Review of Calibration Methods for Scheimpflug Camera

The Scheimpflug camera offers a wide range of applications in the field of typical close-range photogrammetry, particle image velocity, and digital image correlation due to the fact that the depth-of-view of Scheimpflug camera can be greatly extended according to the Scheimpflug condition. Yet, the conventional calibration methods are not applicable in this case because the assumptions used by classical calibration methodologies are not valid anymore for cameras undergoing Scheimpflug condition. Therefore, various methods have been investigated to solve the problem over the last few years. However, no comprehensive review exists that provides an insight into recent calibration methods of Scheimpflug cameras. This paper presents a survey of recent calibration methods of Scheimpflug cameras with perspective lens, including the general nonparametric imaging model, and analyzes in detail the advantages and drawbacks of the mainstream calibration models with respect to each other. Real data experiments including calibrations, reconstructions, and measurements are performed to assess the performance of the models. The results reveal that the accuracies of the RMM, PLVM, PCIM, and GNIM are basically equal, while the accuracy of GNIM is slightly lower compared with the other three parametric models. Moreover, the experimental results reveal that the parameters of the tangential distortion are likely coupled with the tilt angle of the sensor in Scheimpflug calibration models. The work of this paper lays the foundation of further research of Scheimpflug cameras

Enzymatic degradation of the cornea to develop an experimental model for keratoconus: Biomechanical and optical characterisation

The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. It is composed of five layers, in which the stroma is the thickest layer (approximately 90% of corneal thickness) that consists mainly of laminated collagen fibrils associated with proteoglycans. The cornea acts as the eye’s outermost lens that is accounted for approximately two-thirds of the eye's total optical power. Like other lenses, the cornea’s geometrical characteristics, such as the curvature, are important to maintain its functions for clear and stable vision. These geometrical characteristics are highly affected by the biomechanical properties of the cornea. For example, in keratoconus, the cornea is characterised by a progressive and localised thinning in corneal thickness, which is associated with a reduction in stiffness and other biomechanical properties. These alterations happen at the collagenous network, which is mainly responsible for the biomechanical features of the cornea, and are mostly attributed to genetic factors and abnormal enzymatic activity. Histological and biochemical studies suggested the role of amylase and collagenase activities in degradation of collagenous network and progression of keratoconus. However, the role of amylase and collagenase on biomechanical and optical properties have not been investigated. In this study, in vitro enzymatic degradation of porcine corneas was conducted with varying concentrations of α-amylase and collagenase (crude and purified) enzymes for different incubation periods. Several techniques, including atomic force microscopy, nanoindentation and optical coherence tomography, were utilised to assess the effect of the enzymes on biomechanical of corneal tissue at macroscale, microscale and nanoscale levels. Corneal transparency and absorption following enzymatic incubation were also measured using spectrophotometry. The biomechanical techniques that were utilised indicated that amylase and collagenase decrease corneal stiffness and thickness following incubation the corneas with amylase and collagenase. Further reduction in biomechanical properties and thickness of the corneas was found with increased enzymes concentrations and incubation periods. Corneal transparency was increased following incubation with the enzymes. The results suggest depletion of proteoglycans by amylase and digestion of collagen fibrils by collagenase. These results were used to propose an animal biomechanical model for keratoconus