What is Q value of cornea?
The human cornea has an average Q value of -0.26; it would require a value of -0.52 to eliminate all spherical aberration. The Q value of a young adult crystalline lens is -0.25; thus, the combined value for a young phakic eye results in elimination of spherical aberration.
What is the average corneal curvature measurement?
Results. The mean values obtained were as follows: CCT = 548.97 ± 34.28 μm, IOP = 15.61 ± 2.69 mmHg, average corneal curvature (AVK) = 42.98 ± 1.19 D, horizontal corneal diameter (HVID) = 11.39 ± 0.69 mm and vertical corneal diameter (VVID) = 10.51 ± 0.50 mm.
What is Q value in refractive surgery?
The Q value, being the crucial parameter of the mathematical cornea model, is re ective of the shape of the cornea and its optical properties [4,5], which includes the aberration distribution, the spherical aberration, the refractive power, and others.
What is Q value in pentacam?
The corneal Q-value was automatically calculated by the Pentacam software according to the formula Q = −E2 (E-value = eccentricity). A Q-value less than 0 is considered prolate, and a Q-value greater than 0 is oblate. All Q-values included nasal, temporal, inferior, and superior quadrant data.
What is corneal Asphericity?
Asphericity. Asphericity is a measure of shape of a refractive medium and how it affects bending of light, when light passes through an optical medium, the shape of the optical media affects where the central and peripheral rays eventually focus.
What is coma in lenses?
Coma is an aberration resulting from a variance in magnification depending on the ray height at the lens. Negative coma occurs when rays hitting the lens further from the paraxial region focus closer to the axis than rays closer to the paraxial region.
How do you interpret corneal topography results?
Warmer colors represent steeper corneal curvature while cooler colors represent flatter areas. For the elevation maps (anterior and posterior float), warmer colors denote where the cornea is elevated above the best fit sphere and cooler colors denote where the cornea is depressed below the best fit sphere.
What is Belin Ambrosio?
The goal of the Belin / Ambrósio Enhanced Ectasia Display is to combine elevation based and pachymetric corneal evaluation in an all inclusive display. This gives the clinician a global view of the structure of the cornea and allows the physi- cian to quickly and effectively screen patients for ectatic disease.
What is negative spherical aberration?
“Positive” spherical aberration means peripheral rays are bent too much. “Negative” spherical aberration means peripheral rays are not bent enough.
What is apical radius?
Apical radius represents the curvature at the corneal apex, which is the point of maximum curvature on the corneal surface when maximum curvature is found at a single point. Apical radius also determines the size of the conic section model.
What is the difference between coma and astigmatism?
Coma is caused by obliquely inciding parallel rays of light on a spherical lens, astigmatism is caused by obliquely inciding diverging rays of light on the spherical surface. Both errors can primarily be corrected by sophisticatedly combining several lenses and using aspheric lenses.