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    GEK1532

    Interlude: Painters Color and

    Color deficiencies

    Thorsten Wohland

    Dep. Of Chemistry

    S8-03-06

    Tel.: 6516 1248

    E-mail: [email protected]

    Claude Monet, Japanese Bridege over

    water Lily Pond, 1926

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    The Anomalouscope

    From Scientific Amrican, Special on

    Color (German Version)

    a) Normal vision

    b) No red

    c) No green

    d) Red anomalous

    e) Green anomalous

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    Lightness and Color Constancy

    Webers law states that we see brightness in logarithmic scale.

    However, we know as well that we perceive something white always as

    white, no matter how bright the illumination is. This phenomenon is called

    Lightness constancy.

    Lightness constancy thus means that we see objects always in relationto the surrounding. So when the illumination changes, the brightness

    (absolute intensity) changes, but not the lightness (the ratio of different

    brightnesses).

    Good illumination Darker illumination

    http://www.purveslab.net/seeforyourself/

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    Lightness changes not uniformly

    everywhere

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    The rod system

    http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

    Kurt Nassau, Fig. 1-16

    Scotopic vision: night vision, based on rods;

    maximum sensitivity: around 500 nm

    Photopic vision: day vision, based on cones;

    maximum sensitivity around 550 nm

    Mesopic vision: transition from photopic to

    scotopic vision, both systems operate (e.g. at dusk)

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    Scotopic vs Photopic Vision

    http://www.cquest.utoronto.ca/psych/psy280f/ch3/purkinje/ps.html

    Purkinje shift

    Scotopic vision: max sensitivity ~500 nm Photopic vision: max sensitivity ~550 nm

    Mesopic vision: Humans have characteristics of tetrachromat

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    Painters and use of color

    Rembrandt (1606-1669)Leonardo

    da Vinci (1452-1519) Charles-Joseph

    Natoire (1700-1777)

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    Joseph Mallord William Turner

    (1775-1851 )

    1819 Colour Beginning

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    Joseph Mallord William Turner

    (1775-1851 )

    1835 grand-canal

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    Joseph Mallord William Turner

    (1775-1851 )

    ~1845 sunrise with sea-monsters (detail)

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    Pointillism

    Georges Seurat 1884-6 Un dimanche aprs-midi l'Ile de la Grande Jatte

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    Pointillism

    Paul Signac Harbour at Marseille 1906

    Camille Pissaro 1897 Boulevard-Montmartre-Afternoon-Sun-1897

    Paul Signac The bridge at Asniere 1888

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    Pointillism

    http://www.state-of-entropy.com/points.htm

    Add NoiseSharpenReduce Color depth to 2

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    Claude Monet (1840-1926) -

    Impressionism

    1872/3: Impression Soleil levant (sunrise)

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    MonetThe port of the Cathedral at Rouen

    Morning Mist 1893 Sunlight 1894 Sunlight 1892-4

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    Monet

    Japanese Bridge over

    Water Lily Pond 1894

    Japanese Bridge over

    Water Lily Pond 1899

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    Color vision defects

    http://www.psych.ucalgary.ca/pace/va-lab/Brian/acquired.htm

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    Monet

    House seen from the

    rose garden 1924

    House seen from the

    rose garden 1924

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    The organization of the retina

    Rods (R) and Cones (C)

    Bipolar Cells (B)

    Ganglions Cells (G)

    Amacrine Cells (A)

    Horizontal Cells (H)

    Falk: Fig. 7.2

    To optic nerve

    Light

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    The organization of the retina

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    The rod system (scotopic vision):

    The experiment of Hecht, Schlaer

    and Pirenne from 1942

    T.N. Cornsweet, Fig. 2.2

    http://hyperphysics.phy-astr.gsu.edu/

    hbase/hframe.html

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    Dark adaptation

    Dark adaptation is complete within 40 minutes.

    T.N. Cornsweet, Fig. 2.1

    T.N. Cornsweet, Fig. 2.6

    The threshold is defined as the

    intensity at which a subject perceives

    60 % of flashes.

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    Spatial summation

    T.N. Cornsweet, Fig. 2.5

    1st spot: only few rods on average

    2nd spot: smaller than summation area

    3rd

    spot: larger than summation area

    Sensitivity constant

    Sensitivity decreases

    Illuminate spots on the retina of different size

    and determine the number of photons

    needed before the spot can be seen

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    Temporal summation

    Adapted form T.N. Cornsweet, Fig. 2.5

    time (ms)

    0 10 20

    How many photons have to arrive

    in a certain time interval so that the

    eye sees a flash?

    time (ms)

    0 10 20

    time (ms)

    0 10 20

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    Temporal summation

    Adapted form T.N. Cornsweet, Fig. 2.5

    time (ms)

    0 10 20

    How many photons have to arrive

    in a certain time interval so that the

    eye sees a flash?

    time (ms)

    0 10 20

    time (ms)

    0 10 20

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    Spectral sensitivity of rods

    J19

    106.3 }P

    R hchE !!9

    834

    10550

    103106.6

    v!

    We just saw that the eye does not need more than 2 photons to perceive a

    flash (a rod can actually be activated by one photon alone).

    How many photons do we get from a 20 W [=20J/s] light bulb?

    Therefore a light bulb gives us

    sphotonsJ

    sJ/106.5

    106.3

    /20 1919

    !

    56.000.000.000.000.000.000 photons

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    Lateral Inhibition

    + +

    - -

    These two cones/rods inhibit each

    other, that is the higher the signal for

    cone/rod A, the more will be the

    signal for cone/rod B diminished and

    vice versa.

    A B

    + +

    - -

    A =0.5 B = 0.5

    A=B

    + +

    - -

    A =0.5 B = 0.25

    A - B

    + +

    - -

    A =0.75 B = 0.5

    A - B

    With lateral inhibition we are much more sensitive to differences than absolute values.

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    Lateral Inhibition

    STL Fig. 7.12

    rest

    excitation

    inhibition

    No difference -> rest

    Strong excitation

    No difference -> rest

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    Lateral Inhibition

    STL Fig. 7.8

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    If edge information is missing

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    Spatial frequency and tilt

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    AfterimagesYou can have negative and positive afterimages.

    The effect comes from the fact that when a cone/rod is stimulated for a long time

    it desensitizes.

    1) The cones perceiving the black square are not excited, the cones perceiving the

    white surrounding are excited and desensitize with time.

    2) When looking at the white surface on the right, the desensitized cones are lessexcited than the rested cones in the middle and thus you see a white square.

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    Afterimages

    Positive afterimages.

    You can sensitize your retina by closing your eyes and resting your cones

    (remember when you close eyes a long time and open them you seem to be

    blinded first).

    When you open your eyes shortly (seconds) and look at some bright object the

    cones get excited.

    When you close your eyes again the cones will not desensitize and will stay

    stimulated longer and give you a positive afterimage.

    See the TRY IT on page 194 of STL.

    http://www.michaelbach.de/ot/

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    Summary

    Spatial summation in the eye

    Temporal summation

    Lateral Inhibition Edge discrimination

    Afterimages

    Are there non-trivial constraints on colour categorization?

    B.A.C. Saunders, J. van Brakel

    Behavourial and Brain Sciences (1997), 20, 167-179