C = L âL L + L
Light, Eye, Brain and Spatial Vision
! ! ! !
What’s Out there? Electromagnetic radiation: Periodic changes in electric and magnetic field Rays: ravel in straight lines at a constant very high speed Particle: Discrete packets or ‘quanta’ and light particles known as photons Wave: Light has a set wavelength, measured in nanometers
The electromagnetic spectrum
Light Intensity ! Light intensity = intensity of electromagnetic wave? ! But not all wavelengths are visible – need a scale that takes visibility into account • Units we use to measure this are called candelas per m2 and the quantity is know as luminance. • Luminance is the intensity of the light that we can see as relevant to human vision ! Human vision works in a wide range of different light intensities (luminance) ! Objects reflect different percentages of incident light e.g. white paper ~75%, black paper ~5%
Contrast If we want to see things we need to detect objects against its background. In order to be able to do that, there is got to be some contrast between the objects and its background. ! Relative luminance’s is constant regardless of absolute luminance ! Contrast is defined as: where Lmax & Lmin are the largest & smallest luminance values respectively • Varies between 0 & 1 • When Lmax = Lmin, contrast is zero (i.e. there’s nothing there to see)
L max − L min ) ( C= (L max + L min ) Michelson Contrast (C) formula
1
Light, Eye, Brain and Spatial Vision ! Cornea: The transparent ‘window’ into the eyeball ! Pupil: The dark circular opening at the center of the iris in the eye, where the light enters the eye ! Lens: Enables changing focus using ciliary muscles ! Retina: A light sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve ! Aqueous/vitreous humor: the squishy bits.
The eye
Light first passes through the cornea the outermost part of the eyeball where it beings to be focused. Then it enters the pupil a small opening that leads to the lens (the coloured part around is called the iris and it grows and shrinks to protect the pupil and make sure the right amount of light gets in. Once the light makes it past the pupil it hits the lens a surface where it is further focused through the process of accommodation Finally the light hits the retina a layer of tissue that lines the inner part of the eye. The retina beings the process of turning the light into a image. Retina contains two photoreceptors (the rods and cones). After rods/cones have transduced the light, they turned it into an electrical impulses then pass on the impulse to ganglion cells (via bipolar/amacrine/horizontal cells). Ganglion cells have long axons that exist the eyeball via a bundle called the optic nerve. Where the optic nerve leaves the eye there are no photoreceptors (this is the blind spot). After impulses exist the optic nerve; retinal ganglion cells axons terminate in lateral geniculate nucleus (LGN), which is a structure in the thalamus. The points where the signals cross over is called the optic chiasm. LGN projects to primary visual cortex (V1) in the optic lobe via optic radiations.
Focusing ! Light rays from a single point spread in all directions ! Focusing is recombining rays from various directions to form a single point on the imaging surface ! In the eye, this job falls on the cornea and lens • Cornea is curved – light refracts a constant amount. It can’t change its shape and it does the exact same amount of focusing regardless where the object is. That's a problem because for objects further away it needs a different amount of focusing compared to when its close • Lens refracts light variable amount – so the cornea is doing most of the focusing and lens does all the fine tuning • Accommodation: lens can be stretched to allow focusing of far objects • Cornea has greater refractive power
2
Light, Eye, Brain and Spatial Vision Focusing Errors glasses its probably due to problems with accommodation; if If people need your lens doesn't do it naturally, light needs to first pass through artificial lenses in order to be properly focused. Emmetropia (A) ! Normal refractive condition: appropriate focus ! Light waves coming in are being bend, focused inwards so that they all are imaged on exactly the same point on the retina, so that forms a very sharp image Myopia (B) ! Near or short sightedness ! Focal length is too short ! Light is focused in front of the retina ! Need for concave corrective lens Hyperopia/ Hypermetropia (C) ! Far or long sightedness ! Focal length is too long ! Light focused behind retina ! Need for convex corrective lenses Presbyopia ! Occurs with old age ! Inability to change accommodation ! There is stiffing of the ciliary muscles and the lens so that they cant as easily change shape. Therefore your change of focus is impaired Astigmatism ! Different focal lengths for different orientations e.g. ok for vertical line but myopic for horizontal lines Transduction ! Retina contains light-‐sensitive photoreceptors • Rods: High sensitivity (night vision) • Cones: Lower sensitivity (day time vision) When we outside in bright sunlight and then go into a dark room, there is not enough light to get our cones going and we have to reply on our rod. However our rods have just been outside in bright sunlight and are completely bleached. It takes some minutes in the dark for the rods to recover their sensitivity ! After rods/cones have transduced the light, they turned it into an electrical impulses then pass on the impulse to ganglion cells (via bipolar/amacrine/horizontal cells). Ganglion cells have long axons that exist the eyeball via a bundle called the optic nerve. Where the optic nerve leaves the eye there are no photoreceptors (this is the blind spot).
3
! ! ! !
