by plotting theripple ratio r versus the critical frequency.
Abstract: The results of measurements of the critical flicker frequency of the eye depending on both the average luminance of the test field and the time function of this luminance were recorded by plotting the “ripple ratio” r versus the critical frequency; r is defined as amplitude of first Fourier component/average luminance of the stimulus. It is shown that with constant average luminance the points observed for various time functions fit into one smooth curve which for low luminances is monotonous.At high luminances the curve shows a minimum for r, which is smaller than the threshold value of the foveal brightness discrimination ratio 1.35 percent, observed for a frequency f = 1 c/s.
212 citations
TL;DR: The data show that under good experimental conditions the retinal image is virtually stationary for exposures up to 0.01 sec in duration.
Abstract: The ability of an observer to maintain steady fixation on a test object is limited by involuntary eye tremor. Such tremor is characteristic of balanced muscular systems such as those serving to rotate the eye. Rotations of the eye cause proportional displacements of the retinal image of a stationary test object. Data are given for typical excursions of the retinal image during various exposure intervals from 0.01 to 1 sec in duration. The data show that under good experimental conditions the retinal image is virtually stationary for exposures up to 0.01 sec in duration. Exposures of 0.1 sec entail an average displacement of 25 sec of arc (a visual angle corresponding to the diameter of a single foveal cone). Exposures as long as 1 sec permit an average excursion of about 3 min of arc.
210 citations
TL;DR: In this article, a series of films ranging in thickness from 0.03 to 10 microns was prepared on fused quartz, glass, rock salt, and highly reflecting aluminum surfaces, and the optical constants of the films were computed in the wavelength region from −0.24 to 14 microns from reflectance, transmittance, and thickness measurements.
Abstract: Using evaporation conditions adjusted for the deposition of true silicon monoxide, a series of films ranging in thickness from 0.03 to 10 microns was prepared on fused quartz, glass, rock salt, and highly reflecting aluminum surfaces. The optical constants of the films were computed in the wavelength region from 0.24 to 14 microns from reflectance, transmittance, and thickness measurements. Silicon monoxide films exhibit strong absorption in the ultraviolet which extends into the visible, and an infrared absorption maximum at 10 microns which differs significantly from the position of the absorption maximum of fused quartz. In the wavelength region from 0.40 to 0.70 micron, the refractive index of SiO decreases with increasing wavelength from 2.15 to 1.95. Strongly oxidized films show less ultraviolet absorption, lower indexes of refraction, and a shift of the infrared absorption band to shorter wavelength. Decomposition of SiO to Si+ SiO2 results in increased ultraviolet absorption and higher refractive indexes. In the region of the 10-micron absorption a silicon monoxide protective layer of 0.125 micron in thickness has no appreciable effect on the reflectance of a highly reflecting metallic surface, but decreases the transmittance of a rock salt plate by 30 percent.
203 citations
TL;DR: In this article, the theory of the Poincare sphere is presented in detail, and its application to tracing the passage of light through doubly refracting and optically active media is fully illustrated.
Abstract: The parameters necessary to define an elliptically polarized vibration, namely the azimuth, the shape, and the sense of rotation of the ellipse described by the light vector can be represented geometrically by a point on a sphere. The method was suggested by Poincare in 1892. The theory of the Poincare sphere is presented in detail, and its application to tracing the passage of light through doubly refracting and optically active media fully illustrated. A simple model, designed on the principles involved, is described: it is suitable for instruction and demonstration.
TL;DR: In this article, the diameters of polyvinyltoluene latex were determined using three different methods: (1) for particles with diameters less than 276 mμ, the sizes were determined by dissymmetry of light scattering of dilute solutions, (2) for particle sizes between 302 mμ and 481 mμ by diffraction from the dried crystalline samples, (3) for larger particles greater than 595 mμ in diameter by interference haloes from thin dried films of the samples.
Abstract: Polyvinyltoluene latexes having uniform particle-diameters have been prepared. Diameters range from about 100 mμ to 1000 mμ. When such a monodisperse latex is dried down to the form of a friable film, the particles spontaneously arrange themselves in a close-packed crystalline array. The structure of these crystals has been investigated by means of the electron microscope, utilizing a surface replica technique.The diameters of the particles were measured optically by three different methods: (1) for particles with diameters less than 276 mμ, the sizes were determined by dissymmetry of light scattering of dilute solutions, (2) for particles with diameters between 302 mμ and 481 mμ the sizes were determined by diffraction from the dried crystalline samples, (3) for particles greater than 595 mμ in diameter the sizes were determined by interference haloes from thin dried films of the samples. Agreement is good with particle-sizes determined from electron microscopy.The diffraction from the crystalline samples of intermediate particle-size is shown to be that for a two-dimensional grating. It is suggested that these crystalline arrays be used as spectroscopic diffraction gratings.
