Optical Components of the Eye Flashcards
Describe 3 general constraints on eye design
Limited by available radiation
Availability of detection mechanisms (photopigments)
Animal dimensions (eye size/position, brain)
Could we use IR, UV or Radiowave radiation instead of visible light?
IR: Thermal body energy would imapir our VA
UV: damages retinal ph-receptors, most absorbed by cornea (too much causes photokeratitis)
Radio: pupil diameter would need to be 400+m for desired resolution
Describe the difference between an eyespot and eyepit design
Eyespot: single, small, flat spot of photosensitive pigments provides very crude light detection/direction sensitivity
Eyepit: light-sensitive cells arranged in depression for better directional sense but poor resolving power
Describe a pinhole eye and it’s pros/cons
inverted image requiring extra neural processing (e.g. Nautilus Mollusca)
simple designs form single image with much better direction discrimination
Diffraction limits resolving power, limited FoV, small, fixed size results in faint image (poor scotopic vision)
Describe the vesicular eye
epithelial surface (cuticle) grows over pinhole protecting against FBs, also has lens of irregular shape/n and irregular fluid-filled chambers
Describe the lens eye and its pros/cons
cornea (evolved from cuticle), small, regular lens, humours, multiple photosensitive cells arranged into cup, adaptable pupil (iris)
Light hits single point on retina only for great directional sensitivity, bigger/adaptable aperture (pupil) adjusts focussing power for sharper/brighter image
very complex image at photoreceptors requires greater neural processing/time to extract relevant info
Describe apposition compound eyes and the pros/cons
common in diurnals, ommatidia provides multi-feed for motion detection, upright image forms requiring less neural processing/scanning
Con: each cone lens feeds 1 rhabdom limiting eye sensitivity so more suited for diurnal
Describe the structure of an ommatidium
cornea, gradiented cone-shaped lens, Rhabdom (tight-packed transparent rods enhances photon absorption) diverts light outwards to retinular cells (rhodospin photopigments absorb light ~ transmit signals along nerve fibres to CNS
Explain the benefit of a gradiented-index cone lens
relative increase in angle of acceptance of light
Explain superposition eye design
multiple cone-lens array collects/redirects light towards a given rhabdom (array) improving sensitivity
2 piece gradiented cone-lens length is twice as big as apposition lens allowing light to be re-collimated towards single rhabdom
Compare superposition/apposition eye differences
Superposition show larger angle of acceptance (multiple longer cone-lenses provide enhanced sensitivity ideal for nocturnal)
Also have better motion detection
Describe the pre-corneal tear film structure/function
lipid (meibomian), aq (lacrimal), mucus (conjunctival goblet cells) ~ 8um
smooth refracting surface assists cornea optics, lubricates lids/cornea lowering friction, anti-microbial, removes debris, regulates external hydration/O2 supply to cornea
Explain the optical role of the tear film
ignore presence in calculations
varying thickness can induce aberrations (unwanted distortions) e.g. crying blur, cold winds, staring
Describe the corneal stroma ultrastructure
individual parallel collagen fibrils arranged in quasi-regular lamellae (200-250, 12mm long) perpendicular to each other (vital for transparency)/extracellular matrix
Explain Corneal Transparency
stroma acts as a complex 3D grating with ~50nm inter-fibril-spacing: forward incident difracted waves cause constructive interference, obliquely diffracted waves cause destructive
Why are scleral tissues comparatively opaque to the stroma despite both being formed of collagen fibrils?
outer sclera: variable fibril diameter (70-300nm)
middle/inner: irregular inter-fibril spacing
overall scleral tissue can’t act as 3D diffraction grating so signficant light scattered (milky white appearance)
Describe the aspheric corneal contour using the conicoid approximation
aspheric design reduces total spherical aberration for higher quality retinal image
anterior surface approximated to ‘elliptical conicoid’ describes central 8mm
harder to distinguish p values for central 3mm
How does ACD change with age & accommodation? Why is this a crucial pre-op measure?
ACD decreases so required accommodation increases
big influence on IOL power selection for implants/phacoemulsification
Describe pupil function and its adaptation
reactive, controls light, diameter influenced by age/refractive error
size affects aberrations/diffraction/depth of field
diameter changes alone can’t maintain retinal sensitivity to adapt for change from photopic to scotopic so specialised neural/retinal adaptations compensate for this
What is the significance of the aspheric curvature of the lens’ surfaces?
degree id peripheral flattening reduced total spherical aberrations for high-qualiy retinal image
Describe the crystalline lens structure
fibrous, light-scattering bundle of fibres with varying n boundaries, transparency reduces with age due to brunescence (less blue light transmitted)
continous gradiented n (due to packed lens fibres in cortex)
Describe the Vitreous Body
stagnant humour with hyaloid remnants (mainly water, becomes less viscous with age so risk of macula hole) perfectly transparent in youth
Explain how the eye sub-structures absorb EM radiation
cornea absorbs <320nm
lens absorbs blue light better with age due to lenticular brunescence protecting foveal cone pigments
liquid humours absorb IR at specific wavelength absorption bands
macula pigments (LZ) absorb high energy blue light protecting RPE/foveal cones
Describe 3 uses of retinal reflectance
most reflected light in red to near-IR region allows: retinoscopy reflex, non-mydriatic fundus photos, autorefractors
