Walter G. Egan

Retroreflectance measurements of photometric standards and coatings

Using a technique that we have developed, the opposition effect (brightening in the retroreflection direction) has been measured for MgCO(3), BaSO(4) paint, and sulfur in the visual region with incandescent illumination and found to be 1.3,1.5, and 1.3, respectively, independent of wavelength. Nextel red, blue, white, and black paints are generally similar. However, in comparison to incoherent illumination, 0.6328-microM laser illumination shows a reduction in diffuse reflectance for angles less than 40 degrees from the incident direction. In addition, the coherent opposition effect may be very large for dark paints. The opposition effect in photometric standards can lead to calibration errors at near opposition and spurious responses in integrating spheres coated with such materials.

Determination of Absorption and Scattering Coefficients for Nonhomogeneous Media. 2: Experiment

Separation of the scattering contribution from the total optical attenuation is necessary to determine the absorption portion of nonhomogeneous media such as naturally occurring minerals. In order to investigate experimentally the applicability of a previously developed two-flux radiative transfer model that takes into account surface reflections and collimated incidence, we have measured pulverized and sintered scattering standards prepared from a glass of known absorption coefficient variation. The new model produces an accuracy improvement up to a factor of 2.5 over the Kubelka-Munk theory. Off-axis scattering measurements were made with improved instrumentation between 0.33 microm and 2.7 microm. The model was then applied to the mineral rhodochrosite in this range to obtain accurate values of scattering and absorption.

Complex refractive index of Martian dust - Mariner 9 ultraviolet observations

Abstract Mariner 9 ultraviolet spectra of the 1971 Mars dust storm were studied to determine the cloud particle size distribution and complex index of refraction. The method consisted of matching the observed single particle scattering albedo and phase function with Mie scattering calculations for size distributions of spheres of homogeneous and isotropic material. Preliminary results indicate that the effective particle radius is 1 μm with an effective variance (a measure of distribution width) ⪞0.2 . The real component of the index of refraction is ⪞1.8 at both 268 and 305 nm. For the imaginary index, a value of 0.02 was found at 268 nm and 0.01 at 305nm. These ultraviolet refractive indices are compatible with measurements at visible wavelengths which indicate that the real part of the refractive index is 1.75 with a negligible imaginary term. The rapid increase of refractive index and absorption coefficient with decreasing wavelength are indicative of an ultraviolet absorption band. An ultraviolet absorption band is not only diagnostic of the composition of the Mars material, but may have important implications for the development and evolution of life on Mars. A 30 μm layer of material that absorbs uv but transmits visible light can shield organisms from harmful irradiation while providing for photosynthesis. Comparison of the Mars ultraviolet refractive indices with laboratory measurements indicates that none of the terrestrial analog samples of limonite, basalt, andesite, or montmorrillonite have the required ultraviolet properties.

Ultraviolet complex refractive index of Martian dust: Laboratory measurements of terrestrial analogs

Abstract The optical complex index of refraction of four candidate Martian surface materials has been determined between 0.185 and 0.4 μm using a modified Kubelka-Munk scattering theory. The candidate materials were limonite, andesite, montmorillonite, and basalt. The effect of scattering has been removed from the results. Also presented are diffuse reflection and transmission data on these samples.

Integrating Spheres for Measurements Between 0.185 μm and 12 μm

A new system for absolute total diffuse transmission and reflection measurements using two integrating spheres is described; it extends the normal spectral range into the uv and ir. Furthermore, as part of the system, a unique self-calibrating technique has been incorporated. One sphere is coated with BaSO(4) for use between wavelengths of 0.185 microm and 2.0 microm, and the other is coated with sulfur flowers for use between 1.5 microm and 12 microm. The BaSO(4) and sulfur fulfill the diffuseness and high reflectance characteristics necessary for an integrating sphere. The feature of the system for calibration of the spheres uses goniophotometric measurements of diffuse transmission and reflection. These reflection intercalibrations are better in the visual range than a fraction of a percent (absolutely); however, transmission intercalibrations on quasi-specular samples may have absolute differences of several percent, thus requiring a goniophotometric transmission measurement as the valid determination.

