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