and at Clemson University and then joined the Fire Research Section of NBS (1958-1964) and subsequently served as Chief of the Photographic Research Section (1958-1964) and Chief of the Image Optics and Photography Section (1964-1970).

In the Photographic Research Section and later the Image Optics and Photography Section, he designed a nomograph to compute the color filter required to take well-balanced colored pictures with a given film and illumination [11]. It was made available on a single sheet from the U.S. Government Printing Office or NBS and was very popular with amateur and professional photographers. It was the subject of many feature articles in popular and professional photographic magazines and became a common feature of color filter catalogs.

When demonstrations by Edwin Land led to widespread speculation that cheaper and better color television could be possible by using two primary colors rather than three, McCamy demonstrated to the Federal Communications Commission that a two-color system was not acceptable. That lecture-demonstration, by means of three projectors, allowed people to witness many visual phenomena. It generated such widespread interest that he was invited to present it fifty times at NBS, major universities, major industrial research. laboratories, and scientific society meetings in 19591961 [12]. Among other phenomena, he demonstrated that under certain conditions, people perceive colors in black-and-white images. He was invited to repeat that lecture-demonstration forty years later, in February 2000 [13].

Photographic wedges are widely used in photographic science. The wedge may be straight, the density varying linearly with length, or it may be circular, the density varying linearly with rotational angle. Since density is the logarithm of the reciprocal of transmittance, the transmittance varies logarithmically. When the density is gradually changing and is measured over a finite area, it is difficult to know where on the wedge the actual density value is measured. Some finite area is required for measurement. NBS could measure uniform areas precisely, but wedges could not be calibrated because the required theoretical relationships were unknown. McCamy derived the mathematical relationship between the measured density and the location to which it may be assigned, for a rectangular aperture on a straight wedge, a circular aperture on a straight wedge, a sector aperture on a circular wedge, and a circular aperture on a circular wedge [14]. The last case was commonly encountered and, for that case, the mathematical derivation was remarkably complex.

McCamy also designed the resolution target used internationally to test microfilm cameras, and his laboratory made as many as 25,000 per year as standard

reference materials for the industry. He designed and provided other test targets to calibrate instruments used to measure the image structure characteristics of optical and photographic systems. He developed a laboratory camera to measure how much information a photographic film or plate could record on a given area. Scientists involved in manufacturing electronic components came to the Bureau to study the camera, and the general features of it came into widespread use in the production of tiny electronic components. McCamy derived a formula to compute the information storage capacity of a photographic system, in bits per square millimeter, from the measured resolving power [15]. All these activities supported the development and utilization of the U.S. satellite reconnaissance system, which was highly classified during the cold war.

When it was discovered that the vast stores of federal and state government records on microfilm were developing blemishes that might destroy vital archival information, McCamy mustered the support of many government agencies and many private interests to conduct a wide-ranging investigation. His laboratory discovered the cause of the blemishes. The microfilms were stored in cardboard boxes and the aging cardboard emitted minute amounts of hydrogen peroxide, which attacked the film. The task was difficult because the concentration of peroxide was less than 10 mol/L and the molecules were so labile that they were dissipated on passing through two centimeters of air [16-19].

TM

As Vice President for Research of the Macbeth Division of the Kollmorgen Corporation in 1970-1990, after leaving NBS, McCamy continued research on optical design, precise transmission measurements, color measurement, optical filter design, simulation of daylight for color inspection, geometric attributes of appearance, densitometry in photography and color printing, color order systems, color standards, and related mathematics. He substantially improved the classical absolute method of photometry based on the inverse-square law of illumination, and he designed the Macbeth ColorChecker Color Rendition Chart' which is used internationally to evaluate color-imaging systems. At the request of Congress in 1978, he analyzed all known photographs and x rays related to the assassination of President Kennedy and testified before the House Select Committee on Assassinations. His method of analyzing images of long firearms is used routinely by the U.S. Federal Bureau of Investigation. He continued to be active in national and international standardization of photography, color printing, and color science, chairing committees of the American National Standards Institute, the American Society for Testing and Materials, the International Commission on Illumination (CIE), and the International Organization

for Standardization (ISO). He wrote the spectral specifications for optical character recognition for the banking industry and the Universal Product Code for the grocery and other retail industries.

