Opto mid sem

• Seidel aberrations


  Suggest that the second term of sin expansion should be considered as well as the first.
• Seidel aberrations


  • Spherical aberration
  • Coma
  • Oblique astigmatism
  • Field curvature
  • Distortion
  • Chromatic aberrations (axial and lateral)
• Spherical aberration


  Occurs when all incoming light waves focus at different points along the optic axis after passing through a spherical surface
• Coma


  Imaging wavefront is spherical but tilted with respect to the optical axis. These rays may originate from objects that do not fall along the optical axis
• Oblique astigmatism


  Even if the refracting surface is spherical, oblique astigmatism with two focal lines can still occur if the incident light is oblique with respect to the optical axis. Different from astigmatism due to toric surface (two radii of curvature)
• Field curvature


  Aberration of extended objects - object points form perfect point images, but on a curved paraboloid surface
• Distortion


  Aberration of extended objects - the magnification of the image varies over the image plane
• Chromatic aberration occurs when different wavelengths of light after passing through a lens or optical surface refract differently and focus at different points
• Short-wavelength blue light with more energy is refracted more than long-wavelength red light
• Wavefront error


  How the shape of the wavefront is deviated from the reference spherical wavefront at the exit pupil
• Zernicke polynomials


  Represent the total wavefront error in an optical system, and are illustrated as wavefront aberration map
• Diffraction


  The bending of light rays around the edges of the aperture, including the eye
• Diffraction effects


  • Constructive interference (waves are in phase)
  • Destructive interference (waves are out of phase)
• Point Spread Function (PSF)


  Quantifying diffraction
• PSF is the light intensity in an image following diffraction through a circular aperture
• Wavefront from the centre of the aperture interferes constructively and destructively with the wavefront from the edge of the aperture and forms a series of rings with varying intensities: PSF
• Airy disk size


  • Inversely proportional to pupil size
  • Directly proportional to wavelength - bigger for red
• Airy disc


  Diameter of the first diffraction minimum
• First dark band


  First diffraction minimum
• Strehl ratio


  • An image quality metric that defines the effect of optical aberrations in a PSF
  • It includes the effect of both aberrations and diffraction on the image quality
  • The peak of PSF reduces in the presence of aberrations
  • Strehl ratio is typically around 0.2 - 0.4 in the human eye
• Rayleigh criterion


  The maximum resolution of an optical system occurs when the maximum of one PSF coincides with the first minimum of another (or separated half the Airy disc diameter)
• MTF


  Function to specify the image forming capability of an optical system (or how well the contrast details of an object can be transferred to the image)
• Diffraction limited MTF


  Best possible image we can form, affected only by diffraction but not aberrations
• Fourier analysis


  A process of decomposing every pixel of an object into its corresponding series of sin waves of different spatial frequencies
• Stiles-Crawford effect of the first kind

  Light entering the eye at different pupil heights is perceived to be of different luminous intensities
• Stiles-Crawford effect of the second kind

  • The colour of a monochromatic light (one wavelength) entering the eye near the edge of the pupil is perceived to be of different colour compared to the same wavelength light passing through the pupil centre, regardless of the overall intensities of the two lights
  • Could be due to:
  • Light entering the eye from the edges stimulate different proportions of the three coloured cone photoreceptors
  • Different distribution of the angles of three cones at the retina
• Nyquist sampling

  • Nyquist limit: For two image points on a sensor to be resolved as separate, the two stimulated pixels must be separated by one unstimulated pixel
  • Nyquist frequency: The maximum spatial frequency in any image that can be resolved is one cycle per two pixels (two white bars separated by a black bar)
• Lateral inhibition


  • It occurs when an excited neuron suppresses the response of its neighbouring neurons to optimize its response
  • At the photoreceptor level or the outer retina: Stimulation of several photoreceptors is transmitted as a response of a single photoreceptor (or a few photoreceptors) in order to maximize perception of the stimulus
  • At the second synaptic level (the inner retina): Secondary neurons (horizontal cells and multiple synapses of bipolar cells) actively suppress the response of other receptors, adjacent to the main receptor of maximum response
  • It increases the contrast and sharpness in visual response and also colour perception
• Resolution


  • The closeness of two adjacent points or pixels in an object that still be seen as separate in the image (i.e., the finest detail that can be seen or distinguished in an image)
  • Resolution or sharpness of an optical system is compromised due to optical aberrations
  • Measured using visual acuity charts
  • More spatial frequencies = higher resolution, lower contrast
• Contrast


  • The closeness of brightness or luminance of two adjacent points or pixels in an object that still be seen as separate in the image
  • Measured using contrast sensitivity charts
• Pupil Pathology


  • Aniridia: absence of an iris
  • Coloboma: iris not fully formed
• Transmission of light by ocular media

  • UV radiation: does not reach retina. Is mostly absorbed by cornea, and leftover absorbed by lens
  • Visible light: shorter wavelengths are partly absorbed particularly by lens. Absorption increases with age
  • IR: up to 1400nm reaches the retina with little reduction of energy
  • Absorption and scattering are very wavelength dependent
  • About 20% of the light incident on the eye is lost in reflection and absorption
  • About 35 - 40% is lost in light scatter become stray light
  • Only about 40% of the light falling on the eye contributes to the retinal image
• Corneal pathology


  • Pterygium
  • Keratoconus
  • Trachoma
  • Anoxia
• Pterygium

  • A vascularised, fleshy growth that invades the cornea from the nasal canthus
  • Causes the anterior surface of cornea to distort
  • May result in irregular astigmatism without clearly defined focal lines perpendicular to one another
  • May be no satisfactory optical correction of the refractive error so surgery may be necessary
• Keratoconus

  • Bulging (ectasia) of the cornea caused by thinning of central stroma
  • Central rad of curve reduces so the power increases thus the eye becomes myopic
  • Advanced keratoconus usually exhibits an inferior cone and high astigmatism
  • May be irregular with poorly defined focal lines that may not be perpendicular
  • In early stages, vision can be corrected with spectacles and rigid gas permeable CL but may eventually require corneal transplant
• Trachoma

  • An infection of the conjunctiva and lids caused by the bacterium chlamydia trachomatis
  • In the early stage (follicular stage) it manifests as follicles on the inner lid surfaces
  • These form scar tissue which contracts and causes the lids to turn inward
  • The eyelashes then scrape the corneal epithelium (trichiasis) which causes permanent scarring eventually
  • Treatable by antibiotic (requires on-going facial and environmental hygiene as well)
• Anoxia

  Severe response to CL wear, including corneal haze due to edema, a corneal ulcer, and a sub-conjunctival haemorrhage
• Corneal transparency

  • The regular arrangement of the collagen fibrils in the corneal stroma
  • The integrity of the corneal endothelium
• Corneal toricity

  • Where the radius of curvature is different across the 2 meridians of the cornea
  • Non rotational symmetry
  • Toric corneas are also known as astigmatic corneas
  • Very common
  • Corrected used sphero-cylindrical lenses
  • Can also often be corrected with a rigid contact lens with spherical surfaces
• With the rule astigmatism

  The cornea is usually steeper (shorter rad of curve and higher power) vertically than horizontally. The upper and lower eyelids pull the cornea back a little more in the vertical direction