Vision L1 Flashcards
difference between illuminance and luminance?
Light from a light source (e.g. sun) is quantified as illuminance (lux); light reflected from objects as luminance (cd/m2).
Intensity range of human vision.
what is contrast?
measure of stimulus strength is relative intensity or contrast, defined as I/I where I is the mean background and I the increment (or decrement) in intensity.
An _____ image is focused by the ______ and ____ on to the retina, with an aperture controlled by the _____
An inverted image is focused by the cornea and lens on to the retina, with an aperture controlled by the pupil
label the eye
describe how Diffraction limits optical resolution.
what is meant by a ppoint spread function
The process of diffraction causes waves (or light) to spread when they pass through a small aperture.
Because of diffraction, even with a perfect lens, the image of a point source is a blurred circle known as the pointspread function.
The angular diameter of this is d = lamba/D radians, where D is the diameter of the lens (or aperture, if limited by diaphragm or pupil), and lambda the wavelength of light. The smaller the lens (or aperture) therefore, the larger the diffraction limit.
The smaller the lens (or aperture) therefore, the larger the …….
The smaller the lens (or aperture) therefore, the larger the diffraction limit.
describe 3 ways lens aberations degrate the image
- Spherical aberration: for a spherical surface, rays towards the edge are more strongly refracted.
- Chromatic aberration: different colours are focused at different depths. The human eye is well focused for green, but poorly for blue light.
- Glare: small particles in the optical media scatter light in all directions, reducing contrast of the image
when might lens abberations impact image quality - and when mighjt the diffraction limit be the limting factyor
At small pupil diameters the contribution of spherical and chromatic aberration is modest, and so the pointspread function approaches the diffraction limit.
As the pupil dilates, off- axis rays contribute to image formation and aberrations become more significant, broadening the pointspread function despite the reduced effects of diffraction.
lens aberations
Linespread function for small and large pupil diameters.
checek again
emmetropic vs ametropic
If an object at infinity is sharply focused, the eye is called emmetropic.
If this is not the case the eye is referred to as ametropic.
refractive errors
Myopia predisposes to what?
retinal detachment, degeneration and glaucoma
describe how spaical resolution is achieved in the retina?
The detail in the optical image also has to be matched by the “grain” (receptor spacing) of the retina. Theoretically, receptor spacing should be approximately half the width of the pointspread function in order not to sacrifice the detail in the image. This is achieved in the fovea, where adjacent cones are separated by ~0.5 arc min and arranged in a precise mosaic to maximise packing density.
describe the cornea
Cornea: consists of a ~650 micro-m thick layer of transparent collagen fibrils (stroma) enclosed between an epithelium and an endothelium.
describe the lens
Lens: built from long ribbon-like cells, packed with transparent protein (crystallin).
Cells are added from the periphery throughout life; the oldest cells are in the core, which has a greater refractive index to correct for spherical aberration. The lens absorbs strongly in the UV and, increasingly with age, in the blue.
whats clouding of the lens called
Clouding of the lens (cataract) is very common in old age.
blood supply to cornear and lens?
To ensure transparency the cornea and the lens are avascular and are supplied with metabolites by the aqueous humour.
what secretes and drains aqueous humour?
This is secreted by the epithelium of the ciliary body and drained off through the trabecular meshwork and Canal of Schlemm
primary cause of glaucoma?
reduced rate of aqueous humour outflow
define the power of a lens
The power (P) or strength of a lens is expressed in Dioptres (1/focal length measured in meters).
describe accomodation
The lens changes its focal length to focus on objects at different distances - a process known as accommodation.
how does the lens change focal length?
combination of radial elastic ligaments - the suspensory ligaments or zonule, and a circular ciliary muscle.
when the lens is contracted what stuff is in focusa?
close objects
where does most of the power in the eye come from?
and most of the variable power come from?
what is presbyopia
Lens elasticity decreases with age, leading to a reduction in accommodating power known as presbyopia.
what nerves control hte ciliary muscle?
The ciliary muscle is under parasympathetic control via the oculomotor nerve
accommdation is associated with what through which reflex?
simultaneous constriction of the pupil to improve depth of focus and also convergence of the two eyes to fixate on the new target (near reflex)
describe how pupil diameter is controlled?
The iris comprises two antagonistic smooth muscles under control of the autonomic nervous system: the sphincter (parasympathetic) and dilator (sympathetic).
describe the circuitary controlling the pupil diameter
- retina
- pretectum
- bilaterally to preganglionic parasympathetic neurons in the Edinger-Westphal nucleus
- project via the oculomotor nerve
- to the ciliary ganglion innervating the pupillary sphincter muscle.
whats Argyll-Robertson pupil,
In Argyll-Robertson pupil, characteristic of neurosyphilis, the pupil does not react to light but does react to accommodation.
which layer of the retina are photoreceptors in?
furtherst from the surface wierdly
what are these?
Müller cells
retinal glial cells (Müller cells) act as optical waveguides to aid transmission of light.
cell arrangement in the fovea vs periphery
In the fovea (~1.5 mm, or 5o in diameter) the interneurons are displaced to one side - to reduce scatter
describe the fovea pit
Cone density ______ dramatically in the fovea at the expense of rods.
Cone density increases dramatically in the fovea at the expense of rods.
is the foveola avascular?
yes - to reduce scattering
are there any rods in the fovea?
no
The fovea and surrounding region contain ;… what?…… to reduce the effects of chromatic aberration.
The fovea and surrounding region contain a blue absorbing macular pigment to reduce the effects of chromatic aberration.
Rod density peaks about ……….. of the fovea (parafoveal region: the area of most sensitive vision under mesopic and scotopic conditions).
Rod density peaks about 20o either side of the fovea (parafoveal region: the area of most sensitive vision under mesopic and scotopic conditions).
in the parafoveal region, the rods are the same space as the cones in the fovea - why dont thay have the same visual acuity?
Although rods here are spaced as closely as the cones in the fovea, rod signals are summed or pooled together so that spatial acuity is much reduced
what causes the blind spot
There is a blind spot subtending ~5o at the optic disk where the optic nerve exits the retina. Because the CSF in the optic nerve is in continuity with that in the brain, the optic disk can appear swollen with raised intracranial pressure (papilloedema)
fat
mamba
