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Abstract

The calibration standard is attached to a sensing head of an infrared absorption gauge for the purpose of calibrating the gauge. The calibration standard includes spectrally selective absorption means having a spectrum closely resembling that of a substance to be measured, such as water sorbed onto a paper sample. The calibration standard also includes optical means for returning infrared radiation, which has passed through the absorption means, to the absorption gauge substantially along the same path as would normally be followed by the radiation reflected from the sample in the absence of the standard. The spectrally selective absorption means is preferably made of anhydrous material, such as glass containing a rare earth substance, or polypropylene, which is stable. The optical means may include diffusing means and a reflective plane. According to one arrangement, the calibration standard comprises a housing with a window at one end, a plurality of diffusing elements attached to a central portion of the window and a reflective plane spaced from the diffusing elements. The window and/or the diffusing elements and/or the reflective plane may comprise or include the spectrally selective absorption means. Various arrangements are described.

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