Showing papers in "Journal of the Optical Society of America in 1942"

A new calculus for the treatment of optical systems. IV

Abstract: Part IV is divided into two sections. The first is devoted to some additions to the general theory developed in Part I, and the second section to the derivation of the matrices representing two optical elements which were not treated in Parts II and III: (1) plates possessing circular dichroism, and (2) plates cut from crystals of such low symmetry that the principal axes of absorption and refraction are not parallel. In case (2), the discussion is limited to monoclinic and triclinic crystals which do not possess optical activity.

Long Optical Paths of Large Aperture

Abstract: 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

Visual Sensitivities to Color Differences in Daylight

Abstract: An apparatus is described which facilitates the presentation of pairs of variable colors without variation of luminance. With this instrument, various criteria of visual sensitivity to color difference have been investigated. The standard deviation of color matching was finally adopted as the most reproducible criterion. The test field was two degrees in diameter, divided by a vertical biprism edge, and was viewed centrally with a surrounding field of forty-two degrees diameter uniformly illuminated so as to have a chromaticity similar to that of the I.C.I. Standard Illuminant C (average daylight). The luminance of the test field was maintained constant at 15 millilamberts, and the surrounding field was 7.5 millilamberts. These fields were viewed monocularly through an artificial pupil, 2.6 mm in diameter. Over twenty-five thousand trials at color matching have been recorded for a single observer, and the readings are analyzed in detail and compared with previously available data. The standard deviations of the trials are represented in terms of distance in the standard 1931 I.C.I. chromaticity diagram. These increments of distance are represented as functions of position alongstraight lines in the chromaticity diagram, and also as functions of direction of departure from points representing certain standard chromaticities. Such representations are simpler than the traditional representations of wave-length thresholds and purity thresholds as functions of wave-length, and the accuracy of the representations is improved by this simplicity. Chromaticity discrimination for non-spectral colors is represented simultaneously and on the same basis as for spectral colors. Small, equally noticeable chromaticity differences are represented for all chromaticities and for all kinds of variations by the lengths of the radii of a family of ellipses drawn on the standard chromaticity diagram. These ellipses cannot be transformed into equal-sized circles by any projective transformation of the standard chromaticity diagram. The consistency of these data with the results of other investigators is exhibited in terms of the noticeabilities of wave-length differences in the spectrum and of the noticeabilities of purity differences from a neutral stimulus, as functions of dominant wave-length.

Photoelectric Tristimulus Colorimetry with Three Filters

Abstract: The term, photoelectric colorimetry, is commonly employed to designate both photoelectric tristimulus colorimetry, used to evaluate the appearance of materials, and abridged spectrophotometry, often used to assist in chemical analyses. This paper is devoted to the first type of measurement. For a photoelectric tristimulus colorimeter, it is desired to find three or more source-filter photo-cell combinations of such spectral character that they duplicate the standard I.C.I. observer for colorimetry. With an instrument having these combinations, tristimulus values would be obtained by direct measurement. Although no one has duplicated the I.C.I. observer perfectly, several investigators have obtained source-filter photo-cell combinations suitable for the measurement of color differences between spectrally similar samples. To measure color differences as small as those which the trained inspectors of paint, textile, plastic, paper, and ceramic products can see, an instrument must have high precision. If the needed precision is available, a photoelectric tristimulus colorimeter may be used to measure: (1) I.C.I. colorimetric values, x, y, and Y, relative to those of a spectrally similar, calibrated standard; (2) relative values of α and β, components of the chromaticity departure from neutral in a new uniform-chromaticness-scale mixture diagram for representing surface colors; (3) amounts of color difference between pairs of spectrally similar samples; (4) amounts of color change accompanying fading; and (5) whiteness of white and near-white surfaces. In giving examples of the measurement of some of these different properties and in describing the errors of color measurement to which the tristimulus method is subject, reference is made to operations with the author’s recently developed multipurpose photoelectric reflectometer.

Photoelectric Detection and Intensity Measurement in Raman Spectra

Abstract: Preliminary experiments on the photoelectric detection of Raman spectra are described making use of commonly available spectroscopic equipment in conjunction with the new RCA-931 photomultiplier tube. A more eleborate experiment is described using a large planegrating monochromator as the dispersing device. The photoelectric current was suitably amplified and continuously recorded making use of a galvanometer and photographic galvanometer deflection recorder. The limit of detection set by the thermionic emission at room temperature of the photomultiplier has been reached. It is shown that the minimum intensity observable photoelectrically compares favorably with the minimum commonly observed photographically in Raman spectra. A rough order of magnitude experiment was performed with the photomultiplier to ascertain the feasibility of the photoelectric method for measuring intensities in arc and spark spectra under high dispersion. An arrangement of apparatus for quantitative spectroscopic analysis photoelectrically is suggested.

