Optics and the Eye
Optics and the Eye
One nm = one billionth of a meter
The Visible Spectrum
Some similarities between the eye and a camera
Air Glass
Refraction - the basis of optics
Light bends when it goes from one medium to another
The amount or bending, or refraction, depends on theangle incidence, and the nature of the two media
Light Source
• close objects are focused further from the lens • far objects are focused closer to the lens
Light Source
• close objects are focused further from the lens • far objects are focused closer to the lens
Objects-Lens Distance
Optical Power
greater the power the closer the image is to the lens
Convex lenses have positiveoptical power
Concave lenses have negative optical power
The precise distance from the lens to the focused image depends on lens power and the distance to the object according to the following equation:
P = 1/F = 1/do + 1/di
(lensmaker equation)
P = lens powerF = the focal distancedo = the distance to the objectdi = the distance to the image(all distances are expressed in meters)
Focal Point
Focal Length
Optical Axis
(FL)
FL = 0.25 m D = 1/FL = 4 Diopters
FL = 0.5 m D = 1/FL = 2 Diopters
Nodal Point
Cornea - does most refracting
Lens - does fine focussing
Focussing --Accomodation
In humans, fine focussing is controlled through changing the shape of the lens.
Accomodation
• Changing the lens shape is controlled by the ciliary muscles
Average human eye’s power is 60 diopters (cornea and lens together)
Focal distance of such optics is 1/60 = 0.0167 m = 16.7 mm
Posterior nodal distance of average eye is 16.7 mm - good fit!
Near and Far Points
• Near point - closest distance that an object can still be kept in focus.– Will change with age
• Far point– Normally at optical infinity
Astigmatism Target
Sometimes a lens can be emmetropic for light waves in one orientation (say, for horizontal lines), but be badly hyperopic or myopic for other orientations (say, vertical lines). Thus, people with astigmatic vision see sharp lines and contours in some orientations, and blurry contours in others.
Astigmatism
Someone with an astigmatism in the vertical orientation might see the target such that the horizontal lines are in focus and of high contrast, but as the lines become more vertical they go out of focus, becoming blurry and lower in contrast.
The Human Eyeball
The cells of the RETINA act as transducers. A transducer changes one form of energy into another.
The Retina
Membranous discs contain photopigment
Rod Photoreceptors• about 120 million per eye• only one kind• most sensitive to light of about 505 nm• approx. 10 times more sensitive than
cones• used in night/scotopic vision• psychophysical & physiological data
indicate that rods can respond to a single photon!
Cone Photoreceptors
• about 8 million per eye• 3 kinds, each most sensitive to
440, 530 or 560 nm• basis of colour vision• approx. 10 times less sensitive
than rods• used in day/photopic vision
Distribution of Rods & Cones
• rods are most dense in periphery• no rods in the center of the
macula• cones are most dense in fovea• no receptors in blind spot
Eye Movements
One reason we move our eyes is that we have a relatively narrow field of vision and must move our eyes around to sample the visual world extensively.
A second reason we move our eyes is that our retinas are not uniformly sensitive. Our retinas have the highest acuity in the region called the fovea. To see something clearly we orient our eyes so that the image will be projected onto the high resolution fovea.
Why move our eyes?
Two Main Classes of Eye Movements
1. Conjugate (Version) - Both eyes move to the samedegree and in the same direction
2. Vergence - eyes rotate in opposite directions - e.g. inward to look at a close object
Conjugate Eye Movements
Vergence Eye Movements