This is a day late, but worthy of a post!
I've read many things over the years, but I never knew that Our Lady's eyes dilate when light is flashed upon the image.
Scientists from the NASA Research Centre have conducted extensive tests on the fabric and image and have discovered no scientific explanation, since the picture is neither painted, dyed, or woven. Digital enlargements of the Image were made and it was discovered that the reflection of Bishop Juan de Zumárraga looking in astonishment could clearly be seen in the pupil of Our Lady's eyes. Further study of the photographic enlargements and rigorous investigations of Our Lady's eyes (only about 1/3rd inch in size), have revealed not only the minuscule human image of a bearded man clearly identifiable in the eyes that no artist could have painted, but all the optical imaging qualities of a normal human eye, such as light reflection, image positioning, and distortion on the cornea which are impossible to obtain on a flat surface. When the eye of Our Lady is exposed to light, the retina contracts, and when the light is withdrawn, it returns to a dilated state, just as happens with a living eye. Our Lady's eyes have been ophthalmologically determined to be alive on the Tilma!
Here are several links of interest: