Where a metal foil construction is utilized, {TAIL ~ a plurality} of such metal foils may be stacked in layers with orifices in adjacent layers being offset from one another such that the radiation shield is substantially opaque to {HEAD ~ infrared radiation} while permitting the passage of the alkali metal vapor 
Where a metal foil construction is utilized , { TAIL ~ a plurality } of such metal foils may be stacked in layers with orifices in adjacent layers being offset from one another such that the radiation shield is substantially opaque to { HEAD ~ infrared radiation } while permitting the passage of the alkali metal vapor
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