He is on the Advisory Board of the Munsell Color Science Laboratory at the Rochester Institute of Technology and was Adjunct Professor at Rensselaer Polytechnic Institute, President of the Kollmorgen Foundation, and Trustee of the Munsell Foundation. He was elected fellow of the Optical Society of America, Society of Photographic Scientists and Engineers, Royal Photographic Society of Great Britain, Society of Motion Picture and Television Engineers, and the Washington Academy of Sciences and has been honored for his lectures. He received the 1997 Bruning Award of the Federation of Societies for Coatings Technology and the 1999 Godlove Award of the Inter-Society Color Council.

Prepared by Calvin S. McCamy.

Bibliography

[1] C. S. McCamy, Concepts, Terminology, and Notation for Optical Modulation, Photogr. Sci. Eng. 10, 314-325 (1966). [2] Photography-Terms, Symbols, and Notations-Density Measurement, ANSI PH2.16—1984 (R1990), American National Standards Institute, New York.

[3] Photography-Terms, Symbols, and Notations-Density measurements, ANSI/ISO 5/1—1984, International Organization for Standardization (ISO), Geneva, Switzerland. [4] Standard Terminology of Appearance, ASTM E 284-98a, American Society for Testing and Materials (ASTM), West Conshohocken, PA.

[5] Standard Practice for Specifying the Geometry of Observations and Measurements to Characterize the Appearance of Materials, ASTM E 1767-95, American Society for Testing and Materials (ASTM), West Conshohocken, PA.

[6] International Lighting Vocabulary (E) (F) (G) (R), CIE Document No. 17.4 (1987), International Commission on Illumination (CIE), Vienna, Austria.

[7] F. E. Nicodemus, J. C. Richmond, J. J. Hsia, I. W. Ginsberg, and T. Limperis, Geometrical Considerations and Nomenclature for Reflectance, NBS Monograph 160, National Bureau of Standards, Washington, DC (1977).

[8] H. K. Hammond III and H. L. Mason (eds.), Precise Measurement and Calibration, Volume 7, Radiometry and Photometry, NBS Special Publication 300, National Bureau of Standards, Washington, DC (1971).

[9] Calvin S. McCamy (ed.), Precise Measurement and Calibration, Volume 10, Image Optics, NBS Special Publication 300, National Bureau of Standards, Washington, DC (1973). [10] C. S. McCamy, A five-band recording spectroradiometer, J. Res. Natl. Bur. Stand. 56, 293-299 (1956).

[11] C. S. McCamy, A nomograph for selecting light balancing filters for camera exposure of color films, Photogr. Sci. Eng. 3, 302-304 (1959).

[12] C. S. McCamy, A demonstration of color perception with abridged color-projection systems, Photogr. Sci. Eng. 4, 155-159 (1960); C. S. McCamy, Colors perceived with abridged color projection systems (Abstract), J. Opt. Soc. Am. 50, 510(A) (1960).

[13] C. S. McCamy, Abridged Color Revisited-or Sleeping Beauty II, invited presentation at the Inter-Society Color Council, 2nd Panchromatic Conference on "Color in Its Surround," Savannah, GA, February 20, 2000.

[14] C. S. McCamy, Theory of optical wedges as flux modulators, J. Opt. Soc. Am. 66, 1350-1355 (1976).

[15] C. S. McCamy, On the information in a microphotograph, Appl. Opt. 4, 405-411 (1965).

[16] C. S. McCamy, Inspection of processed photographic record films for aging blemishes, NBS Handbook 96, National Bureau of Standards, Washington, DC (1964).

[17] C. S. McCamy, S. R. Wiley, and J. A. Speckman, A survey of blemishes on processed microfilm, J. Res. Natl. Bur. Stand. 73A, 79-99 (1969).

[18] C. S. McCamy and C.I. Pope, Current research on preservation of archival records on silver-gelatin type microfilm in roll form, J. Res. Natl. Bur. Stand. 69A, 385-395 (1965). [19] C. S. McCamy and C. I. Pope, Redox blemishes-their cause and prevention, J. Microgr. 3, 165-170 (1970).

