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• Chapter 5: The Human Eye and Vision - I: Producing the Image

• Using what we have learned about lenses and cameras to understand how the eye works

• The eyeball is a kind of camera but– Two lenses (cornea and eyelens)

– Focusing by accomodation

– Iris instead of diaphragm• Pupil is the adjustable hole

– Interior filled with liquids (humors)

– Retina instead of CCD• Fovea, optic nerve & blind spot

• Photoreceptors: rods and cones

• Dark adaptation

• Image processing (Ch. 7)

Physics 1230: Light and ColorChapter 5

Parts of the eye and the iris

• Iris acts like diaphragm in camera– Aperture of iris is called the pupil – Iris and pupil are alive– Irises are as different as

fingerprints • Iris scans can be used for

identification (e.g., airport)

• Your iris widens when the light goes down and narrows when the light is bright (video camera demo)

– The f-stop of the fully-opened iris is between f/2 and f/3

– Stopped down, it can reduce light intensity by up to a factor of 20

• However, the main function of the iris is not to control the intensity of light coming into your eye

– The range of light intensities to which eye responds varies by 1013

• Main functions of iris– Reduce aberrations, sharpen image.– Increase depth of field (e.g.

threading needle)

Parts of the human eye

How and where is the image produced in your eye?

• Dual (compound) lens system

– Cornea (on outside) does most of the ray bending because speed of light is slow inside cornea.

– Eyelens does fine tuning: • speed of light inside

eyelens is not much larger than in humors which fill the inside of the eyeball

• Hence, not much ray-bending is done by the eyelens (it has a long, adjustable focal length)

– The image is focused on your retina

– We'll use an equivalent single lens in examples

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Final imageis onretina

Cornea does most of ray bending

Eyelens behind cornea does fine-tuning;focuses by changing its (long) focal length

Retina

The eye focuses in a different manner from the camera

• A camera is focused by changing the distance, xi, from the lens to the image at the back on the film or CCD as the distance to the object changes.

• A camera focuses on an object any distance xo from the lens (of fixed focal length) by changing xi so that xi is a solution to the lens equation

• 1/f = 1/xo + 1/xi

• The eyelens is a fixed, unchanging distance, xi, from the retina at the back of the eyeball where the image is created• As the object distance, xo,

changes, focus is achieved by the eyelens changing its focal length, f, so that f is a solution to the lens equation

• This method of focusing is called accomodation

• 1/f = 1/xo + 1/xi

fixed numbervariesvaries

changes as xchanges as xoo changes changesvariesvaries fixed number

Accommodation is achieved by the eyelens' ability to change its focal length by changing its bulge

– Eyelens shape is changed by ligaments. This changes its focal length

• Less bulge means less bending power and longer focal length

• More bulge means more bending power and shorter focal length

• Demonstration

large f

smaller f

How and why your eyelens accommodates to put images in focus

• Your eyelens has a small depth of field – You can't see something

close and far with both objects in focus at the same time

• Hold out your thumb about 15" just below me– Then, alternately focus on

thumb and me (right above your thumb)

– Do you feel your eyelens changing its bulge?

• Note that you cannot see both me and your thumb sharply (in focus) at the same time– You focus on one or the

other by changing the bulge of your eyelens

• How do you focus?

What does accommodation of the eye have to do with looking at me or your thumb? How does it work?

(Lens represents combined cornea-eyelens system)

large f

smaller f

Focusing your eye on a nearby thumb requires shorter focal length (more bulgy) eyelens than focusing on the Prof far away, since rays must be bent more for image to fall on retina.

Thumb is out of focus

Prof. is in focus

Thumb is in focus

Prof is out of focus

Human Eye: Review of what we learned about the Human Eye

Try in home

When we are looking at objects, the image is formed

The retina is where the image falls at the back of your eyeball

• Image falls on retina instead of CCD.

• Rods & cones packed into retina. – Sensitive to light like camera film

• Optic nerve– Nerve fibers connect rods & cones to

brain. (transform light into electrical signals)

– Blind spot is where optic nerve leaves eyeball. Demo.

• How are rods and cones distributed in the retina? Fig. 5.12

– The fovea is the small region near the center of the retina

• Used for sharp, detailed viewing.• Has the most cones (precise, color vision)• Has no rods (used for low light, less

precise viewing).

– Looking at someone means their image is on your fovea

• If their image is not on your fovea you see them "out of the corner of your eye."

• Eyeball moves to see a sharp image– It scans to make all parts of an image

eventually fall on your fovea

There are no light detecting rods and cones where the optic nerve leaves the eye. This is called the blind spot.

Adaptation of the rods and cones to the dark: darkness adapation

• The retina is able to change its sensitivity to light as the light intensity varies by a factor up to 1013.

– This is similar to a camera (but the range is enormous)

• After you walk into a dark movie theater your eyes gradually "adjust" to the darkness because your rods and cones can detect lower and lower intensities as time goes on!

– At any given time the minimum light intensity you can just barely see (called your detection threshold) gets lower and lower

•How does the lowering of your detection threshold work?

– When you are outside the theater you don't need to see very low light levels. Your detection threshold is high

• Outside in the sun you are mainly seeing by using your cones.

• They enable you to see in full color. • This is called photoptic vision

– After you enter the theater your detection threshold gradually goes down

• After about 7 minutes you can no longer see with your cones

•You switch to rods, which are more sensitive

•Your rods don't allow you to see colors•This is called scotopic vision•Your rod sensitivity continues to improve in the dark for another 25 minutes (detection threshold goes down)