We establish theoretically the relationship between the lateral magnification m and the object distance for a gradient refractive index (GRIN) square lens and a GRIN rod lens. Through the linear fit of the object distance and the reciprocal of the magnification for different image locations, we use the imaging method to measure the central refractive index and focusing constant α of the GRIN square lens and the GRIN rod lens. The center refractive indices and focusing constant α of the GRIN square lens and the GRIN rod lens are 1.611 and 1.609, 0.0926 and 0.100, respectively. For the GRIN square lens and the GRIN rod lens, the measured central refractive indices are accurate to .
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