The Far Ultraviolet Reflectivities of Metallic Films.

Abstract: The reflectivities of evaporated films of aluminum, copper, silver, and beryllium have been measured spectrographically in the region 1200–2200A before the films were exposed to air. Copper reflection was considerably higher before contamination by air than afterward; the other metals showed no effect. The pronounced minimum in beryllium reflectivity found by previous investigators did not occur here; it is believed that this minimum is due to a thin film of tungsten evaporated over the beryllium.

The Control of Photographic Printing by Measured Characteristics of the Negative

Abstract: A large amount of experimental data has been obtained to determine what portion of the D-log E curve of photographic paper is useful in making high quality prints Psychophysical statistical methods were necessary in solving this problem Special attention was given to selecting a procedure which would yield results of maximum practical significance Prints were made from about 200 negatives using five different exposures on each of three contrast grades of paper for each negative Most of the work was done with a normal, commercially available set of semi-matte surface papers having six different contrast grades Additional work was done using glossy and matte surface papers Every set of prints was judged by a number of observers to determine the best print The location of the “first-choice” prints on their respective D-log E curves was then obtained from maximum and minimum density readings on the prints and on the negatives The data for second-choice and third-choice prints were also obtained as a matter of general interest It was found that the minimum or shadow density of a negative is a much more reliable guide for determining correct printing exposure than the maximum or highlight density The integral or average density of the negative is nearly but not quite as satisfactory for this purpose as the minimum density Of great interest is the fact that the useful maximum density of the printing paper was, on the average, considerably below the available maximum density, while the useful minimum density of the paper was usually very close to, and often equal to, zero density The primary requirements, however, appear to be expressed by gradients rather than by densities It was found that the limits of the useful portion of the D-log E curve of the positive material are determined very closely by fractional gradients of 01 Ḡ on the toe of the curve and 10 Ḡ on the shoulder of the curve, where Ḡ is the average gradient measured between these two limiting points This conclusion has an important bearing on the establishment of a significant method for expressing the sensitometric characteristics of photographic papers It leads to an evaluation of the effective printing speed and the useful exposure scale of the positive material The proposal is made that speed be defined as 104/E, where E is the exposure corresponding to the limiting gradient on the shoulder of the curve The useful log exposure scale of the paper is defined as the log exposure interval lying between the two limiting gradients The application of these results to the making of prints by sensitometric control is discussed In general, a negative should be printed so that its minimum density falls on the shoulder of the D-log E curve at the point where speed is measured A method is given whereby the density scale of the negative can be used with reasonable success to determine the proper contrast grade of paper for making the print

Testing and Accurate Use of Abbe-Type Refractometers

Abstract: For high accuracy in commercial refractometry, refractometers should be used more strictly as comparison instruments and series of standard samples with known refractivities over a range of room temperatures should be available for frequent check measurements. Tests for various mechanical and optical defects are necessary and compensators should be examined for their possible deviation of sodium light. Care in the selection of suitable contact liquids for solid media is advisable for several reasons. Owing to inherent difficulties in the design of illuminating prisms there are possibilities of getting different readings on solids and liquids of identical refractivity. Since dispersion measurements as usually made with the compensator are necessarily less reliable than is generally realized, it is recommended that line spectra sources be employed whenever accurate dispersions are desired and that the scale be calibrated for each wave-length that is to be used.

First Spark Spectrum of Thorium—Classification and Zeeman Effect Data of Th II

Abstract: Five sets of exposures to the thorium arc in silver, burning in a Bitter electromagnet at fields up to 93,000 oersteds, have given resolved Zeeman patterns for more than 800 lines of Th II. Data obtained from these plates, measured with an automatic comparator and used in connection with improved wave-length measurements, have made possible classification of 1091 lines of Th II as arising from 219 levels. These levels belong to two groups of terms between which no combinations have been found. Group A contains even terms arising from d2s (with a4F112 lying lowest), from d3, and from ds2, and odd terms from d2p, dsp, and fds. About half of all terms expected from these configurations have lceen identified. Group B contains odd terms expected from fs2 (with a2F°212 lying lowest), and fds, and even terms from fsp and fdp. g values are given for all levels, in many cases to within 0.002 unit, which makes available a large amount of fairly exact experimental data for evaluation of perturbations. Partial tests of the g sum rule show good agreement with theory, though all terms are not yet available for inclusion. Perturbations of even those terms which lie low are rather large.

An Investigation of the Properties and Applications of the Geiger-Müller Photoelectron Counter*

Abstract: The construction of a reliable photoelectron counter with a nickel cathode and a quartz envelope is described and an account is given of comparative tests on the measurement of the relative intensities of spectral lines with the counter and by means of photographic spectrophotometry. The counter has a linear response to variations of intensity of light over a wide range. For measuring the relative intensities of spectral lines the counter tube is mounted on a spectrograph behind a slit which moves on the focal plane of the instrument so that spectral lines are measured singly. The application of the counter in spectrochemical analysis is demonstrated and comparative analyses of the same materials by use of the counter and by the method of spectrochemical analysis are given. Studies on the constancy of several spectroscopic sources are described.

