3c. Image Quality on the Retina Flashcards
Pupil Constriction
- Muscle
- Innervation
Circumferentially arranged fibres of the iris sphincter muscle under parasympathetic innervation
Pupil Constriction
- Pathway
- Cranial nerve II (optic)
- Pretectum of midbrain
- Bilateral projection to Edinger-Westphal nucleus
- Preganglionic parasympathetic neurones to ciliary ganglion
- Postganglionic parasympathetic neurones to iris sphincter muscles
Pupil Constriction
- Inhibition
Atropine
Pupil Dilation
- Muscle
- Innervation
Radially arranged fibres of the iris dilator muscle under sympathetic innervation
Focussing Light
- 3 ways
- Convex surfaces
- Varying refractive indices
- Accommodation
Accommodation
- Process
- Parasympathetic stimulation of the circular ciliary muscle via cranial nerve III (oculomotor)
- Contraction of ciliary body
- Relaxation of zonules of Zinn
- Lens becomes more spherical with a higher refractive index
- Focusses light from nearby objects
Accommodation
- Different Types
Cats:
Cats move the lens backwards and forwards without changing its shape to accommodate
Chickens:
Change the shape of the cornea as well as the lens to accommodate
Mice:
Don’t accommodate at all
Presbyopia
Decreasing lens elasticity with increasing age, leading to a reduction in accommodating power
Nyquist Limit
The requirement for photoreceptor spacing to be at least twice as fine as the width of the point-spread function to differentiate 2 separate objects as distinct.
3 Factors Limiting Spatial Resolution
- Diffraction
- Lens abberations
- Refractive defects
Diffraction
Waves form as light passes through a small aperture, forming a blurred circle known as the point-spread function on the retina
Reducing Diffraction
- 2 Methods
- Make aperture larger
- Use shorter wavelength light
Point Spread Function
3D diffraction pattern of light emitted from an indefinitely small point source that is represented on the retina
Caused by:
- Diffraction
- Lens Aberrations
Lens Aberrations
- Description
- 3 Types
Imperfections in the eye that degrade the image
3 types:
- Spherical aberration
- Chromatic aberration
- Glare
Spherical Aberration
- Description
Spherical lens brings peripheral light rays into focus closer to the lens than central light rays, meaning that peripheral light rays will have focussed before reaching the retinal, spreading out again and leaving a blurry fringe around the image in focus
Spherical Aberration
- Prevention
- Flattening the outer margins of the cornea
- Changing the refractive index of the lens centre so it is stronger than in the periphery
Chromatic Aberration
- Description
The refractive index of ocular material varies as a function of wavelength, so different wavelengths of light focus at different focal lengths.
Blue light is refracted more strongly than red and yellow wavelengths so focuses much closer to the lens
The human eye is well adapted to focus green light:
- Blue light will have focussed before the retina and spread out again
- Red light will not have focussed as it passes through the retina
Glare
- Description
Scattered particles in the ocular material causes light scattering, giving rise to glare that reduces the contrast of the retinal image
Diffraction vs Aberration
Small pupil diameter:
- High diffraction
- Low aberration
Large pupil diameter:
- Low diffraction
- High aberration
Ideal Pupil Size
- Human
2mm
- Aberration contributes a small amount to point-spread function
- Central region is similar to the diffraction limit
Refractive Errors
- 2 Examples
- Myopia
- Hypermetropia
Refractive Errors
- Description
When light is brought into sharp focus on the retina = emmetropic
Refractive errors are where light is not brought into sharp focus on the retina = ametropic
Myopia
- Description
Short-sightedness
- Optics are too strong
- Eyeball is too long (more common)
Rays come into focus before the retina so begin to diverge again, forming a blurry image for distant objects
Myopia
- Prevalence
20% of the human population
Myopia
- Correction
Diverging lens to spread rays so that they appear to come from the eye’s appropriately close far point and form a perfect image on the retina
Hypermetropia
- Description
Long-sightedness
- Optics are not strong enough
Light focusses after the retina, so even when the object is at infinite distance and light rays are parallel objects seem blurry
Hypermetropia
- Prevalence
30% of the human population
Hypermetropia
- Correction
Converging lens to add extra optic power and allow light rays to come into perfect focus on the retina
Stiles-Crawford Effect
The ellipsoids of cones act as waveguides, reducing cone absorption of oblique light rays to yield a sharper retinal image.
Rods absorb oblique light rays.
