George W. Luckey

Optical Sphere Paint and a Working Standard of Reflectance

Specially prepared barium sulfate powders and coatings of these powders with polyvinyl alcohol have been used as working standards of reflectance and for coating integrating spheres. These materials are satisfactory for use in the wavelength range 0.20-2.00 micro. Since the refractive index of barium sulfate is rather low, it is necessary to use fairly thick layers in order to obtain good results. The absolute value of luminous reflectance of the barium sulfate powder, described in this paper, is 0.995 +/- 0.001, and that of the paint, when properly applied, is 0.992 +/- 0.001. The spectral reflectance of this material is higher than that of magnesium oxide, particularly in the uv region of the spectrum. By using barium sulfate as reference standard and as a coating on the integrating spheres of spectrophotometers, it is now possible to measure spectral reflectance reliably in the uv region to 200 nm. Because the reflectance of barium sulfate paint is significantly greater than that of magnes um oxide in the shortwave region, there is a corresponding gain in instrument sensitivity when barium sulfate is used as a sphere coating and as a comparison material.

Effect of Bromine on the Dark Conductivity of Silver Bromide

The dark conductivity of silver bromide in a bromine atmosphere increases with increasing bromine pressure. The conductivity increases are proportional to the square root of the halogen pressure in the range between 10 and 200 mm of mercury, at temperatures between 25°C and 200°C. The increases agree with those calculated from the corrosion data of Wagner and Teltow, and they are probably caused by positive holes. The increases are affected by the purity of the crystals, and the rate of equilibrium attainment depends on the structure and purity of the samples. In particular, the presence of small amounts of silver in the crystals prevents attainment of equilibrium until all the silver is removed. The rate of silver removal depends on the positive‐hole conductivity. The results are in good agreement with the corrosion mechanism suggested by Wagner.

Vacuum Photolysis of Silver Bromide and Silver Chloride

The quantum yields of halogen production in the vacuum photolysis of silver bromide and silver chloride have been measured at various temperatures, wavelengths, and light intensities. Silver‐bromide crystals containing small amounts of copper and cadmium bromide have also been studied. At room temperature, the yields of thick crystals follow the equation, φ=Q(1−e−ka), where Q is the surface efficiency, k is the absorption coefficient, and a is a parameter which is defined below. At room temperature, in the halogen evolution range studied, the value of a for silver bromide is about 0.4 micron and that for silver chloride is about 0.24 micron. The values of Q are usually between 0.5 and 1.0.The quantum yield of halogen production at room temperature is similar to the yield of a fast wall reaction. Both depend on the diffusion of reactants to the surface. The parameter a is related to the thickness of the diffusion layer. A numerical solution of the diffusion equation by C. A. Duboc has shown that a=1.154D12...