Projective Transformations of Color-Mixture Diagrams*

Abstract: Projective transformations of a standard coordinate system can be written in the form: u=c3+(e1x+e2y)/(c7x+c8y+1),υ=c3+(e4x+e5y)/(c7x+c8y+1).The shapes and relative sizes of all geometrical figures subjected to such transformations will be invariant if the values of c7, and c8 are invariant, and if the following quantities are also invariant: (e1e5−e2e4)/(e1e2+e4e5),(e22+e52)/(e12+e42).The sizes of such figures are invariant when the numerators and denominators of these expressions are themselves invariant.Colors having equally noticeable differences can be represented by equidistant points in some projective transformation of the standard color-mixture diagram only if in the standard diagram the separations of the points on any line representing a series of equally noticeably different colors are proportional to the square of the distance from some unique point on that line, and if the locus of such unique points for all linear series of colors is a straight line. The locus of colors equally noticeably different from any standard color can be represented adequately in the standard color-mixture diagram by an ellipse. All such ellipses can be transformed by a single projective transformation into equal-sized circles only if the common tangents of every pair of ellipses intersect on a single straight line. This line is the same as the locus of unique points for the series of colors just described. The equation of this straight line is (c7x+c8y+1 = 0), where the constants are those appearing in the denominators of the successful transformation formulas. If the separations of points representing equally noticeably different colors in the standard color-mixture diagram cannot be represented adequately in the manners described, then such colors cannot be represented adequately by equidistant points in any projective transformation of the standard coordinate system.

A Logarithm-of-Wave-Length Scale for Use in Absorption Spectrophotometry

Abstract: Some advantages of a logarithm-of-wave-length scale for spectrophotometric absorption curves are indicated. A system arranged so as to provide 100 units per spectral octave has been found especially convenient, particularly so in connection with instruments which can plot over a number of different ranges and at different magnifications.

Electro-Optical Properties of Colloids

Abstract: This paper deals with investigations of the electric birefringence of colloidal solutions in alternating fields; the most detailed measurements were carried out with monodisperse bentonite sols. In general the double refraction consists of a direct and an alternating part, both of which depend on field strength and frequency of the applied field, particle size, temperature, and concentration of the solution. The direct birefringence, which can be positive or negative, can reverse its sign with a change of any of the five parameters mentioned. The measurements are interpreted by means of a relaxation theory which can explain all phenomena connected with the vibrating component in a satisfactory way. Likewise this theory is capable of explaining the variations and the reversal of the direct birefringence with changes of the frequency, field strength, particle size, and temperature. The character of the dependence of the steady birefringence on concentration which has not hitherto been observed with other colloids was first discussed by H. Mueller. The changes and reversal of sign of the direct component with concentration are accounted for by the assumption that the micelles are surrounded by a compressed water hull; the pressure of the water hull depends on particle size and concentration. Direct and alternating birefringence have different relaxation times.

The Photographic Reciprocity-Law Failure and the Ionic Conductivity of the Silver Halides*

Abstract: A method is given for determining from reciprocity-law failure measurements at different temperatures the value of the energy exponent ∊ in the equation for ionic conductivity of silver bromide (1)Ni=Ae−∊/kT ions/sec.According to present theory, the reciprocity-law failure at high intensities is due to the sluggishness of the secondary process of latent-image formation consisting of migration of Ag+ ions to the specks. The free electrons produced in the grains by exposure can move practically instantaneously and independently of temperature. However, if the Ag+ ions cannot move up to the specks at a sufficient rate to keep them discharged, an inefficiency is introduced. This inefficiency may be said to depend upon the number of free electrons produced in the grain per second, Ne, compared with the number of free Ag+ ions, Ni, available for ionic conduction. The number of free electrons produced can be controlled by the light intensity, I, whereas the number of conducting Ag+ ions can be controlled by temperature in accordance with Eq. (1). By choosing points on the reciprocity curves made at different temperatures such that the change of efficiency arising from a change of intensity I1 to I2 is equal to the change of efficiency arising from a change of temperature T1 to T2, the relation is shown to hold (2)logI2/I1=0.434∊k(1T1−1T2).Substitution of experimental values in this equation for I1, I2, and T1, T2 gives a value of ∊ = 0.69. Direct measurement of the ionic conductivity at different temperatures on a single crystal of AgBr gives the value of ∊ = 0.66. The close agreement between the ∊ values obtained by photographic means and by direct measurement affords quantitative evidence that the high intensity reciprocity-law failure is due to the ionic conductivity effect alone.