The present invention provides a thermally stable reference member comprising,
at least one radiation attenuating element and at least one radiation scattering
element. The radiation attenuating element comprising at least one aperture for
transmission of radiation therethrough. The attenuating and scattering elements
placed in series so that radiation transmitted through the reference member passes
through each of the attenuating and scattering elements. The attenuating and scattering
elements of the reference member may further comprise a thermally stable mount
to hold the elements in a selected position relative to each other, and in relation
to an instrument, or the elements may be bonded together. The radiation attenuating
element may be comprised of a material selected from the group consisting of INVAR,
tungsten, brass, and a material substantially non-transparent for incident radiation,
and the radiation scattering element may be comprised of a radiation scattering
material selected from the group consisting of opalescent glass, SPECTRALON,
PTFE, ZERODUR, fused silica, quartz, sapphire, diamond,
and a transparent material with essentially low thermal expansion.
