Optics and the Eye

The Visible Spectrum

Some similarities between the eye and a camera

Refraction - the basis of optics Light bends when it goes from one medium to another The amount or bending, or refraction, depends on the angle incidence, and the nature of the two media

Objects-Lens Distance

Optical Power greater the power the closer the image is to the lens

Convex lenses have positive optical 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 power F = the focal distance do = the distance to the object di = the distance to the image (all distances are expressed in meters)

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 = 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 same degree 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