Victor R. Weidner
National Institute of Standards and Technology
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Applied Optics | 1965
David M. Gates; Harry J. Keegan; John C. Schleter; Victor R. Weidner
The spectral properties of plant leaves and stems have been obtained for ultraviolet, visible, and infrared frequencies. The spectral reflectance, transmittance, and absorptance for certain plants is given. The mechanism by which radiant energy interacts with a leaf is discussed, including the presence of plant pigments. Examples are given concerning the amount of absorbed solar radiation for clear sky and overcast conditions. The spectral properties of desert plants are compared with those of more mesic plants. The evolution of the spectral properties of plant leaves during the early growing season is given as well as the colorimetric behavior during the autumn.
Journal of the Optical Society of America | 1981
Victor R. Weidner; Jack J. Hsia
The reflection properties of pressed polytetrafluoroethylene powder have been under investigation by the Radiometric Physics Division at the National Bureau of Standards for the past five years. This material has a great potential use, both as a standard of diffuse reflectance and as a coating for integrating spheres for applications in reflectance spectrophotometry and other signal-averaging devices. It possesses certain physical and optical properties that make it ideal for use in these applications. Techniques are given for preparing reflection standards and coating integrating spheres with the pressed powder. The effects of powder density and thickness on its reflectance are reported, and observations of possible problems with fluorescence that are due to the presence of contaminants in the powder are discussed. The absolute reflectance (6°/hemispherical reflectance factor relative to a perfect diffuser) is reported for the spectral range of 200–2500 nm. The directional/hemispherical reflectance factor relative to 6°/hemispherical reflectance is given for several wavelengths in the ultraviolet and visible spectrum and for angles of incidence between 5 and 75°. The bidirectional reflectance factor is reported for 300, 600, and 1500 nm at angles of incidence of −10, −30, −50, and −70° and at viewing angles at 10° intervals from −80 to +80°.
Applied Optics | 1985
Victor R. Weidner; Jack J. Hsia; B. Adams
The object of this paper is to present results of several experiments relating to the preparation and use of pressed polytetrafluoroethylene (PTFE) powder as a diffuse reflectance standard for the 200–2500-nm spectral range. These experiments include two round-robin intercomparisons involving nine laboratories. These round-robin experiments provided data on the variability of the reflectance of pressed PTFE reflectance standards prepared in different laboratories. Results of these measurements provided insight into the problems associated with the PTFE standards and helped to establish what practices needed to be standardized to improve interlaboratory agreement for diffuse reflectance measurements.
Applied Optics | 1976
K. L. Eckerle; William H. Venable; Victor R. Weidner
Through the use of an efficient design and a newly available sphere coating material, a simple, passive, sturdy averaging sphere was made that operates effectively over the wavelength range from 200 nm to 2000 nm. Data are reported for a sphere of this type in which the sphere transmittance is 0.32 at 200 nm and rises rapidly to near the maximum theoretical value of 0.56 over the remainder of the wavelength range. The several orders of magnitude reduction in error due to beam displacement more than compensate the slight reduction in signal for many spectrophotometric and radiometric applications.
Metrologia | 1981
Jack J. Hsia; Victor R. Weidner
A 45°/normal reflectometer has been constructed and tested for calibrating the absolute reflectance factor of diffuse samples over the 380-770 nm spectral range using polarized radiation. The measurement equations have been derived for the method used. The method using a step-down technique and view factor calls for the measurements of the ratio of two fluxes and, in addition, some linear dimensions. The uniformity of the receiver system is achieved by means of a double-sphere signal averager. Uncertainties of the absolute-reflectance-factor measurements obtained with this system are estimated to be ±0.3% of the measured value. For all the samples that have been tested, the 45°/normal reflectance factor was found to be higher than the 6°/hemispherical reflectance factor. The higher reflectance values for 45°/normal geometry were confirmed by additional gonioreflectometer measurements.
Applied Optics | 1980
Victor R. Weidner; Jack J. Hsia
A specular reflectometer has been constructed and tested for calibrating the reflectance of mirror standards over the 250-2500-nm spectral range. This instrument is a measurement accessory to a reference spectrophotometer, which is also used for diffuse hemispherical spectral reflectance and 45 degrees /0 degrees spectral reflectance. The specular reflectometer is designed to measure mirror reflectances at angles of incidence between 5 and 80 degrees using both vertically and horizontally polarized radiation. Absolute reflectance measurements are obtained by an optical system, which provides for direct measurement of the incident beam and for the sample mirror reflectance using the same beam. This is accomplished by means of a beam tracking system through which the beam is directed into a signal averaging sphere. The sphere rotates with the beam tracking optics, and the stationary detector views the interior of the sphere. Control of the beam tracking optical system is accomplished by a computer-controlled stepping-motor-driven precision turntable. Uncertainties of the reflectance measurements obtained with this system are estimated to be +/-0.2% of the measured value.
Applied Optics | 1982
Klaus D. Mielenz; Victor R. Weidner; R. W. Burke
A new method of estimating the amount of heterochromatic stray light in UV spectrophotometers is described. The method uses the same solution filters with sharp UV absorption edges as ASTM Test Method E387, but one measures the apparent absorbance of a 10-mm path-length cell in the sample beam relative to a 5-mm cell in the reference beam. Scanning toward shorter wavelengths, one records an apparent absorbance maximum which is a direct measure of the stray light. This method was found to be in satisfactory agreement with the ASTM method in comparative tests of several spectrophotometers at different wavelengths between 200 and 390 nm, using KC1, KI, Nal, acetone, and NaNO(2) solution filters. The new method proved to be simpler, the main advantage being that the apparent absorbance maximum occurs at considerably lower scale values than the corresponding absorbance plateau measured by the ASTM method. This reduces the need for successive attenuations of the reference beam every time the spectrophotometer runs off scale. In many instances the new method required no attenuation at all.
Applied Optics | 1979
Victor R. Weidner; Jack J. Hsia
A research hazemeter has been developed by the Radiometric Physics Division of the National Bureau of Standards for the quantitative analysis of light scattering by optical materials. The instrument is designed to measure percentage haze as set forth in the prescribed conditions recommended in ASTM Test Method D 1003. The hazemeter has several special features which make it possible to vary the geometrical conditions of the collimated light beam and the integrating sphere detector system for detailed studies of light scattering. Light scattering associated with the instrument source and collimating optics has been reduced to less than 0.1% of the total light beam by using a diffraction limited off-axis parabolic mirror to collimate the hazemeter light beam. The source optics provide a collimated light beam of approximately 6800 K. The detector is a silicon cell photodiode with a V(lambda) window. This combination gives the system a response corresponding to the visual luminosity function. The detector nonlinearity and instrumental errors associated with the accuracy of the integrating sphere geometry have been analyzed. The over-all uncertainty in percentage haze has been reduced to +/-0.2%, and light scattering of less than 0.5% can readily be measured.
Applied Optics | 1980
K. L. Eckerle; Jack J. Hsia; Victor R. Weidner; William H. Venable
A long-range retroreflectance instrument has been built in the photometric range of the Radiometric Physics Division of the NBS. It is designed to measure photometric properties of retroreflectors for different geometries. It satisfies many needs of the measurement community, and it is planned to use this instrument as the basis for a Measurement Assurance Program (MAP) and for Standard Reference Materials (SRM). This paper describes the design and testing of the instrument. Some estimated uncertainties for typical samples are given.
Journal of research of the National Bureau of Standards | 1977
William H. Venable; Jack J. Hsia; Victor R. Weidner