TL;DR: In this article, it was shown that the absorption of an unsupported metal film in air is increased by supporting the metal on a dielectric of high refractive index, provided that the radiation passes through the dielectrics to the metal.
Abstract: The maximum infrared absorption that can be obtained in an unsupported metal film in air is 50 percent. It is shown that the absorption may be increased by supporting the metal on a dielectric of high refractive index, provided that the radiation passes through the dielectric to the metal. Under certain conditions there is then no reflection from the layer. If the combination is reversed so that the radiation is incident on the metal first, a low absorption results. If the supporting dielectric is of low refractive index neither direction will lead to high absorption; in this case the use of another system, with metal on each side of the support is preferable. Using whichever of the two systems is appropriate to the supporting dielectric, an absorption of 70 percent is always obtainable.
TL;DR: The practical importance of a simply administered, comprehensive test, such as is provided by the H-R-R plates, to vocational selection and guidance is discussed, and an example given of their application in the radio and television industry.
Abstract: A description is given of the H-R-R plates and evaluation of the test as a device (a) to separate those with defective color vision from those with normal color vision, (b) to classify the type of defect (qualitative diagnosis), and (c) to indicate the extent of the defect (quantitative diagnosis). For this evaluation 780 subjects were used, not selected on the basis of a statistical survey. The H-R-R plates are shown to be in complete agreement with the A.O. test (revised selection) in separating the subjects into two groups: 150 with defective and 630 with normal red-green vision. The test agrees remarkably well with the Nagel anomaloscope in classifying the subjects with defective red-green vision into deutan and protan types. It rates 23 percent of these subjects as having mild defect, 31 percent as having medium defect, and 46 percent as having strong defect. For the subjects with defective color vision, a detailed comparison is presented of the qualitative classifications into deutan and protan types yielded by the H-R-R plates with those made by the Nagel anomaloscope, the Tshihara test, and the dichotomous test. A further comparison is made for these subjects between the quantitative estimates of extent of defect by the H-R-R plates and performance on various aptitude tests which were designed to select those color-defective persons who can meet the color requirements of specific but different tasks. It is shown that the percentage who pass these tests significantly and consistently decreases as the severity of defect shown by the H-R-R plates increases. The practical importance of a simply administered, comprehensive test, such as is provided by the H-R-R plates, to vocational selection and guidance is discussed, and an example given of their application in the radio and television industry.
TL;DR: Studies were made of the visual functions of subjects with complete and incomplete achromatopsia, with particular emphasis on measurements of the rates of dark adaptation in a parafoveal region of the retina and at the fovea.
Abstract: Studies were made of the visual functions of subjects with complete and incomplete achromatopsia, with particular emphasis on measurements of the rates of dark adaptation in a parafoveal region of the retina and at the fovea. When adequate preadaptation to a high luminance is achieved, the dark-adaptation curves reveal the presence of some form of photopic receptors in addition to the normal scotopic rods. Both types of receptor are present, not only in the paracentral retina, but also at the fovea.
TL;DR: In this article, a controlled source of excitation has been developed to eliminate selective volatilization and element interference effects encountered in the spectrochemical analysis of powders with the dc arc.
Abstract: A controlled source of excitation has been developed to eliminate selective volatilization and element interference effects encountered in the spectrochemical analysis of powders with the dc arc. A circular air jet, concentric with the sample electrode, was used to cool the unburned portion of the sample throughout the exposure and to stabilize the arc by surrounding it with a directed current of air. The effectiveness of the air jet was increased by placing the samples in deep narrow craters drilled in high-purity graphite electrodes. Uniform volatilization of even the volatile constituents was achieved and the spectral sensitivity of the elements was made relatively independent of the sample matrix. A maximum deviation from the mean of less than a factor of two was obtained for the sensitivity of any element when determined in a number of matrix materials including graphite, SiO2, NaCl, Al, and Zn. A significant improvement was made in the linearity of the intensity-concentration curves.
TL;DR: Results are described by the following equation: T=1B logI/I0+K,where B, I0, and K depend upon the field luminance.
Abstract: Reaction time is measured from the onset of a signal flash to the manual release of a microswitch. Responses are made by two trained subjects over wide ranges of stimulus intensity and under varying conditions of signal flash and background field luminance. Two types of experimental conditions prevail: (1) variable flash luminance at each of three field levels, and (2) variable field luminance at three constant flash intensities. Thirty-six reactions are collected for every flash-field combination at each of two fixation positions (foveal and peripheral).The results are described by the following equation: T=1B logI/I0+K,where B, I0, and K depend upon the field luminance.