What’s Out there? Electromagnetic radiation: Periodic changes in electric and magnetic field Rays: ravel in straight lines at a constant very high speed Particle: Discrete packets or ‘quanta’ and light particles known as photons Wave: Light has a set wavelength, measured in nanometers
The electromagnetic spectrum
Light Intensity ! Light intensity = intensity of electromagnetic wave? ! But not all wavelengths are visible – need a scale that takes visibility into account • Units we use to measure this are called candelas per m2 and the quantity is know as luminance. • Luminance is the intensity of the light that we can see as relevant to human vision ! Human vision works in a wide range of different light intensities (luminance) ! Objects reflect different percentages of incident light e.g. white paper ~75%, black paper ~5%
Contrast If we want to see things we need to detect objects against its background. In order to be able to do that, there is got to be some contrast between the objects and its background. ! Relative luminance’s is constant regardless of absolute luminance ! Contrast is defined as: where Lmax & Lmin are the largest & smallest luminance values respectively • Varies between 0 & 1 • When Lmax = Lmin, contrast is zero (i.e. there’s nothing there to see)
L max − L min ) ( C= (L max + L min ) Michelson Contrast (C) formula
1
Light, Eye, Brain and Spatial Vision ! Cornea: The transparent ‘window’ into the eyeball ! Pupil: The dark circular opening at the center of the iris in the eye, where the light enters the eye ! Lens: Enables changing focus using ciliary muscles ! Retina: A light sensitive membrane in the back of the eye that contains rods and cones, which receive an image from the lens and send it to the brain through the optic nerve ! Aqueous/vitreous humor: the squishy bits.
The eye
Light first passes through the cornea the outermost part of the eyeball where it beings to be focused. Then it enters the pupil a small opening that leads to the lens (the coloured part around is called the iris and it grows and shrinks to protect the pupil and make sure the right amount of light gets in. Once the light makes it past the pupil it hits the lens a surface where it is further focused through the process of accommodation Finally the light hits the retina a layer of tissue that lines the inner part of the eye. The retina beings the process of turning the light into a image. Retina contains two photoreceptors (the rods and cones). After rods/cones have transduced the light, they turned it into an electrical impulses then pass on the impulse to ganglion cells (via bipolar/amacrine/horizontal cells). Ganglion cells have long axons that exist the eyeball via a bundle called the optic nerve. Where the optic nerve leaves the eye there are no photoreceptors (this is the blind spot). After impulses exist the optic nerve; retinal ganglion cells axons terminate in lateral geniculate nucleus (LGN), which is a structure in the thalamus. The points where the signals cross over is called the optic chiasm. LGN projects to primary visual cortex (V1) in the optic lobe via optic radiations.
Focusing ! Light rays from a single point spread in all directions ! Focusing is recombining rays from various directions to form a single point on the imaging surface ! In the eye, this job falls on the cornea and lens • Cornea is curved – light refracts a constant amount. It can’t change its shape and it does the exact same amount of focusing regardless where the object is. That's a problem because for objects further away it needs a different amount of focusing compared to when its close • Lens refracts light variable amount – so the cornea is doing most of the focusing and lens does all the fine tuning • Accommodation: lens can be stretched to allow focusing of far objects • Cornea has greater refractive power
2
Light, Eye, Brain and Spatial Vision Focusing Errors glasses its probably due to problems with accommodation; if If people need your lens doesn't do it naturally, light needs to first pass through artificial lenses in order to be properly focused. Emmetropia (A) ! Normal refractive condition: appropriate focus ! Light waves coming in are being bend, focused inwards so that they all are imaged on exactly the same point on the retina, so that forms a very sharp image Myopia (B) ! Near or short sightedness ! Focal length is too short ! Light is focused in front of the retina ! Need for concave corrective lens Hyperopia/ Hypermetropia (C) ! Far or long sightedness ! Focal length is too long ! Light focused behind retina ! Need for convex corrective lenses Presbyopia ! Occurs with old age ! Inability to change accommodation ! There is stiffing of the ciliary muscles and the lens so that they cant as easily change shape. Therefore your change of focus is impaired Astigmatism ! Different focal lengths for different orientations e.g. ok for vertical line but myopic for horizontal lines Transduction ! Retina contains light-‐sensitive photoreceptors • Rods: High sensitivity (night vision) • Cones: Lower sensitivity (day time vision) When we outside in bright sunlight and then go into a dark room, there is not enough light to get our cones going and we have to reply on our rod. However our rods have just been outside in bright sunlight and are completely bleached. It takes some minutes in the dark for the rods to recover their sensitivity ! After rods/cones have transduced the light, they turned it into an electrical impulses then pass on the impulse to ganglion cells (via bipolar/amacrine/horizontal cells). Ganglion cells have long axons that exist the eyeball via a bundle called the optic nerve. Where the optic nerve leaves the eye there are no photoreceptors (this is the blind spot).
3
