Vision L1

Question 1

difference between illuminance and luminance?

Answer 1

Light from a light source (e.g. sun) is quantified as illuminance (lux); light reflected from objects as luminance (cd/m2).

Question 2

Intensity range of human vision.

Answer 2

Question 3

what is contrast?

Answer 3

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.

Question 4

An _____ image is focused by the ______ and ____ on to the retina, with an aperture controlled by the _____

Answer 4

An inverted image is focused by the cornea and lens on to the retina, with an aperture controlled by the pupil

Question 5

label the eye

Answer 5

Question 6

describe how Diffraction limits optical resolution.

what is meant by a ppoint spread function

Answer 6

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.

Question 7

The smaller the lens (or aperture) therefore, the larger the …….

Answer 7

The smaller the lens (or aperture) therefore, the larger the diffraction limit.

Question 8

describe 3 ways lens aberations degrate the image

Answer 8

1. Spherical aberration: for a spherical surface, rays towards the edge are more strongly refracted.
2. Chromatic aberration: different colours are focused at different depths. The human eye is well focused for green, but poorly for blue light.
3. Glare: small particles in the optical media scatter light in all directions, reducing contrast of the image

Question 9

when might lens abberations impact image quality - and when mighjt the diffraction limit be the limting factyor

Answer 9

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.

Question 10

lens aberations

Answer 10

Question 11

Linespread function for small and large pupil diameters.

Answer 11

checek again

Question 12

emmetropic vs ametropic

Answer 12

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.

Question 13

refractive errors

Answer 13

Question 14

Myopia predisposes to what?

Answer 14

retinal detachment, degeneration and glaucoma

Question 15

describe how spaical resolution is achieved in the retina?

Answer 15

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.

Question 16

describe the cornea

Answer 16

Cornea: consists of a ~650 micro-m thick layer of transparent collagen fibrils (stroma) enclosed between an epithelium and an endothelium.

Question 17

describe the lens

Answer 17

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.

Question 18

whats clouding of the lens called

Answer 18

Clouding of the lens (cataract) is very common in old age.

Question 19

blood supply to cornear and lens?

Answer 19

To ensure transparency the cornea and the lens are avascular and are supplied with metabolites by the aqueous humour.

Question 20

what secretes and drains aqueous humour?

Answer 20

This is secreted by the epithelium of the ciliary body and drained off through the trabecular meshwork and Canal of Schlemm

Question 21

primary cause of glaucoma?

Answer 21

reduced rate of aqueous humour outflow

Question 22

define the power of a lens

Answer 22

The power (P) or strength of a lens is expressed in Dioptres (1/focal length measured in meters).

Question 23

describe accomodation

Answer 23

The lens changes its focal length to focus on objects at different distances - a process known as accommodation.

Question 24

how does the lens change focal length?

Answer 24

combination of radial elastic ligaments - the suspensory ligaments or zonule, and a circular ciliary muscle.

Question 25

when the lens is contracted what stuff is in focusa?

Answer 25

close objects

Question 26

where does most of the power in the eye come from?

and most of the variable power come from?

Answer 26

Question 27

what is presbyopia

Answer 27

Lens elasticity decreases with age, leading to a reduction in accommodating power known as presbyopia.

Question 28

what nerves control hte ciliary muscle?

Answer 28

The ciliary muscle is under parasympathetic control via the oculomotor nerve

Question 29

accommdation is associated with what through which reflex?

Answer 29

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)

Question 30

describe how pupil diameter is controlled?

Answer 30

The iris comprises two antagonistic smooth muscles under control of the autonomic nervous system: the sphincter (parasympathetic) and dilator (sympathetic).

Question 31

Answer 31

Question 32

describe the circuitary controlling the pupil diameter

Answer 32

1. retina
2. pretectum
3. bilaterally to preganglionic parasympathetic neurons in the Edinger-Westphal nucleus
4. project via the oculomotor nerve
5. to the ciliary ganglion innervating the pupillary sphincter muscle.

Question 33

whats Argyll-Robertson pupil,

Answer 33

In Argyll-Robertson pupil, characteristic of neurosyphilis, the pupil does not react to light but does react to accommodation.

Question 34

which layer of the retina are photoreceptors in?

Answer 34

furtherst from the surface wierdly

Question 35

what are these?

Müller cells

Answer 35

retinal glial cells (Müller cells) act as optical waveguides to aid transmission of light.

Question 36

cell arrangement in the fovea vs periphery

Answer 36

In the fovea (~1.5 mm, or 5o in diameter) the interneurons are displaced to one side - to reduce scatter

Question 37

describe the fovea pit

Answer 37

Question 38

Cone density ______ dramatically in the fovea at the expense of rods.

Answer 38

Cone density increases dramatically in the fovea at the expense of rods.

Question 39

is the foveola avascular?

Answer 39

yes - to reduce scattering

Question 40

are there any rods in the fovea?

Answer 40

no

Question 41

The fovea and surrounding region contain ;… what?…… to reduce the effects of chromatic aberration.

Answer 41

The fovea and surrounding region contain a blue absorbing macular pigment to reduce the effects of chromatic aberration.

Question 42

Rod density peaks about ……….. of the fovea (parafoveal region: the area of most sensitive vision under mesopic and scotopic conditions).

Answer 42

Rod density peaks about 20^o either side of the fovea (parafoveal region: the area of most sensitive vision under mesopic and scotopic conditions).

Question 43

in the parafoveal region, the rods are the same space as the cones in the fovea - why dont thay have the same visual acuity?

Answer 43

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

Question 44

what causes the blind spot

Answer 44

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)

Question 45

fat

Answer 45

mamba