He found that corneal depolarization is not significant, contrary to depolarization due to retina. They mentioned Pelz, who calculated the contribution of the central cornea to the polarization properties using the light reflected from the first lens surface measured in vivo for his PhD thesis. Bueno and Jaroński 17 also reported that dichroism properties and effects of depolarization are negligible. They also stated that the effect of the depolarization is not significant, especially measured with light shorter than 500 nm. This means that human corneas should be characterized as an elliptically birefringent medium 31. suggested that considering the cornea as a linear birefringent medium is insufficient because it does not consider optical rotation. using PS-OCT during in vivo and in vitro studies concluded that the cornea in the center can be described as a linear birefringent biaxial crystal, whereas in the periphery, there is an almost circularly symmetric high-birefringence area 20, 21. 30 and Misson 1 using polarizing microscopy in in vitro measurements, stated that cornea might be described as a biaxial medium with low optical retardance. In 2008, Knighton confirmed this hypothesis using Purkinje’s images 29. suggested that in the central area, the cornea behaves as a biaxial crystal with a fast axis perpendicular to its surface and a slow axis located in nasal-lower directions based on their Mueller-matrix ellipsometry 24. 27, 28, using their measurements, concluded that the birefringence in the corneal center is the lowest and that it increases towards the limbus. Most researchers describe corneal polarization properties ditto, which is confirmed via various experiments 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24. Stanworth and Naylor 13 using polarimetry technique stated that the cornea can be described as curved uniaxial crystal plate with its optic axis perpendicular to its surface. He said that human corneas depolarize in all positions. The first mention of corneal birefringence came from Brewster in 1815 12. In their model, the central fibrils are orthogonal to each other and change direction in the peripheral cornea to merge with the tangential fibrils of the highly reinforced limbal annulus. They showed a significant difference in lamellae distributions in the nasal and the temporal part of the cornea. proposed the lamellar distribution model 11. Over time, several models of lamellar orientations were proposed 3, 4, 5, 6, 7, 8, 9, 10. His assumption did not find anatomical confirmation. For instance Rollet in his work suggested that the shape of the interference fringes may reflect the radial character of lamellar orientation in the human cornea (mentioned in Stanworth et al. The issue of lamellae location and corneal birefringence concerned many researchers. Early detection of these changes with non-invasive methods may help detect and treat their development. Changes in lamellar location hence changes in anisotropic distribution, may indicate lesions or corneal structural diseases, such as keratoconus, that lead to reduced visual acuity or even the need for corneal transplantation. This unique stromal organization causes the cornea to become a birefringence medium. Each lamella contains collagen fibrils embedded in ground substance, each with different refractive indices. This creates stronger birefringent properties of this structure 1. The resultant lamellar orientation in the stromal center is random, but towards the limbus, this orientation becomes more ordered, and a preferential axis appears. It mainly consists of hundreds of lamellae. The stroma is the layer with the most robust anisotropic properties in the cornea.
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