of the present invention incorporates manual rotation of the selectively rotatable segment 110 but it is understood that such rotation may be automated fig7 9 graphically illustrate the operation of the polarizer pair fig7 shows a transmission plot of the polarizer pair when second polarizer 220 is rotated fully parallel to first polarizer 210 likewise fig8 shows a transmission plot of the polarizer pair when second polarizer 220 is in a fully crossed position relative to first polarizer 210 fig9 shows a typical normalized transmission plot of the polarizer pair as a function of the rotational angle of second polarizer 220 with respect to first polarizer 210 the selection of circular polarizers rather than standard linear polarizers serves to reduce internal specular reflections specifically the quarter wave plate which forms part of any circular polarizer is a retardation element that changes the phase of the incident plane polarized light by one quarter wavelength if the exitant light is reflected back from a specular surface its phase will again be retarded one quarter wavelength by the quarter wave plate so that it will then be one half wavelength out of phase with the linear polarizer and will not be transmitted see photometry and radiometry for engineers by allen stimson john wiley amp sons new york 1974 and optics by hecht and zajac addision wesley publishing reading mass 1974 in this manner internal reflections within the indicator arising from external light sources will not be transmitted back out externally the light radiation which is passed by second polarizer 220 is.