What is the sensitivity of geometric moire? Recall that geometric moire refers to moire resulting from grids where the pitches p and p' are large enough so that the effect of overlapping the grids can be explained by simple occlusion, without recourse to any diffraction effects. To determine the strain sensitivity of these grids, simply use the definition of exx = du/dx = p(dN/dx) in the principal direction x. Then depending on the experimental system, image sizes, camera film, etc., determine the largest moire spacing that is practical. If this moire fringe spacing is called h, then the strain sensitivity of the moire is given by exx = p(dN/dx) = p/h. As an example, suppose that the largest moire spacing you can use is 1.0 inch. The strain in metals is generally measured in microstrain (me), so what grid pitch p is required to measure a strain of 50 me in metals? Simple substitution gives the answer p = eh = (50 x 10-6)(1.0) = 5 x 10-5 inch or the fringe density must be 20,000 lines/inch! This is far too dense for geometric moire and there are no suitable mechanical methods for producing such grids. Such grids can be produced optically, however, and the use and interpretation of moire at this level is called interferometric moire, about which more later. Thus it appears that geometric moire is too insensitive to be used for measuring small strains. What then is it good for?
Also, it should be noted that it is possible to make moire measurements more accurate by sharpening or narrowing the fringes, thus making it easier to locate their centers and improving the accuracy of interpolation.
Up to now, it has been assumed that all gratings have been Ronchi gratings, i.e., bars and spaces of equal width. However, it is not necessary to have this equality. One can use arbitrary ratios of bar width to space width. Factors of 3:1 are common. Indeed, it is advantageous to have reverse ratios for the master and specimen gratings. In this case, the gratings are said to be complementary. The advantage is in the production of sharper, narrower moire fringes.
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