This review of light scattering theory [1] brought together a range of concepts which had been developed over more than half a century. Light scattering has been used to measure thermal properties of liquids and solids ever since Einstein showed that the intensity of light scattered by density fluctuations is proportional to the isothermal compressibility of the fluid. Density fluctuations, in turn, can be considered to be a sum of pressure fluctuations and entropy fluctuations. Independently, Brillouin and Mandel'shtam realized that the pressure fluctuations are associated with acoustic modes and would shift the frequency of the scattered light by an amount proportional to the speed of sound. In 1935, Landau and Placzek published a short note in which the contribution of the entropy fluctuation component of density fluctuations to the scattering of light was identified and shown to be unshifted in frequency. The intensity ratio of the two components of the scattered light (entropy fluctuation component divided by the pressure fluctuation component) is equal to the ratio of (C,-C)/ C, the specific heat difference divided by the constant volume specific heat. (This has come to be known as the Landau-Placzek ratio.) They also noted that the spectral width of the light scattered by the non-propagating density fluctuations would be proportional to the thermal diffusivity of the fluid.

Prior to the advent of the He-Ne gas laser, it was not possible to exploit these ideas fully because the spectral width of available light sources was larger than the frequency broadening due to the non-propagating fluctuations. It was only possible to estimate sound speed and heat capacity ratios if a great deal of care was taken. This changed when He-Ne lasers became available as light sources that were nearly monochromatic and coherent. Heterodyne and homodyne detection schemes now made it possible examine the structure of scattered light in detail. In short order, light scattering became an active research topic and tool with particular attention paid to scattering near critical points, where the scattering intensity is large. It would not be an understatement to say that the prospect of using a new technique to study dynamics near critical points was exciting.

The theoretical ideas mentioned above were not organized in a form that would be readily accessible to the students entering the field. The necessary theory can be extracted from three of the volumes of the Landau and Lifshitz series, Statistical Physics, Fluid Mechanics, and

Electrodynamics of Continuous Media, but only if one knows where to look. Thus, a review article putting the pieces together was timely. At the urging of M. S. Green, Chief of the Statistical Physics Section, Raymond Mountain prepared the review article Spectral Distribution of Scattered Light in a Simple Fluid [1] that brought these theoretical ideas together. The emphasis of the article was on how the spectra are related to fluid properties. The review also showed how light scattering theory was related to the theory of sound propagation in a liquid. It was an opportunity to put statistical mechanics in contact with experiments. In structuring the article, use was made of the work of I. L. Fabelinskii, S. M. Rytov, and R. Pecora.

The underlying idea in the theory of scattering by fluctuations is that scattering provides an average over the fluctuations in both space and in time. Statistical mechanics provides the theoretical framework needed to relate the observed scattered light to fluid properties, and that is the approach followed in the paper. The result is an explicit formula connecting the intensity and frequency distribution of the scattered light to the thermodynamic and transport properties of the fluid.

This theory of light scattering is phenomenological. It provides the connection between the spectrum of the scattered light and various thermodynamic and transport properties of the fluid. The use of the inherently macroscopic equations of hydrodynamics to describe events on the scale probed by light scattering may seem to be an unjustified extrapolation. It is not for two reasons. First, the fluctuations that lead to light scattering have a size on the order of the wavelength of light, that is micrometers. The distance between near neighbor molecules in a liquid is on the order of tenths of a nanometer, over a thousand times smaller. Second, the time scale of these fluctuations is much longer than the time intervals associated with molecular scattering processes. With these considerations in mind, the use of hydrodynamic fluctuation theory is quite reasonable. This approach would break down for the case of scattering from a gas, where the mean-free-path of a molecule is on the order of the wavelength of light. (Although not part of the review, this has been demonstrated experimentally. Agreement between theory and experiment is recovered when kinetic theory of gases is used in place of hydrodynamics to describe the time evolution of the fluctuations [2].)

The spectrum of the scattered light consists of a central, unshifted component with a width proportional to the thermal diffusivity and two BrillouinMandel'shtam components that are shifted in frequency by an amount proportional to the adiabatic sound speed. The width of the shifted components is proportional to the acoustic attenuation coefficient. If a He-Ne source is used and the scattered light is observed at an angle of 90° from the direction of propagation of the incident light, the widths of these spectral features are on the order of megahertz and the shifts are on the order of gigahertz.

When the fluid is close to the liquid-vapor critical point, the density fluctuations are large enough to make the scattering visible to the naked eye. The fluid takes on a milky color, called critical opalescence. Since critical opalescence was a "hot topic," the review utilized the theory to predict how the scattered light would appear for states close to the critical point of carbon dioxide, a substance for which the relevant thermodynamic and transport property data were available. There followed some speculations on how these predictions would be modified very close to the critical point. (These speculations were later shown to be only crude approximations to the observations since renormalization ideas were not included [3].) Finally, a cautionary note was sounded about carefully characterizing the state of the system being studied. "It is important to keep in mind that light-scattering experiments are only as good as the PVT data used to specify the thermodynamic state of the scattering system."

