Color Vision and Color Specification Systems

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                       Color Science
                                             

 




        Color Vision and Color
        Specification Systems
Mechanism of color vision,

Chemistry of color vision,

Color specification and terminology,

Munsell color system


Color system using additive color mixing,




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                       Color Science
                                                 

 




          Mechanism of Color Vision

                 Unwelcomed mechanisms

* Existence of about 100 types of photoreceptors

(not accepted)

* Presence of six or seven types of photoreceptors

             Most accepted mechanisms

* Trichromtic theory presented by Thomas Young and

Hermann von Helmholtz

* Opponent color theory presented by Ewald Hering




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          Young – Helmholtz Theory




Three types of photoreceptors named cones, sense
red, green and blue colors
The degree of response of cone is important issue

                  Advantage?

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                      Hering Theory
                                 




     Three types of photoreceptors which respond to
    white-black, red-green and yellow-blue opponencies

    Colors are characterized by the degree of response
                 of these photoreceptors

                      Opponent colors
                      White and Black
                      Red and Green
                      Yellow and Blue


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         Color Vision Model
             Stage theories:

    Trichromatic + opponent theories




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     Chemistry of Color Vision


     Eye response and photography,

    Rhodopsin (purple pigment) in rods

    Idopsin (Violet pigment) in cones




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              Color Specification
    One color different names? Subjective data




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            Color Specification Systems


    * Color appearance or perceptual systems (Munsell)



              * Color mixing systems (CIE)




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          Color Appearance Systems


             Based on color perception,

                   Standard materials

        Psychological impression (subjective)

      Specified attributes are perceived color

    Extendable to higher levels of sensory such as

                        texture




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             Color Mixing Systems


Base on the amounts of mixed colored light necessary

to obtain a color match to a test color in a color

mixing experiments




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        General Concepts and Comparison between

    Color Mixing and Color Appearance Systems

                        Color Mixing System                 Color Appearance System

Category                Psychophysical color                Perceived color

Basis                   Color sensation                     Color perception

Principal               Color mixing of light               Color appearance of material

                                                            standards (e.g. color chips)

Object of expression Color of light                         Color of object

Typical Example         CIE                                 Munsell

Specification Values    Colorimetric       values      (e.g Color appearance values (e.g.

                        tristimulus values)                 hue, lightness, chroma)

Process     for   Color Converting      color       stimulus Visual color matching with

Expression              function into psychophysical material             standard         or

                        values using color matching converting from colorimetric

                        function                            values

                                                 




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         Classification of Samples in Color
                  Appearance System
         Hue: different colors such as red, blue, ..




    Lightness: difference between bright violet and dark
                           violet

                      or grays with different lightness




     Chroma: difference between a yellowish gray and
                 yellow with same lightness




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      Color Appearance Systems



         Color appearance values?

     How many color appearance values?

    A standard color appearance system?




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       Differences between

       Brightness and Lightness?

    Brightness and the light sources,

             Lightness and objects

       What about illuminance?




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        Color Mixing System

              Color sensation and,

                Color stimulus

    Same responses for matched colors




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        Color Mixing System


    Same responses for matched colors




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                 Color Mixing System

      * Can actual reference stimuli match all colors?

    * If yes, the color stimulus can be expressed by the

               amounts of reference stimuli

          * Amounts are called tristimulus values




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             Munsell Color System
The most attractive color specification system base
on the color appearance model
              Different perspectives




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    Different Perspectives




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    Coordinates in Munsell Space

                 Munsell Hue

                Munsell Value

               Munsell Chroma




                  However,

             (Don't forget HV/C)




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                         Munsell Hue




    RED        YELLOW        GREEN            BLUE        PURPLE

          YR            GY               BG          PB                RP

10 basic hues which could be also divided to subgroups




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                Munsell Value




    V=0 (ideal black) to V=10 (ideal white)




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                     Munsell Chroma




    From a gray with equal Munsell Value to as pure as

                      possible colors

                         (open direction)




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    Munsell Color Solid




    Is not ideally a sphere




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      Munsell Chips Are Available
    Schematically (for educational purpose)




                Atlas and book




                     Cards




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                         Color Science
                                                   

 


           Munsell Notation of Samples
Could be determined by visual matching of sample with
                      Munsell chips.