TL;DR: Except for integrate capacity in space in the fovea, a considerable decrease in these capacities is demonstrated for higher levels of adapting brightness, and the amplitude of variation in sensitivity during adaptation is estimated.
Abstract: The interaction of subliminal effects arising in the retina from two stimuli presented at adjustable time and distance apart from each other is studied (with the aid of measurements of frequencies of seeing) for each stimulus separately and for the combined flash. The chance for successful interaction as a function of time and distance between the stimuli is deduced quantitatively. Experiments refer to red and green flashes on red and green adapting fields, in peripheral and foveal vision. Except for integrate capacity in space in the fovea, a considerable decrease in these capacities is demonstrated for higher levels of adapting brightness. The amplitude of variation in sensitivity during adaptation is estimated.
TL;DR: In this paper, a device that can be used for the measurement of the absolute spectral reflectivity or reflectance of metals, paints, etc. is described, in conjunction with a Perkin-Elmer Model 83 monochromator equipped with an NaCl prism for the wavelength range 1.0 to 15.0 microns.
Abstract: A device that can be used for the measurement of the absolute spectral reflectivity or reflectance of metals, paints, etc. is described. The unit is used in conjunction with a Perkin-Elmer Model 83 monochromator equipped with an NaCl prism for the wavelength range 1.0 to 15.0 microns. Preliminary results for MgO, polished copper, molybdenum, and electrolytic gold are presented.
TL;DR: There is more than enough stray light in the human eye to account for the effects of glare stimuli, and it is concluded that the principal component of stray light is scatter.
Abstract: A technique is described whereby it is possible to measure, by a direct physical method, stray light in the excised eye of the cat, steer, and human Systematic changes in the measures obtained are noted as a function of time following removal When time correction factors, based on a cat eye, are applied to cat, steer, and human data, no significant difference in the distribution of stray light in the eyes of the three species is obtained Experiments with steer eyes lead to the conclusion that the principal component of stray light is scatter The results of this research are compared to those of three studies where estimates of intraocular stray light were based on psychophysical judgements of the effects of glare stimuli It is concluded that there is more than enough stray light in the human eye to account for these effects The discrepancy is attributed mainly to the directional sensitivity of the cones
TL;DR: In this paper, the index of refraction of fused quartz glass was determined for 24 wavelengths from 0.34669 to 3.5078 microns, using the minimum deviation method.
Abstract: The index of refraction of fused quartz glass was determined for 24 wavelengths from 0.34669 to 3.5078 microns, using the minimum deviation method. The whole range is covered with a single instrument. The variation in index between samples was determined and no definite variations in dispersion were observed. Possible relations between purity and index are discussed.
TL;DR: The CIE normal observer properly predicts equality of brightness under the experimental conditions used if the observer is allowed to respond only to brightness (flicker method).
Abstract: Two screens viewed side by side were illuminated by radiations approximating those of blackbodies of different color temperatures and matched in brightness at photopic levels. A group of 73 observers found that less light of the higher color temperature was needed to produce equality of brightness than would be required by the CIE normal observer. The average gain in apparent brightness was 1 percent per 100°K color temperature difference in the range 5400–6300°K. The gain decreases with the age of the observer.When 27 of the observers re-matched the screen colors using a “flicker” method, no gain in apparent brightness was obtained. Thus the CIE normal observer properly predicts equality of brightness under the experimental conditions used if the observer is allowed to respond only to brightness (flicker method). If the observer has time to color adapt, as in continuous side by side screen viewing, an increase in the apparent brightness of the higher color temperature screen results.
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TL;DR: In this paper, a general treatment of the effects of slit widths on emission lines, absorption lines, and continuous absorption spectra is given, based on a triangular shaped slit function of base 2seff, where the effective slit width seff is related to the geometrical slit-width sg as seff2=so2+sg2, so being a function of the frequency characteristic of individual spectroscope.
Abstract: A general treatment is given of the effects of slit-widths upon emission lines, absorption lines, and continuous absorption spectra. The fundamental approximation is a triangular shaped slit function of base 2seff, where the effective slit-width seff is related to the geometrical slit-width sg as seff2=so2+sg2, so being a function of the frequency characteristic of the individual spectroscope.A formula is given for the observed shape of an emission line. The observed width L is derived from its ideal width l as L2=l2+seff2. The same formulas are valid for absorption lines for sufficiently small values of the optical density D. For larger D the mathematical difficulties of a general treatment are insuperable, the more so because the superposition principle does not hold. For the maximum point it is shown that neither Beer’s law nor Lambert’s law is experimentally fulfilled in many cases. A correction term is given for the effect on absorption spectra linear in log∊ versus cm−1, where ∊ is the molar extinction coefficient. The possibilities of an experimental determination of the function so are discussed on the basis of the formula L2=l2+sg2+so2.
