For such multiple beamline systems, wavelength selection consists simply of opening and closing the appropriate mechanical beam blocks and changeover between wavelengths requires less 30 seconds or less. Absolute accuracy of prism coupler measurements are often limited to the .0005 range by uncertainties in the measuring prism angle and index. We are currently working to develop calibration standards which will permit absolute index accuracy comparable to the rotary table effective index resolution. Accurate and reproducible coupling pressure without sample rotation or prism damage is obtained with a pneumatic coupling cylinder. A conveniently mounted pressure regulator permits rapid and quantifiable adjustments in coupling force.
Coated prisms are available for precise control of the air gap.
Standard coupling geometry is a single-prism with a section of a spherical stainless ball to dimple a small portion of the sample against the coupling face of the prism. If desired, the coupling fixturing can be made modular to permit quick interchange of a variety of coupling arrangements on the rotary table. Metricon offers four standard coupling prisms of rutile, GGG, and cubic zirconia to span the effective index range from 1.0 to 2.60. Low index prisms can be coated or depolished slightly to control the size of the air gap between guide and prism and prevent overcoupling effects. Prisms are normally supplied in convenient mounting clamps which permit interchange of prisms in less than 30 seconds and which provide four redundant coupling locations per prism.