Since hydrodynamic fluctuation theory is also pertinent to acoustic wave propagation and attenuation, a section discussing the connections between light scattering and sound propagation was included. In both

situations, the same dispersion relation is used. The difference is that in light scattering the wavelength is fixed and the frequency varies, while in acoustic wave propagation the frequency is held fixed and the wavelength is allowed to vary. The lesson is that the interpretations of the two types of measurements are different, a point that had not been widely appreciated.

The article provides an introduction to the theory that can be readily understood by most physical scientists. The review was well received and was widely referenced. It has been reprinted in collections. In 1980 it was highlighted in Science Citation Index as a "Citation Classic," meaning that it had been cited more than 250 times since the article was published. Many of the features of the review had been incorporated into books by then [2,4].

Some of the theory expounded in the review did not conform to the experimental results on the scattering of light in molecular liquids. In many cases, the ratio of the intensity of the central component to the Brillouin components (the Landau-Placzek ratio) exceeded the specific heat ratio for the liquid. This experimental finding indicated that the spectrum of the scattered light contained additional features. This led Mountain to develop a hydrodynamic fluctuation theory for the spectrum of the scattered light that included internal molecular degrees of freedom, such as vibrational states. This theory predicts that some of the intensity associated with the sound modes is transferred to an unshifted, but quite broad, spectral feature which is separate from the entropy fluctuation component, which has a spectral width proportional to the thermal diffusion coefficient. The existence of this component was demonstrated experimentally at the same time the short note describing the feature appeared [5-7]. This feature is sometimes called the "Mountain peak" although it is a fairly weak, broad feature in the spectrum of many liquids. The story is different for polymers and for strongly supercooled liquids, where there are many internal degrees of freedom that have a major influence on the density fluctuations, so that the additional feature is large.

The same sort of theory developed for one-component liquids has been developed for mixtures. In that case, the dominant fluctuations that lead to scattering are composition fluctuations. The spectrum of these fluctuations are more complex due to the additional variable (composition) in the problem [8-10].

Raymond D. Mountain came to NBS as a Postdoctoral Research Fellow in 1963 and became a regular staff member of the Heat Division in 1965. The review article was prepared while he was a postdoc. He became chief of the Statistical Physics Section in 1968 when M. S. Green left NBS for Temple University, and served

in this position until 1982. In 1986 he was named a NIST Fellow. He received the Department of Commerce Gold Medal in 1983 for his research on the liquid state. In 1974 he was named a Guggenheim Fellow and spent a year at the Institute for Theoretical Physics in Utrecht, The Netherlands.

Prepared by Raymond D. Mountain.

Bibliography

[1] R. D. Mountain, Spectral Distribution of Scattered Light in a Simple Fluid, Rev. Mod. Phys. 38, 205-214 (1966).

[2] J.-P. Boon and S. Yip, Molecular Hydrodynamics, McGraw Hill, New York (1980).

[3] P. C. Hohenberg and B. I. Halperin, Theory of dynamic critical phenomena, Rev. Mod. Phys. 49, 435-479 (1977).

[4] B. J. Berne and R. Pecora, Dynamic Light Scattering: With Applications to Chemistry, Biology, and Physics, John Wiley and Sons, New York (1976).

[5] R. D. Mountain, Interpretation of Brillouin Spectra, J. Chem. Phys. 44, 832-833 (1966).

[6] R. D. Mountain, Thermal relaxation and Brillouin scattering in liquids, J. Res. Natl. Bur. Stand. 70A, 207-220 (1966). [7] R. D. Mountain, Density fluctuations in fluids having an internal degree of freedom, J. Res. Natl. Bur. Stand. 72A, 95-100 (1968). [8] R. D. Mountain, Spectral structure of critical opalescence: Binary mixture, J. Res. Natl. Bur. Stand. 69A, 523-525 (1965). [9] R. D. Mountain and J. M. Deutch, Light scattering from binary solutions, J. Chem. Phys. 50, 1103-1108 (1969).

[10] L. Fishman and R. D. Mountain, Activity coefficients of solutions from the intensity ratio of Rayleigh to Brillouin scattering, J. Phys. Chem. 74, 2178-2182 (1970).