TL;DR: A flash-photography technique for determining the statistical distribution of slopes of wind-created waves is described in this paper, which consists of taking photographs with a flash camera directed vertically downward toward the water surface.
Abstract: A flash-photography technique for determining the statistical distribution of slopes of wind-created waves is described. The technique consists of taking photographs with a flash camera directed vertically downward toward the water surface. The resulting photographs are flash reflection patterns that can be readily interpreted in terms of wave slopes. Wave-slope dispersion curves, as determined from daytime flash photographs taken from a bridge about 45 feet above the water, are given for wind velocities up to 20 knots. For example, the standard deviation of the across-wind wave slopes was found to be approximately 2.5, 5, and 7.8 degrees for wind velocities of 5, 10, and 20 knots, respectively. Under the same conditions the with-wind wave-slope standard deviation was found to be 4.2, 7.5, and 10 degrees.
TL;DR: In this paper, the Moon and Spencer formula for the luminance distribution of the completely overcast sky can be taken to apply to northern latitudes, and the blue-sky brightness distribution agrees with other similar measurements and with a formula derived from first principles by G. Pokrowski.
Abstract: Measurements of the distribution of sky luminance have been made at Stockholm.The fully overcast sky appears to have the same luminance distribution in the latitude of Stockholm (60° North approximately) as it has in the latitude of London (50° North approximately), Washington (U.S.A.) (40° North approximately), and elsewhere. It would therefore appear that the Moon and Spencer formula for the luminance distribution of the completely overcast sky can be taken to apply to northern latitudes.The blue-sky brightness distribution agrees with other similar measurements and with a formula derived from first principles by G. Pokrowski, but less well with Pokrowski’s own modified formula which he adjusted to agree with measurements made in Moscow.
TL;DR: An apparatus is described which automatically controls the presentation of light stimuli and records subjects’ responses in visual threshold experiments and makes a permanent record of the conditions of each light stimulus and of the responses made by each of four subjects.
Abstract: An apparatus is described which automatically controls the presentation of light stimuli and records subjects’ responses in visual threshold experiments. The apparatus includes: (a) A tape reader which controls the magnitude and time of occurrence of light stimuli, in accordance with the location of holes in a programming tape, and (b) a punch recorder which makes a permanent record of the conditions of each light stimulus and of the responses made by each of four subjects. The accuracy of the stimulus-producing equipment is monitored. Electric counters tally the correct responses made by each subject for each magnitude of light stimulus.
TL;DR: A nearly complete analysis of the third spectrum of nickel is given, based on 800 measured lines extending from 600 A to 3000 A as discussed by the authors, which includes all but 1S from 3D8, and all but the higher 1D from 3d7 4s.
Abstract: A nearly complete analysis of the third spectrum of nickel is given, based on 800 measured lines extending from 600 A to 3000 A. A partial term list was published in “Atomic Energy States,” National Bureau of Standards Circular No. 467, 1952. The 168 levels now given include all but 1S from 3d8, and all but the higher 1D from 3d7 4s. The discovery of the higher 3D is noteworthy. High even levels allow the calculation of an I.P.=36.16 volts. The group of odd levels due to 3d7 4p lacks only a few of low J and some members of 3d6 4s 4p are present. The important term 3d6 4s2 5D has not been found. Intensities are very irregular, especially where terms from different families are involved.
TL;DR: In this paper, a unique optical instrument for the measurement of light scattering in physically nonhomogeneous systems is described, which provides a basis for the size characterization of optical inhomogeneities in the radius range 0.1 to 100 microns.
Abstract: A unique optical instrument for the measurement of light scattering in physically nonhomogeneous systems is described. The scattering data provide a basis for the size characterization of optical inhomogeneities in the radius range 0.1 to 100 microns. The useful angular range of the instrument extends from about 0.05° to 140°. Angular resolution of 0.02° is readily attainable for characterizing large inhomogeneities at small angles. Scanning rates are 0.1, 0.4, 1.0, 4.0 and 20°/min. Light intensities varying by a factor of 108 are measured through the use of an optical attenuator which facilitates use of a 1P21 multiplier phototube at a low level of illumination. The phototube output is charted by a special high-impedance recorder having a full-scale sensitivity of 0.004 microampere. Angles are marked by an auxiliary pen on the recorder.