192 RADIATION BIOLOGY 



scratched unless protected by an evaporated film of silica or other trans- 

 parent coating. Evaporated films of chromium, nickel-chromium alloys, 

 and rhodium (Claiborne, 1947; Sabine, 1939) produce surfaces that are 

 much more scratch-resistant and have reflecting properties equivalent to 

 those of aluminum in the visible and near ultraviolet. 



"White" coatings with high diffuse reflectance throughout the spec- 

 trum are often required for the interiors of irradiation chambers and 

 integrating, or Ulbricht, spheres. The spectral characteristics of a thick 

 white coating depend on the spectral transmittance of the "pigment" 

 crystals and the vehicle. The "hiding power," which is inversely propor- 

 tional to the thickness required for complete opacity, primarily depends 

 upon the refractive index of the pigment; the higher the refractive index, 

 the thinner the layer of crystals required. Basic lead carbonate (white 

 lead), with a refractive index of 2.1, is one of the oldest of the white 

 pigments. The two titanium dioxide pigments have almost replaced 

 white lead in modern finishes, partly because they are nontoxic and 

 because both crystalline forms have a higher refractive index (2.61, rutile; 

 2.54, anastase). However, the titanium pigments have low ultraviolet 

 reflectance (Judd, 1949; Mattiello, 1946) beyond 400 mp.. White lead 

 has high visible and ultraviolet reflectance and is therefore a much more 

 versatile pigment for the near ultraviolet. Aluminum and other metalHc 

 paints that contain a powdered white metal dispersed in a vehicle have 

 anomalous reflection properties. Aluminum-coated surfaces never appear 

 quite white, and yet their reflectance may approach that of surfaces 

 coated with white crystalline pigments (Coblentz, 1912; Mattiello, 1946). 



Vehicles of the drying oils and resins yellow with age, and the near- 

 ultraviolet reflectance decreases. Therefore the most permanent whites 

 are obtained with vehicles of the clear, colorless synthetic resins such as 

 the polymethacrylates. These resins also impart a high degree of chemi- 

 cal resistance to the finish and are used in many industrial coatings. 

 "Glossy" finishes contain a high proportion of vehicle in relation to 

 pigment, and the surface is smooth and exhibits considerable specular 

 reflection. Many "flat" commercial finishes contain a minimum of 

 vehicle and a "flatting" agent that tends to produce a roughened surface. 

 The flat finishes, especially those of the low-vehicle type, are preferable 

 for obtaining the maximum diffusion of the incident flux, but the surface 

 is less durable. 



Magnesium oxide deposited from burning magnesium ribbon is the 

 primary standard of white reflectance and is the coating most often used 

 in small integrating photometer spheres (Benford et at., 1948; Middleton 

 and Sanders, 1951, 1953; National Bureau of Standards, 1939). Since 

 the refractive index of magnesium oxide is only 1.74, a deposit several 

 millimeters thick is required for opacity; therefore many layers of coating 

 must be deposited on a white metal or porcelain-enameled surface. A 



