Infrared Filters of Controllable Transmission

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Abstract
When using an echelette grating for work in the infrared it is necessary to eliminate overlapping, higher orders. Since no adequate series of true filters covering the entire infrared is known, a new type of filter has been developed. Essentially, this type of filter consists of a powder spread uniformly over a surface of polished speculum. Short wave-length radiations are reflected diffusely while long wavelength radiations are largely transmitted since the Rayleigh scattering for long waves is feeble. The region of transition from opacity to transparency is controlled by choosing powders of proper particle-size and by depositing these in layers of appropriate thickness.

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