Principle Optical Components Flashcards
Summarise the functions of (pre-corneal) tear film
smooth refracting surface (assists cornea optics)
lubrication between lids/cornea (less friction)
removes debris
Regulates corneal hydration (O2 supply)
Anti-microbial function
Describe the structure of the pre-corneal tear film and its influence on optical quality
Lipid (meibomian)
Aq (lacrimal)
Mucus (conjunctival goblet cells)
8um ~ dramatic thickness changes induce unwanted distortions (aberrations, blur from crying/cold wind exposure/prolonged staring)
Describe the functions and structure of the cornea
curved refractive surface (2/3 eyes total power)
transparent stroma (90% CCT) transmits >90% incident light
Explain the corneal stroma ultrastructure
parallel collagen (n=1.47) fibrils arranged into 200-250 tightly packed lamellae (12mm long/2um thick/perpendicular to each other) in a quasi-regular fashion ~ crucial for transparency
extracellular matrix (n=1.34)
Explain corneal transparency
Equal sized/spaced Fibrils (30nm diameter) & Inter-fibril spacing (50nm) gaps provide highly-regularised structure to maintain corneal transparency
Explain corneal transparency using Young-Slit theory
stroma~ complex 3D diffraction grating (lattice array of slits)
forward-incident diffracted waves produce 2’ in-phase constructive wavelets (360’/2pi rads)
non-forward-incident waves produce 2’ out-of-phase destructive wavelets
Explain scleral (fibrils) collagen arrangement
Outer: variable fibril diameter (70-300nm)
Middle: irregular inter-fibril spacing (quasi-random)
Scleral tissues NOT regular 3D diff.grating so suffers from light scattering ~ looks milky white
Explain the aspherical corneal contour
reduced total spherical aberration produces higher quality retinal image
anterior surface approximated to elliptical conicoid (central 8mm/max pupil diameter)
Explain the conicoid approximation of the cornmeal contour
assume cornea is centred, rotationally symmetrical about optical axis (z
p (peripheral corneal steepness)
Perfect Sphere: p=1
PROLATE (flattening ellipsoid) 0<p<1 typical cornea!
OBLATE (steeping ellipsoid) p>1 corneal disease!
Explain AC depth and why its important to measure pre-op
AQ n = 1.336
ACD ~ 3-4mm in normal eyes, decreases with age/accommodation
Measure ACD via OCT pre-operative: Phacoemulsification/Phakic/IOL implantation
ACD influences IOL power selection
Explain how pupil structure relates to function
aperture controls light entering (reactive) ~ age/refractive error changes diameter
size affects aberrations/diffraction/field depth (crucial measurement before RGP CL fitting/LASIK surgery)
Explain how pupil size affects retinal sensitivity
assume diameter 2-8mm (in different light) ~ area can only chnage by max. x16
specialised retinal/neural adaptations compensate to maintain sufficient retinal sensitivity (light/dark adaptation)
Explain the crystalline lens structure and each surface’s curvature
non-homogenous onion-like
aspherical curvatures ~ ant/post both have peripheral flattening to reduce total spherical aberration producing high-quality retinal images
Describe the real lens structure
fibrous structure scatters light at varying n boundaries (lower n in cortex (1.386), higher n at nucleus (1.406))
new fibres added with age (30y ~ 3.6mm thick, 80y ~ 5mm thick)
How does lens transparency/cataract risk change with age?
lens transparency reduces with age (yellow to brown (brunescence) so less blue light transmitted)
nuclear (age-related), cortical, post. sub-capsular
Explain how high quality retinal images are maintained
continuous gradiented refractive index (with cornea/lens aspheric curvatures) reduced eyes total spherical aberrations
Describe the vitreous body
gelatinous consistency (mainly h2O)
stagnant, hyaloid remnants remain (unreplaced unlike aq humour)
transparent in youth, less viscous with age (potential risk of macular hole)
Why should we appreciate the proportion of EM light radiation reflected?
crucial for imaging/measuring ocular components (slit-lamp, ret, keratometry)
Define transmittance and reflectance
T: fraction of light transmitted forward through each optical surface
R: fraction of light reflected back off each (polished) optical surface
Describe UV transmission through the cornea
stroma x20-40 thicker than epithelium/Bowman’s so had most significant role in filtering UV
Explain how the cornea, lens, humours and macular pigments absorb/transmit
Cornea: absorbs <320nm EM radiation
Lens: absorbs blue light (increased with age due to lenticular brunescence) protecting foveal cone pigments
Humours: liquid H2O absorbs IR at specific wavelengths (absorption bands)
Pigments: L/Z in both plexiform layers absorb high-energy blue light protecting foveal cones/RPE (reflex looks bright yellow)
Explain retinal reflectance
most light reflected in red to near-IR region
allows us to perform ret (red reflex), take fundus pics with non-mydriatic camera, measure autos
What are the benefits of tapetum lucidum?
specialist reflective tissue layer behind photoreceptors
> 1 transparent, iridescent layers utilise thin-film optics for constructive interference capturing max. photons ~ great night vision
70% reaches TL then reflected out (30% absorbed by phreceptors)
Describe the Stiles-Crawford effect: Type 1
apparent brightness landing on foveal cones depend on entry point through pupil
energy through periphery perceived less bright than through centre (retinal effect rather than pupil)
perpendicular beams brightest
Describe the SCE Experiment
variable density filter adjusted to reduce light transmission of axial beam so 2 halves appear equally dim ~ filter density value recorded
peripheral hole position changed (axial hole constant)
brighter beam = darker filter needed
Describe the experimental SCE results for cones and rods
Cones: photopic conditions/axial beam on fovea stimulates cones only
Rods: scotopic conditions/axial beam pointing parafoveally (8 deg) stimulates rods only
Cones show narrower angle of acceptance to cause TIR in photoreceptor outer segments
Is the SCE good or bad?
reduces effect of oblique scattered light at foveal cones (photopic) so improves retinal image contrast/visual performance