    Interpolation between neighborhood chips would be
                         possible

                     Munsell notation

              Don't forget H V/C system

                      e.g., skin color
                       1.6YR 6.3/3.9

         Munsell notation for achromatic colors

                (white, grays and black)

                             e.g.

                      N3, N 6, N 8




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    Conditions for Determination of Munsell

                  Codes of Samples

              Important factors are:

                  Type of light source,

                   Viewing condition,

                      Backgrounds




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     Advantages of Munsell Color System

            * Reasonable color position,

            * Uniform color differences,

                   * Unlimited boundary

    Disadvantages of Munsell Color System

        * Inconvenience for some application,

         * Inaccuracy due to interpolation,

       * Extrapolation for saturated samples




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    Additive Color Mixing System
        R, G and B primary lights

       Color matching experiment




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    Typical Primaries in Color Matching
                Experiment




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                     Color Equation

                [F1]=[F2]       and         [F3]=[F4]

    = signs shows that left-hand side stimulus matches

                 right-hand side stimulus




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     Grassman Laws for Additive Mixtures

                  Proportionality:

          α[F1]=α[F2] and α[F3]=α[F4],




                    Additivity:

    [F1]+[F3]=[F2]+[F4] and [F1]+[F4]=[F2]+[F3],




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           General Form of Color Equation

                       [F]=R[R]+G[G]+B[B]

    * Values of R, G and B depends on the type of [R], [G]

                            and [B]

    * [R], [G] and [B] should be standard to compare the

           results of color matching experiments




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So, Two Critical Points Should Be Answered

            Colorimetric values of[R], [G] and [B],

    The relative values of the luminances of each primary,

                        i.e. R, G and B




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                     Primary Stimuli,

                               or

             Reference Color Stimuli?

                      Tristimulus values

    The value of each primary i.e. R, G and B in color

                          equation




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    Matching of Monochromatic Light Sources

    The rλ ,   gλ   and   bλ   are the amounts of [R], [G] and

     [B] primary stimuli which match the spectral light

                               source [Fλ]




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    Tristimulus Values of Polychromatic

                        Stimulus

           If two monochromatic lamps [Fλ1]


    f1[Fλ1 ] = f1 (rλ1[R ] + g λ1[R ] + bλ1[B])
                         and [Fλ2],

    f 2 [Fλ 2 ] = f 2 (rλ 2 [R ] + g λ 2 [R ] + bλ 2 [B])

                     match the color[F]




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               In Mathematical Form:
    [F] = f1[Fλ1 ] + f 2 [Fλ 2 ]
        = f1 (rλ1[R ] + gλ1[R ] + bλ1[B]) +
           f 2 (rλ 2 [R ] + g λ 2 [R ] + bλ 2 [B])
        = (f1rλ1 + f 2 rλ 2 )[R ] + (f1g λ1 + f 2 g λ 2 )[G ] +
           (f b
             1 λ1       + f 2 bλ 2 )[B]




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    The Tristimulus Values of Stimulus [F]

             R = (f1rλ1 + f 2 rλ 2 )
             G = (f1g λ1 + f 2 g λ 2 )
             B = (f1bλ1 + f 2 bλ 2 )
    In the Case of n Monochromatic Lights:
                              n
                   R = ∑ f i rλi
                            i =1
                        n
                  G = ∑ f i g λi
                                      and
                       i =1

                            n
                   B = ∑ f i bλi
                          i =1
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    Replacement of Radiant Power with
     Spectral Power Distribution P(λ)
                    n
               R = ∑ P(λ )rλi Δλ
                   i =1

                    n
               G = ∑ P(λ )gλi Δλ
                   i =1

                   n
           B =    ∑ P (λ )rλ i Δ λ
                  i =1




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    Change the Summation with Integral

            R=       ∫ P(λ )r(λ )dλ
                     vis

                G=    ∫ P(λ )g (λ )dλ
                     vis

            B=       ∫ P(λ )b(λ )dλ
                     vis




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    Tristimulus Values of Object Colors

          R=    ∫ P(λ )R(λ )r (λ )dλ
                vis

           G=    ∫ P(λ )R(λ )g (λ )dλ
                vis

           B=    ∫ P(λ )R(λ )b (λ )dλ
                vis




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               Color Science
                                      

 




    The most important additive

        color mixing system is

      CIEXYZ which defines its

     primaries in a manner that

     match all actual colors with

    the non-negative values of its

         imaginary primaries




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