Flashcards in 2.0 Visual System Deck (117)
What is the wavelength of visible light?
390nm → 700nm
(violet) → (far red)
What are the 4 variables that the visual system can analyse?
Illuminance vs Luminance
Illuminance = Quantification of light from light source (lux)
Luminance = Quantification of light reflected from objects (cd/m²)
A property of a material that dictates the proportion of reflected light from an illuminating source
Contrast is defined as the ratio of incremental light intensity to the mean background intensity (∆I/I)
What is pointspread function?
Pointspread function = blurred image of a point source on the retina
Diameter of pointspread function = wavelength of light/diameter of aperture
4 factors which may limit the optical quality/visual resolution:
2) Chromatic aberration
3) Spherical aberration
Spreading of waves as they pass through an apperture.
Diffraction causes pointspread function
Define Spherical aberration
In a spherical surface, rays towards the edge are refracted more
Define Chromatic aberration
Different colours are refracted by different amounts (thus focused at different depths)
Human eye = well focused for green. Poor focus for blue
Small particles in optical media scatter light in all directions → ↓ contrast of image
Object at infinity is sharply focused
Inability to sharply focus at an object at distance infinity
Short sight (inability to focus on distant objects)
Light is focused at a point before the retina → ↑ pointspread function
Long sighted. Inability to focus on nearby objects
A progressive ↓ in the ability of the eye to focus on near objects
Caused by ↓ accomodation because of ↓ lens elasticity
Define Grain (receptor spacing)
To see two points as separate points, receptor spacing needs to be at least 1/2 the width of the pointspread function
(this is usually achieved in the fovea)
What is the receptor spacing in fovea?
0.5 arc mins apart
What is cataracts?
Clouding of the lens
What is the power of a lens?
The strength of the lens
Expressed in dioptres
How are the lens and cornea supplied with metabolites?
They are avascular. Supply is via the aqueous humour
(secreted by ciliary body and drains in the canal of schlemm + trabecular meshwork)
What is glaucoma?
A reduction in the outflow of aqueous humour → ↑ intraocular pressure
What is accommodation?
Lens changes its focal length to focus on objects at different distances
Due to combination of:
1) Radial elastic ligaments (suspensory ligaments)
2) Circular ciliary muscle
Near reflex accomodation requires:
1) Constriction of pupils
2) Convergence of the two eyes (to fixate on new target)
What are the two muscles that control pupillary size?
What is their innervation?
1) Sphincter pupillae (PSNS)
2) Dilator pupillae (SNS)
What is the control pathway from the retinal to the pupillary sphincter (light reflex)?
Retina → Pretectum (midbrain) → Bilateral Edinger-Westphal nucleus → CN III → Ciliary ganglion → Pupillary sphincter
What is Argyll-Roberston pupil?
Characteristic of neurosyphilis
Pupil does not react to light but does react to accomodation
What are Muller cells?
Retinal glial cells that act as optical wave guides.
How wide is the fovea?
Features of foveola:
Middle (260um/1 degree) of fovea
Centre of foveola has a cone spacing of 0.5min arc