The Human Eye

Anatomy and

Detection Characteristics

Anatomy of the Human Eye

The Eye

The human eye is a camera!• Iris - colored annulus with radial muscles

• Pupil - the hole (aperture) whose size is controlled by the iris

• What’s the “film”?– photoreceptor cells (rods and cones) in the retina

Slide by Steve Seitz

The Retina

Cross-section of eye

Ganglion cell layer

Bipolar cell layer

Receptor layer

Pigmentedepithelium

Ganglion axons

Cross section of retina

Retina up-close

Light

© Stephen E. Palmer, 2002

Cones cone-shaped less sensitive operate in high light color vision

Two types of light-sensitive receptors

cone

rod

Rods rod-shaped highly sensitive operate at night gray-scale vision

Rod / Cone sensitivity

© Stephen E. Palmer, 2002

Distribution of Rods and Cones.

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Visual Angle (degrees from fovea)

Rods

Cones Cones

Rods

FoveaBlindSpot

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tors

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Night Sky: why are there more stars off-center?

Why do we see light of these wavelengths?

© Stephen E. Palmer, 2002

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Wavelength (nm)

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VisibleRegion

…because that’s where theSun radiates EM energy

Visible Light

The Physics of Light

Some examples of the reflectance spectra of surfaces

Wavelength (nm)

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400 700

Yellow

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Blue

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Purple

400 700

© Stephen E. Palmer, 2002

© Stephen E. Palmer, 2002

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M L

Three kinds of cones:

Physiology of Color Vision

The Human Eye as an Astronomical Instrument The eye is a camera with:

Focal length f = 18 mm Aperture variable 2 – 7 mm Fast scanning and focus adjustment A high-resolution color sensitive center: the fovea with cone

cells Lower resolution peripheral vision, both cones and rods Separate day and night vision detectors:

o Cones for color vision during the day o Rods for low-light monochromatic vision

Redundant system Stereoscopic Rangefinding system Powerful image processing and object identification system

connected

Empirical Starting point: Experienced observers under ideal conditions can just barely see stars of 6 th magnitude. Calibration of the magnitude scale: A star of 0 magnitude in the visual band emits: 3.75 10-11 J m-2 s-1 nm-1

(Joules per square meter collecting area (0.38 10-4 m2), per second collecting time (0.15 s), and per nm filter bandpass (100nm) ) This makes 2.14 10-14 J of energy received by the eye in one reaction time.

Each photon carries an energy of hc/λ = 3.61 10-19 J This means the eye receives 59000 photons per second from the 0 magnitude star. A 6 mag star receives a factor of 251 less Photons, i.e. 235 photons The eye also receives 1711 competing photons From every square degree of the sky. Assume light from 0.1 square degrees actually interferes or competes with the detection of The star, i.e. 171 competing photons.

Quantum efficiency of the eye is about 5% under optimal conditions: Healthy eye, perfectly dark adapted, using peripheral(rod) vision, having enough oxygen, good nutrition (vitamin A), experienced in the mental evaluation of faint signals. Under these conditions, in the reaction time interval, we have: 12 photons detected from the star competing with 9 photons from the sky. The 12 photons have to be detected against a total noise of sqrt (12+9) = 4.6 photons. The 6 mag star is thus a 2.6 σ detection, which is just a quantitiative way of saying: “barely able to see it”

Resolving Power of the Eye Resolution (daylight viewing with fovea): 1 arcmin Projected diameter of fovea: 100 arcmins Sensor density: 30 106 rods / steradian = 2.7 rods/arcmin2

1.2 106 cones / steradian = 0.1 rods/arcmin2

In the fovea: 50 106 cones / steradian = 4.2 rods/arcmin2

Diameter of individual cones: 2 μm (25”)

Diameter of individual rods: 1 μm (12”)

Comparison to Diffraction Limit Pupil diameter: 2.5 mm Wavelength: 500 nm (green light) Diffraction Limit: 1.22 λ/d = 0.000244 radian = 0.84 arcmin Under optimal bright daylight conditions, the eye is capable of nearly diffraction-limited resolution. At night, the pupil is larger (up to 7 mm) and the resolution is limited by rod-cell density.

Visual Observations

• Navigation

• Calendars

• Unusual Objects (comets etc.)

