Handout 6 Flashcards
Aberrations of the eye are taken into account in clinic for
Keratorefractive procedures, corneal grafts, keratoconus
Aberrations of the eye can be measured with
Wavefront sensors
Paraxial ray
A ray which makes a small angle to the optic axis of the system and lies close to the axis throughout the system
Paraxial approximation
Incident rays are close to the optical axis yielding point images for point objects
Is paraxial approximation tru
Not always
Marginal ray
A ray peripheral to the optic axis and is bent more than axial or paraxial rays
When paraxial optics dont apply 2 aberrations will occur called
Monochromatic and chromatic
Monochromatic aberrations aka __. Include : (5)
Seidel Spherical coma oblique ast. Curvature of field Distortion
spherical aberrations, when a pencil of light is refracted by a large-aperture optical system, different zones of the aperture have different focal lengths
Peripheral/marginal rays are focused differently than centra/ paraxial rays which affects sharpness
Large aperture aberrations are mitigated by
The pupil because of its small size only accepts paraxial rays
Spherical aberrations are considered in what lenses
High powered and convex lenses -> correct with aspheric
Spherical aberrations can be negative
Peripheral rays refracted less than central (peripheral hyperopia).
Spherical aberrations can be negative
- Peripheral rays have a shorter focal length/ are refracted more than central rays (peripheral myopia)
- More common than negative
Hen oblique/ off axis rays are refracted by a large aperture optical system at different points than those incident on or near the optical axis produces a
Coma (comet)
Affects the sharpness of image points
-rarely a problem
-significant aberration of the eye itself in keratoconus and can increase after refractive surgery
Factors than can control coma
Aperture size
Lens form
Angle of obliquity
Increase in pupil size leads to
A decrease in image quality owing to increased aberration
-in high powered lenses, it is best to use aspheric lenses
Whats an aspheric lens
Rotationally symmetric surface whose radius of curvature varies with position from lens center -> changes constantly across lens surface
Modifies lens surfaces without changing lens power
Aspheric lenses
Occurs when light passes obliquely through a spherical surface to form an interval of sturm rather than a point focus
Oblique astigmatism
Oblique astigmatism aka ___ forms an __ instead of ___.
Radial astigmatism/marginal astigmatism
Interval of strum rather than a point focus
Relation of oblique astigmatism and pupil/aperture size
Not dependent on each other
All of I together form paranoid all surfaces
Teacup (tangential) and saucer (sagittal) surface. Tan is always steeper
Curvature of field AKA
curvature of image aka power error
-ophthalmic lenses form curved not plane images
A true Petzval surface can only exist when
Oblique astigmatism has been eliminated
What keeps curvature of field to a minimum
Optimum base curvature
Distortion occurs when
Magnification of an extended image varies with the distance of the corresponding object from the optical axis (pincushion and barrel)
Distortion tends to
Falsify positions of objects and cause vertical lines to wave as the line of sight intersects different zones of the lens
Distortion is mainly a problem for what lenses lenses
High power
Pincushion disorder occurs with __ and __ are more magnified
Plus
Corners are more magnified
Barrel distortion occurs with ___ and
Minus
-the corners receive less magnification than the middle
What are orthoscopic lenses
Barrel distortion of one lens is counteracted by the pincushion distortion of the other lens
-used for optical devices
Chromatic abbe ration affects image focus when
Polychromatic light goes through a lens
Longitudinal (axial chromatic aberration
On axis aberration that occurs when different wavelengths of light are focused at different distances from the lens
The eye has ~+1.50 D
Longitudinal aberration
LCA
Diopters/abbe
Test that takes advantage of LCA of the eye
Bichrome
In bicrome test when red and green charts appear equally distinct it means
Eye is in for us for the yellow portion of the spectrum
Transverse (lateral=) chromatic aberration
Occurs because equivalent focal length of a lens is different for components of light having different refractive indices
When there’s TCA the wearer will see
Color fringes around objects, its more important for spectacles than LCA
Chromatic aberration cannot be eliminated in an optical element made of a single material it requires
Achromatic systems
How do Achromatic systems work
The focal lengths are the same for fraun C and F lines = dispersion will be zero
A prism is considered achromatic if
It produces deviation withou dispersion -placing 2 prisms of different material in direct contact with bases in opposite directions