Photometric and polarimetric properties of the bruderheim chondritic meteorite

Abstract Photometric and polarimetric laboratory measurements were made as a function of phase angle in the U(0.36 μm), G(0.54 μm) and R(0.67 μm) bands for 0°, 30° and 60° incident illumination on four particle size ranges of Bruderheim, an L6 olivine-hypersthene chondritic meteorite. The four particle size ranges were: 0.25–4.76 mm, 0.25–4.76 mm coated with

Spectral reflectance of particulate materials: a Monte Carlo model including asperity scattering.

A six-flux Monte Carlo approach to modeling of the reflectance of particulate surfaces is presented that includes the scattering as well as absorption of asperities of the constituents. For spherical particles greater, similar lambda, the forward, backward, and transverse scattering is calculated. Then using a radiative transfer model, the reflection of a particulate surface may be determined. An application of the theory is made to four particle size ranges of a nonhomogeneous, multiconstituent sample-the Bruderheim meteorite-of olivine-hypersthene chondritic composition. The theoretical values of reflectance, using the experimentally determined optical complex indices of refraction, compare well with the experimental reflectance observations.

Polarimetric and photometric simulation of the Martian surface

Abstract An approach is presented to obtain a truly rational interpretation of the Martian surface and atmosphere. Polarimetric and photometric properties of ore specimens (limonite, siderite, desert varnish, and serpentine), considered good prospects for the Martian surface, are presented for the colors B(0.48 μ), G(0.54 μ), and I(1.0 μ). This data was obtained with a large-scale analyzer. Other measurements to better define the specimens are also described. It is concluded that, assuming a small atmospheric influence, the best optical match to the Martian bright areas occurs with Venango County, Pennsylvania limonite, with a dominant particle size range between 1 and 2 mm, and a subsidiary minor distribution of a smaller particle size. The smaller particles are suggested as μ limonite or μ siderite.

The rings of saturn: A frost-coated semiconductor?

Abstract Optical complex indices of refraction for the iron-sulfur semiconductors troilite and pyrrhotite have been measured between 0.3 and 1.1 μm, and at 1.5 and 3.0 cm in the microwave region. The behavior of the absorptive and refractive components of the index in the visual and ultraviolet regions suggests a reasonable match to published data on Saturns rings. A combination of the iron sulfur with water ice and the orthopyroxene enstatite is consistent with an equilibrium condensation model for the formation of the solar system; the water ice and an orthopyroxene are also suggested by near-infrared observational data. A combination of these materials could explain all spectral features seen in the ring spectra to date. The microwave behavior of a small quantity of troilite embedded in water ice at low temperatures is consistent with the radar reflectivity and radio emissivity data.

Coherence-polarization phenomena in remote sensing

The importance of polarization phenomena in remote sensing by electromagnetic means is discussed briefly. With active sensor systems using microwave or laser sources the radiation is very likely to be coherent and plane polarized. Both intuition and measurement point to anticipated differences in the scattering functions of particles and surfaces interacting with incoherent and with coherent radiation. Polarization is one important aspect of these scattering functions that we have undertaken to investigate in the laboratory. Data selected from our measurements on silica beach sand, Haleakala volcanic ash, and magnesium carbonate have been used to point out significant differences in polarization, depolarization, and photometric functions related to the geometry of incidence and observation. Filtered tungsten light (λ eff =0.48 µ, λ eff =0.54 µ, λ eff =1.0 µ) results are compared with He-Ne laser (0.6328 µ) results on the same photometric-polarimetric analyzer, and possible explanations are offered. A very interesting aspect of the magnesium carbonate data is the marked deviation from the cosine law reflection that makes it such a useful photometric reflection standard for incoherent light.