Chemistry of Photography

1) The light sensitive emulsion

2) The latent image

3) Developing the image

4) Fixing the image

Black & White Film

Black and white film is composed of 4 layers. *An upper protective coat. *A layer of gelatin that contains silver halide (AgBr, AgCl, or AgI) crystals. (The type and proportions of the different silver halides determining the speed of the film)

*The film base, usually made from a flexible polymer. *And the anti-halation backing to prevent light from reflecting back onto the emulsion.

Color Film

The Color film “emulsion” is actually made up of 3 different layers of emulsion.

* Each is sensitive to different wavelengths of light. *The emulsions still contain silver halide crystals but are now coupled with dyes. *The dyes are the compliments to the colors too which that layer is sensitive. *There is a yellow filter between the first and second emulsion layer to prevent blue light from getting through to the lower layers because all silver halides are sensitive to blue light. *The film base is an orange color to reduce the contrast of the negative and to correct for sensitivities in the red and green layers. *The anti-halation layer in color film serves the same purpose as in black and white film

Exposure, Development of Black and White Film - Overview

• A. Unused film in camera

• B. Exposure of film to light (photons)

• C. Formation of silver ions (latent image)

• D. Development changes silver ions to metallic silver

• E. Fixing – removes unreacted silver halides from the emulsion.

• F. Wash – rinsing with clean water. Removes all by-products of development process.

The emulsion• AgNO3 + KBr = AgBr + KNO3 in gelatin

• AgBr precipitates (WHY??) and remain in the gelatin to form minute grains.

• AgBr is light sensitive, forming a latent image that can be “developed”

• But how?

• The sensitivity of the grains are proportional to their sizes. If all the grains were the same size, there would be no shades of grey at all! Typical densities of grains are about 5 x 108 grains per cm2. If you consider a grain to be equivalent to a “pixel”, you see that photographic film (taken by itself) it quite a bit more capable of “resolving” detail than our current digital cameras.

The latent image

• For many years, it was thought that 2AgBr + light = Ag2Br + Br (the “sub-haloid” hypothesis…). But there was never evidence of a chemical change. Less than 5 silver atoms are involved at any site!!

• X-ray spectroscopy finally showed that silver is liberated

Br - + light Br + e –

The electron then migrates to a shallow “trap” (called a sensitivity site).

Ag + + e - Ag

Species produced include: Ag2+, Ag2

o, Ag3+, Ag3

o, Ag4+, Ag4

o

Why doesn’t it go the other way? i.e. why is it stable?

Converting Silver Halide Crystals to Metallic Silver

Ag+Br- (crystal) + hv (radiation) ® Ag+ + Br + e-

Ag+ + e- ® Ag0

Silver Crystals – Sensitivity Centers

• The silver halide crystal contains imperfections called sensitivity centers.

Effects of light on the film

• Within a crystal the Silver atoms have a positive charge and the halide atoms a negative.

• Light (photons) striking the halide atoms within the grains causes excitation of electrons which move within the crystalline structure.

• Those electrons are attracted to the Sensitivity Centers.

Ag+ Br - (crystal) + hv (radiation) Ag+ + Br + e-

Latent Image Formation

• The silver ions are attracted to the negative charge of the electrons at the sensitivity center.

• As more light (photons) hit the halide atoms silver ions build up on the sensitivity centers.

• The silver ions acquire and additional electron and become metallic silver.

• These sites form development centers and make up what is called the “latent image”.

Ag+ + e- Ag0

Developing the image• All of what we’ve discussed so far has gone on

within your camera.• Now we’ll go to the process of “developing” your

film.

• Black and white film is handled in complete darkness as the film is sensitive to all wavelengths of light.

• The Steps of processing/developing film are:

Development

Stop

Fix

Wash

Hardening bath (optional)

Development• Photographic Developers are generally

Reducing agents. The silver ions are reduced to silver metal. The developer donates electrons to the positive silver ions.

• The greater the number of silver nuclei attracted to the sensitivity centers the faster the developer will reduce the silver ions to silver metal. So the more light a crystal is exposed to the faster it will develop and the darker it will be.

• Developers need to be somewhat selective so as not to turn unexposed silver dark. A process known as fogging.

