Ultraviolet transmission changes in glass as a function of the wave length of the radiation stimulus

Abstract

abstract Continuing the research previously reported in Research Paper 113 further consideration is given to methods of testing the photochemical stability of window glass made for transmitting short wave length ultraviolet solar radiation. It is shown that accelerating the stabilization of the transmission of window glass (equivalent to the stabilization produced by sunlight) by means of filtered ultra-violet radiation from the quartz mercury arc is impracticable principally because of the antagonistic action of radiation of wave lengths in the region of about 365 inn which reverses, in varying degrees, the photochemical reaction produced by the shorter wave lengths. A new phenomenon in the ultraviolet transmission behavior of certain glass is reported. The phenomenon may be briefly described as follows: When irra-diated with light of a given wave length (in the region 250 to 365 mn) the sample of glass reaches an equilibrium state in which the transmission (at 302 m/x, for example,) attains a certain value, which is a constant for the given wave length stimulus. The equilibrium value of the transmission is different for each wave length of the radiation stimulus, increases with increasing wave length of the stimulus, and is independent of the previous order of treatment of the glass. In place of a chemical analysis, the absorption spectrum (which is a charac-teristic of every chemical compound), is used as a criterion for judging the mag-nitude of photochemical action produced by ultraviolet radiation of different wave lengths of homogeneous radiation. In the soda-lime-silica glasses examined (whether or not they contained an appreciable amount of iron oxide) the photochemical equilibrium was found to be different for each wave length of homogeneous radiation to which the glass reacts. To attain this equilibrium the reaction, caused by a given wave length, is in the direction to either increase or decrease the transmission, depending upon the existing condition developed by previous heat treatment or by previous exposure to other wave lengths to which the glass reacts. Using homogeneous radiation, no wave length (at least not for wave lengths shorter than and including 365 m/x) was found that has the exclusive property of either increasing or decreasing the ultraviolet transmission in the soda-lime-silica glasses examined. The longest wave length having an appreciable photochemical action on a soda-lime-silica glass is at about 405 m/x. For wave lengths 365 rn/x and shorter, the photochemical action as determined by the equilibrium levels in spectral absorption, is a function of the wave length (the frequency) of the radiation stimulus; the shorter the wave length the greater the photochemical action in the direction of greater absorption. In contrast with a soda-lime-silica glass, a phosphate-lime glass (containing oxides of Al, B, Na, Mg, Si, and Fe as minor constituents) is depreciated in ultra-violet transmission only by wave lengths shorter than about 290 m/x. After depreciation, the ultraviolet spectral transmission curve of the calcium phosphate glass is similar to that of a soda-lime-silica glass. Nevertheless, exposure of the depreciated calcium-phosphate glass to homogeneous radiation of wave lengths 297 to 365 m/x does not appreciate the transmission, as is the case in the soda-lime-silica glass. In other words, in contrast with the soda-lime-silica glass, certain wave lengths have the property of decreasing the ultraviolet transmission; but in the spectral range investigated no reverse reaction was observed.