Vision definitions Flashcards
(38 cards)
Illuminance
Light from a light source
Measured in lux
Luminance
Light reflected from objects
Measure in cd/m^2
Pointspread function
Angular diameter, δ = λ/D radians
Because of diffraction (causes waves to spread when they pass through a small aperture), the image of a point source, even through a perfect lens will be a blurred circle- the pointspread function.
Emmetropic
Object at infinity is sharply focussed
Ametropic
Object at affinity is not sharply focused
Myopia
Short sight, ~20% pop
Corrected with converging lens.
Presisposes to retinal detachment & glaucoma
Hypermetropia
Long sight, ~30% pop
Corrected with diverging lens
Presbyopia
Reduction in accommodating power of the lens.
Occurs as lens elasticity decreases with age.
Power of a lens
1 / focal length (m)
Measured in dioptres
To a first approximation the power of a compound lens system is given by the sum of the powers of the individual components.
Aqueous humour
Supplies the avascular cornea & lens with metabolites.
Secreted by the epithelium of the cilliary body, drained through the trabecular meshwors and Canal of Schlemm. Reduction in rate of outflow = primary cause of glaucoma
Near reflex or ‘Triple response’
- Lens accommodation
- Constriction of the pupil to improve depth of focus
- Convergence of the 2 eyes to fixate on new target
Argyll-Robertson pupil
Characteristic of neurosyphillis
Pupil does not react to light, but does react to accommodation.
Foveola
Central ~1degree of the fovea with the highest acuity.
Avascular, completely rod-free.
Parafoveal region
Area of most sensitive vision under mesopic & scotopic conditions.
Rod density peaks ~20degrees either side of the fovea.
Papilloedema
Optic disk can appear swollen with raised intracranial pressure
Night blindness
Can be caused by
- deficiency of vitamin A (of which retinal is a derivative)
- hereditary retinitis pigmentosa
Negative internal transmitter
e.g Phototransduction G protein-coupled cascade that culminated in the destruction of cGMP.
Hereditary retinitis pigmentosa
~1/3000, Progressive hereditary retinal degradation
Gradual onset of night blindness in adolescence, loss of all peripheral vision in adulthood, in extreme cases total blindness.
5-10% cases are caused by mutations in rhodopsin gene.
Dark current
Continuously circulating in dark.
Na+ and Ca++ ions flowing into outer segment of photoreceptor, depolarizing to ~- 30mv. Circuit completed by outward current in the inner segment, mainly through K+ channels.
Photoreceptor light adaption
= a reduction in sensitivity as the steady light intensity increases.
Believed to be alrgely mediated by calcium ions.
Principle of univariance
Although each cone absorbs maximally at a particular wavelength, alone it provides no colour information.
e.g although a green cone might absorb 10x fewer red photons than green photons, 100 incident red photons would be indistinguishable from 10 green photons
Young-Helmholz trichromacy theory
Colour is defined by the ration of excitation in the three cone classes.
Colour vision is achieved by comparing the outputs of different cones. A trichromatic system can distinguish over 2m colours.
Protanopia
Type of dichromatic vision (red-dichromacy)
No red cones/ no red pigment.
Gene is on X chromosome- sex linked
Deuteranopia
Type of dichromatic vision (green-dichromacy)
No green cones/ no green pigment
Gene is on X chromosome- sex linked
