W11 Visual optics and Aberrations Flashcards
Monochromatic aberration
aberration produced by a single wavelength of light.
Chromatic aberration:
aberrations produced by multiple wavelengths (polychromatic) light
Why is it important to consider aberrations?
Aberrations are important in design of spectacle and correction of refractive errors
Paraxial Assumption2
Light rays with very small angle of incidence with the refracting surface are called paraxial rays.
Paraxaialk rays are close to the optical axis sin(theater) = theartre in radians
seidel aberrations:
These interrelated deviations are referred as seidel or classic aberrations.
1. Spherical aberrations
2. Coma
3. Oblique (radial or marginal) astigmatism
4. Curvature of field (power error)
5. Distortion
Longitudial spherical aberrations:
- Spherical lens suffer from positive (undercorrected) longitudal spherical aberrations
- Non paraxial rays striking peripheral of the lens focus closer to the lens than those striking the centre.
- Peripheral rays are more rafractacted than central rays.
Coddington shape factor/ bending factor
σ, is related to the amount of
spherical aberration
What surface is best to minimise longitudinal spherical abberations
Planoconvex
LSA & SPECTACLES LENS DESIGN
Peripheral rays of that emerge from a spectacle lens are blocked by the iris, preventing them from reaching to retina and participating in image formation, LSA in spectacles lenses generally does not reduce retinal image quality
Describe Ashperical lens
A plus spherical lens has a single radius of curvature, the radius of curvature of a plus aspheric surface lens increases from the lens’s centre to periphery (the surface becomes flatter)
Aspheric surface improves the cosmetic appearance of a plus lens not only by allowing it to be thinner, but also because the flatter front surface results in less spectacle magnification; this reduces the magnified appearance of the patient’s eyes that may occur with plus lenses
Longitudinal Spherical Abberations & HUMAN EYES
SA would be greater if not the aspheric nature of the cornea- the periphery of the cornea is
flatter than its centre as the eye accommodates, the amount positive SA decreases
o Clinical Implications: nighttime vision
o Under dim light, pupil dilates, exposing the retina to non-paraxial rays, which focus in front
of the retina, making the eye myopic
o This is contributing factor to night myopia- that is present only under low illumination
o Clinically, prescribing slightly more minus power (or less plus) for those patients who do
considerable nighttime driving!
What is a COMA? How does it occur?
Coma results when light rays are oblique with respect to the optical axis. (Remember for spherical aberration light rays are parallel to the optical axis)
o Because of coma, an off-axis point source results in image with a comet-like shape
o When the tip of the comet is pointed towards the optical axis, the coma is said to be positive and when it is pointed away, the coma is negative
- it can occur with on-axis objects when the
optical components are non-centred and tilted with respect to each other
If an aperture is placed close to a lens and the diameter of the aperture is decreased, coma and
spherical aberration decreases.
Oblique astigmatism
When a point-source is off axis, the horizontal diverging rays strike the surface of the lens at a different angle than vertically diverging rays, even when passing through centre
wrap arouhnd tilted sunglasses tilted to the horizontal plane of the face = face foam (induces cylinder whose axis is 90 degrees and sign +/-)
- It is the reason that undercorrected myopic patients sometimes intentionally tilt their spectacles to improve distance vision
Curvature of field
Not all points on an extended object are the same distance from the spherical converging lens. The off axis components are further away and therefore the image will be closer for these parts and further away for those on the x axis.
- forms curvarture of field from a straight line.
- Minimised by propper selection of lens front surface power.
Distortion and what are the two types produced?
Central and peripheral regions of a spherical lens do not produce the same amount of lateral magnification
For an extended source, this causes distortion: barrel and pincushion distortion.
Barrel distortion: found with minus lenses as minification of periphery of the a minus lens is
greater than in its centre
Pincushion distortion results from greater magnification in the periphery of a plus lens compared to its centre.