• Photographic developers contain carefully balanced levels of the developing agents, “accelerators” such as Sodium or Potassium Hydroxide, and Sodium or Potassium Carbonate. There are also restraining agents built in such as Potassium Bromide. These restrainers slow down development in areas that received less exposure.

The Mechanism of Development• The photographic process depends upon the fact that the reaction:

Ag + + e Ag

(i.e. the reduction of silver ion to metallic silver by a developing solution), proceeds much more easily for an exposed silver halide grain than for an unexposed grain. The “gain” can be ~109.

Development- Continued…

The reduction potential of the developer must be such that it will develop those exposed silver halide grains, but not large enough to develop them all. (A “fogging” developer…)

What actually happens?

C6H4(OH)2 + Na2SO3 + 2AgBr +NaOH C6H3(OH)2SO3Na +2NaBr+H2O +2Ag

Hydroquinone sodium sulphite silver bromide sodium hydroxide hydroquinone sulphonate sodium bromide water SILVER! | | stabilizer ya gotta do something for the bromine! (plus it adjusts the pH)

Chemical velocity: ΔT = 1o C Δvchem = 10%.

Stop Bath• Photographic developers are generally of a pH

greater than 10.• A “Stop bath” usually made from a weak acid

such as acetic acid is used to stop the development, and prevent fogging of the unexposed silver.

Fixing

• Undeveloped silver halide crystals remaining in your film will darken with time if exposed to light.

• To prevent this, film is “fixed” or has the undeveloped silver halide crystals removed from the film.

• Sodium Thiosulfate, usually referred to as “Hypo” is one of the most common fixing agents though others are used depending on the specific characteristics wanted in the fixing solution.

• The silver halides have a low solubility in water. To remove them they need to be turned into more soluble forms that can be removed in the water wash.

Fixing the image

The biggest problem after the invention of photography in the 1830’s was the lack of permanency. You have to get rid of that remaining bromide, or eventually the photograph will go black. There are no true solvents of AgBr. When sugar is dissolved in water, and then evaporated, the sugar is recovered. This never happens with AgBr. The residue left behind is always a transformed salt. So what we need to do is make sure the transformed salt is soluble, so it can be washed away.

AgBr + Na2S2O3 = AgNaS2O3 + NaBr (only slightly soluble)

But if we have a more liberal solution of sodium thiosulphate:

2AgBr + 3 Na2S2O3 = Ag2Na4(S2O3)3 + 2 NaBr (bingo!)

Does anything else work? KCN. We won’t go there….

Washing

• The final wash of a photographic negative needs to be lots of fresh clean water to remove any residual developing agent, fixative or silver complexes as these can cause degradation of the image with time.

• The ability of a film to withstand this degradation is referred to as it’s Archival Quality.

• Depending on the film, and processing methods film can remain unchanged for many decades.

• An optional hardening bath can be used after the wash to try and minimize scratches to the dried emulsion.

Reciprocity Failure of Photograhic Plates

Cross section of a photographic plate

Emulsion Hypersensitization

• Baking of plates drives out Water• Cooling during exposure slows competing chemical

processes• Soaking in Nitrogen drives out Oxygen and Water• Soaking in Hydrogen reduces AgBr and produces a

few Ag atoms per grain• Pre-Flashing generates a few Ag atom per grain

All this worked surprisingly well for a low-sensitivity, fine grained emulsion originally developed for technical photography, making gas-hypered Kodak Technical Pan 2415 competitive with the specialized astrophotography emulsions of the late photographic era. It is no longer produced today.

Disadvantages of photographic plates:

• Low quantum efficiency. The best plates have a QE of about 3%

• Long exposure times, inefficient use of time

• Reciprocity failure. It becomes less effective as exposure time increases

• Non-linear color sensitivity. Plates are more sensitive to blue light

• Hypersensitising and Developing. Hypersensitising involves baking plates to increase efficiency (up to 10%). You cannot see results until after developing usually many hours later.

• Storage. They are fragile and take up space. They also decay with age.

• Digitisation. Must be scanned to put the data in digital form

• Cost and availability. In 1996 a single 30 cm x 30 cm plate costs $100 USD. Kodak no longer makes plates.

So why do we not still use plates?