Lecture 11 Monochromatic Aberrations in the human eye part 1

How does diffraction affect the retinal image?

For the eye, only a section passes through the pupil; the iris is an obstruction that diffracts the light. This diffraction blurs the image. For a system limited obly by diffraction, the image radius for a point object = 1.22λ f/d meters.

Diffraction affects image quality for

small pupils

Aberrations affect image quality for

large pupils

Pupil size controls impact of

ocular aberrations and diffraction on retinal image quality (emmetrope)

What pupil size provide the best retinal image quality?

~3mm

Due to optical aberrations, a plane wavefront becomes aberrated spherical wavefront after passing through

a spherical lens optics

Optical aberrations can be measured as

-an 'aberrated wavefront' on the pupil plane
or
-a 'point spread image' at the image plane

Significant optical aberrations in the human eye are

-Defocus
-Astigmatism
-Spherical Aberration
-Coma
-Distortion
-Field Curvature

Aberrations degrade the

quality of image

Aberrations are important in

-the design of spectacles, contact lens, IOLs and Refractive surgeries
-Correction of refractive errors

Positive Longitudinal Spherical Aberrations (LSA)

light rays striking the periphery of the lens (non-paraxial rays) are focused closer to the lens than those striking near its center (paraxial rays)

Positive LSA is present in both

plus and minus lenses

Negative LSA

light rays striking the periphery of the lens (non-paraxial rays) are focused farther away to the lens than those striking near its center (paraxial rays)

What is the amount of LSA dependent on?

the radii of curvatures of the front and back surfaces of a lens

What lenses minimize LSA?

lenses with approximately plano-convex shapes that are oriented so that the front surface is more convex

The unaccommodated eye manifests positive spherical aberration, which tends to increase with

age

As the eye accommodates, the amount of positive spherical aberration

decreases

Night myopia

under dim lighting conditions the pupil dilates, exposing the retina to non-paraxial (or marginal) light rays. These light rays may be focused in front of the retina, making the eye myopic
-The eye's spherical aberration may have clinical implications for nighttime vision
-Clinically, prescribe lenses with slightly more minus power for those patients who do considerable nighttime driving.

Both spherical aberration and coma occur because

the refractive power of a spherical surface is not uniform

Off-axis Coma

-results when the light rays are oblique with respect to the optical axis (Spherical aberration result when light rays are parallel to the optical axis)

-Because of coma, an off-axis point source results in an image with a comet-like shape

How do you tell if the coma is positive or negative?

When the tip of the comet is pointed toward the optical axis, the coma is said to be positive, and when it is pointed away, the coma is negative.

Light distribution in Coma image

The light distribution is not uniform across the comatic patch, but is highest at the pointed end.

On-axis decentered coma (induced)

occurs when the optical components are non-centered and tilted with respect to each other
-A decentered eye, where coma may be major foveal aberration.

If the pupil is decentered by 0.25 mm, the magnitude of induced coma will be

same as spherical aberration

If the pupil is decentered by 0.50 mm, the magnitude of induced coma will be

twice as spherical aberration

How to reduce coma?

If an aperture is placed close to a lens and the diameter of the aperture is decreased, the amounts of coma decrease.

Coma is dependent on the

lens shape factor
-It can be minimized by making the curvatures of the lens surfaces similar to those that also reduce spherical aberration

Oblique astigmatism

occurs when light rays emerging from an off-axis object pass through the center of a lens
-It is of clinical significance in the design of spectacle lenses
-Correct selection of the front surface power can minimize this aberration

Curvature of Field

Not all points on the extended object are the same distance from the spherical converging lens. An image plane that is not flat but curved.
-Off-axis rays that pass through the center of a lens can cause curvature to field
-This aberration is clinically important in the design of spectacle lenses.
-It can be minimized by the proper selection of the lens front surface power.

Distortion

the central and peripheral regions of a spherical lens do not produce the same amount of lateral magnification
-a consideration in the design of spectacle lenses

barrel distortion

distortion found with minus lenses
-occurs because minification in the periphery of a minus lens is greater than in its center

pincushion distortion

distortion found with plus lenses
-results from greater magnification in the periphery of a plus lens compared to its center