72 as the polarizing filter wheel 80 rotates the linear polarizing filters 80 a 80 b or the neutral density filter 80 c is inserted into the path of the probe beam at predetermined time intervals which can be the same it should be noted that the order that the filters 80 a 80 b and 80 c is inserted sequentially is a matter of design choice that is the polarizing filter wheel 80 can be rotated clockwise or counter clockwise the birefringence thickness and other characteristics of the web 88 can be calculated using the differences in the spectral fringes of unpolarized and linearly polarized light due to large thickness variability in the md direction of the web it is preferred to employ fast detectors with high signal to noise ratio such as ingaas detectors and to use fast chopping frequency or modulated super luminescent diodes slds for the ir source fig4 a and 4b illustrate another configuration of a multichannel detector 90 wherein a polarizing filter wheel 80 is positioned between the moving web 88 and the detector 90 along the probe beam path from the radiation source 82 the system is essentially the same as that shown in fig3 a and 3b in that it includes a multichannel detector 90 that is positioned one side of the moving web 88 with a source of radiation 82 that can be positioned on either of the moving web 88 in this embodiment the polarizing filter wheel 80 is positioned in front of lens 86 along the path of the probe beam so that either transmitted fig4 a or reflected fig4 b radiation that emerges from the moving web 88 will pass through the polarizing filter wheel.