In this study, we propose a method to expand the dynamic range of expansion or strain measurement using statistical interferometry. Statistical interferometry is a very accurate interferometric technique that is applicable to practical rough surface objects [Opt. Lett. 16, 883 (1991); J. Opt. Soc. Am. A 18, 1267 (2001)]. It is based on the statistical stability of a fully developed speckle field and was successfully applied to measure the growth of plants in our previous study [Environ. Exp. Bot. 64, 314 (2008); J. For. Res. 12, 393 (2007)]. However, the measurable range of the expansion of the object was restricted to less than one wavelength of the light used. Improvement of the dynamic range is confirmed experimentally in this work by introducing a large expansion up to while keeping the precision of measurement high. Next, the improved system is applied to monitor plant growth from the subnanometric scale to several hundreds of micrometers under some environmental conditions. These features of the method make it especially worthwhile in botanical and agricultural studies.
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