Ophthalmic Instrumentation & Optical Perfomance Restraints Flashcards
Explain the Scheimpflug principle
purposely tilting the image-receiving plane/lens plane to both intersect at the same point, along the object plane, we can capture a sharp, in-fcous image of the object without any blur
How does the Oculus Pentacam work?
rotating scheimpflug camera with thin, blue slit-beam (illumination) simultaneously rotate through 360 degrees
specially designed to image curved cornea with 25-50 corneal slit-section images which generate 3D images of the cornea with highly specialised software
Describe key features of the oculus Pentacam
Quick and simple to use (2sec scan)
non-contact (hygienic)
Non-invasive (pain-free)
measures corneal topography/k readings for ant/post surfaces, CCT, corneal pachymetry map across whole cornea
measures ant/post corneal p value (asphercity)
measures ACD
Describe the key features of OPTOMAP - Scanning Laser Opthalmoscope
based on hemi-ellipsoidal mirror
uses dual-wavelength source (red633nm low f scans down to choroid for comparatively deeper retinal tissue penetration, green532nm high f scans down to RPE only)
gives high-res digital fundus images, ultra-wide FoV (200degs)
Non-contact/invasive and easy, quick
Mydriasis not always required
How does a hemi-ellipsoidal mirror work?
ellipsoid has 2 principal Rc and thus 2 principal focal points
with a light source at 1st focal point, all emitted rays striking the mirror are reflected to pass through conjugate focal point (F2)
eye’s 1st nodal point N positioned to coincide with F2 so light reflected enters eye to facilitate retinal imaging
Explain the principle of confocal microscopy
confocal aperture allows ‘improved image contrast’ as dfferent layers can be imaged independently from any surrounding layers
OPTOMAP sophisticated software collects info from different layers to piece together producing colour-composite images
Explain 5 theories regarding presbyopia
- increased stiffness in lens’ capsule/main body
- age-related loss of zonular fibre function (thinning, less volume)
- ciliary ring diameter decreases with age reducing the gap between lens equator/ciliary body so ciliary body can’t influence curvature of lens surfaces efficiently
- Slow progressive movement of zonular fibre attachments from original position towards centre of either lens face
- lens grows thicker reducing mechanical efficiency as the zonules no longer act perpendicular to lens OA
Define Rayleigh’s resolution limit
2 neighbouring points become just resolvable when central maximum of airy disc image of 1 point directly overlaps with the 1st minimum of the neighbouring points airy disc image
How does pupil size affect diffractive blurring assuming the absence of monochromatic aberrations?
Larger pupils cause comparatively less diffractive blurring
Explain how dispersion causes longitudinal chromatic aberrations and how the eye counters this
dispersion tells us as a given medium n increases the wavelength decreases so retinal image can’t be precisely focussed for all wavelengths at the same time
macular xanthophylls absorb blue light to partly suppress the large myopic blur in the blue range
blue cones virtually absent on foveala central is so visual system relatively insensitive to blue light in photopic conditions
Which factors limit the impact of longitudinal spherical aberrations on final retinal image quality?
both cornea/lens have aspherical surfaces and lens has a gradiented n structure
Describe how monochromatic spherical aberrations form
purely spherical convex surfaces induce SA forming bright, blurry central spot surrounded by dark/light concentric rings
aspherical surfaces reduce SA forming single sharper point image
Explain coma aberrations
off-axis aberration due to small tilts/decentration usually between cornea/lens surfaces
typically larger/variable in keratoconic eyes reducing contrast sensitivity/ visual performance
How are comas/SA linked?
typically occur together in normal eye
~3mm pupil both are usually small causing little retinal image degradation
both directly proportional to pupil diameter (larger pupil=larger aberrations)
How do aberrations change with accommodation and what does the eye to balance this?
SA/coma reduce in size due to pupillary meiosis but this increases diffractive blurring
eye balances effects of diffraction against effects of monochromatic aberrations to maintain optimal retinal image quality
Explain intraocular scattering its 4 primary sources
scattering occurs due to microscopic spatial variations in n of a given medium causing unwanted absorption (forward/back scattering)
corneal stroma tissue highly transparent scattering <10% normal incident light but epith/endo and connective tissue membranes cause total 20-30% of eye’s total scatter
iris/scleral tissue pigmentation causes 5% of total scatter
lens causes 35-40% and retina accounts for 25% total scatter
Explain the retinal-neural factors impacting optimal visual performance
SCE Type 1: reduced intraocular scattering keeping foveal image contrast as high as possible
physical spacing size/sampling freq between foveal ph/receptors imposes principle limitation on eyes resolving power
