Appearance of Colour Appearance of colour is governed by three things. • Nature of Object • Nature of Light source • Observer ( Human Eye)
NATURE OF OBJECT • Even and polished and opaque - REFLECTION • Transparent - Refraction • Uneven - Scatter • Somewhat transparent and dull - Diffuse
INCIDENT BEAM Rflected beam Scattered light Diffused Light Refracted ray
LIGHT SOURCE • BRIGHTNESS ( measure of Energy ) 25 WATTS 100 watts
LIGHT SOURCE • Spectral composition- Relative percenatge of different wavelengths present. Example : day light or filament type Bulb is richer in yellow when compared to a fluorescent tube light.
PRIMARY COLOURS • BLUE ( 3000 A ) • YELLOW ( 5500 A ) • RED ( 6500 A)
HUMAN EYE • 2-cornea • 17 blind spot • 12 vitreous Humour • 11.Retina • 14 optic nerve • 4 iris • 5 lens • 10 choroid
FUNCTIONS OF THE EYE • Formation of the image and interpretation • Chromatic aberration and red blue response • RETINA distribution of Rod and Cone cells • Fovea and blind spot
RELATIVE SPECTRAL SENSITIVBITY • The relative amount (Energy) needed to stimulate the EYE is plotted against Wavelenths, we get Two sets of Curves At two levels of illuminations
SATURATION SENSITIVITY • Curve showing number of steps between white and spectral colour • This curve differs from person to person
We will discuss now: • Opponent Colour Theory • Trichromatic system • Grassmann’s Laws • Maxwell’s triangle • CIE coordinate system • Tistimulus vales • Chromaticity diagram .
What is the opponent theory of colour vision ? • The opponent-colour theory of colour vision, proposed by Hering, seemingly contradicts the Young-Helmholtz trichromatic theory ?
OPPONENT COLOUR THEORY • It explains various phenomena that could not be adequately accounted for by trichromacy. Examples of such phenomena are the after-image effect • For an example, if the eye is adapted to a yellow stimulus, the removal of the stimulus leaves a blue sensation or after-effect
OPPONENT COLOUR THEORY • Hering proposed that yellow-blue and red-green represent opponent signals • It also explains why there were four psychophysical colour primaries red, green, yellow, and blue and not just three.
OPPONENT COLOUR THEORY • Hering proposed that yellow-blue and red-green represent opponent signals • It also explains why there were four psychophysical colour primaries red, green, yellow, and blue and not just three
OPPONENT COLOUR THEORY • It is now accepted that both the trichromatic theory and the opponent colours theory describe essential features of our colour vision • Opponent Theory describes the perceptual qualities of colour vision that derive from the neural processing of the receptor signals in two opponent channels and a single achromatic channel.
SUBSTARCTIVE MIXING • This is what happens during a dyeing process, as each dye reflects only a part of the spectrum, which it does not absorb. • Yellow dyed fabrics reflect most of the yellow spectral color and little of the other spectral colours
SUBTRACTIVE MIXINGïÊàËèº •Êò¸ • Consider a yellow dyed textile material where it would be over dyed by a BLUE dye.
SUBSTRACTIVE MIXING • Formation of colour in this manner is called SUBTRACTIVE MIXING
ADDITIVE MIXTURES • If we flash a Blue and Yellow beams of lights , produced by two filters, from the same light source, the visual response we get by the addition of two response is GREEN.
Munsell Color System • The Munsell Color System, first published in 1905, was created by Albert H. Munsell (1858-1918) an American painter and art teacher.
In the Munsell Color System, a single color is represented by three separate parameters (three dimensions) of color: hue(H), value(V) and chroma(C). Munsell Color System
Munsell Atlas • Crossectional view
ADDITIVE MIXTURES • If two lights are having same spectral distribution, then the visual response or COLOUR of the two lights will be same to the Eye. When two different spectral colours are entered in to the eye, it combines the response to give a resultant colour
ADDITIVE MIXING¡å¨È¾ •Êò¸ • If a colour stimuli C is matched by using three primary stimuli Red (R), Blue (B), and Yellow(G) , using some units u, v, and w respectively, For the two COLOUR stimuli C1 and C2 • we can say C 1 = u 1R + v 1 B + w 1 G C 2 = u 2 R+ v 2 B + w 2 G • Then the Additive Mixture C = C 1+ C2 = (u1 + u2) R + (v1 + v2 ) B + (w1 + w2)G
VECTOR ADDITION • We know how to add two Forces to get the resultant force, using the law of Parallelogram of Forces. R y x y X
VECTOR ADDITION • We also can show one more force adding to the same system on a third dimension as shown here.
HENCE we can represent colour in a three dimensional coordinate system, with all Hues of same brightness on a plane as shown here
MAXWELL’S TRAINGLE • Now we use an important theorem in geometry. “ The sum of lengths of the three perpendiculars drawn to three sides of an equilateral triangle is a constant ”.
MAXWELL’S TRAINGLE B r B g R c p b r R G q G IT COULD BE SHOWN THAT p : q : r = b : r : g
MAXWELLS TRIANGLE • Now we use an important theorem in geometry. “ The sum of lengths of the three perpendiculars drawn to three sides of an equilateral triangle is a constant ”. h1 h2 h3 h1 +h2 + h3 = k
MAXWELL’S TRIANGLE • Hence if we take the centroid of an equilateral triangle as the white point “ W ” We have a situation matching above white point and the points inside the triangle representing mixtures of three primaries kept at vertices of the triangle. • This triangle is called Maxwell's TRIANGLE G w B R R +G +B = W
COLOUR SPACE • Combining the Three Dimensional coordinate system of RGB with the Maxwells triangle for r,g,b we get the Colour space.
MAXWELLS TRIANGLE • THE TRAINGLE BGR Is named as MAXWELLS TRIANGLE • When the three primaries R,G,B are selected from the spectrum, many of spectral colours will get negative values.
CIE system • In order to keep the spectral colour in positive values, the coordinate system is rotated to assume three imaginary primaries.
CIE CORDINTE SYSTEM • ALYCHNE IS MADE TO Lye ON BR or (XZ) PLANE The values X, Y, Z measured in this system are called TRI STIMULUS VALUES
CIE Chromatically diagram • Y AXIS MADEPERPENDICULAR TO ALYCHNE • Thereby the Luminosity is also indicated by Y value.
STANDARD OBSERVER • (a) Spectrum, When three Imaginary primaries are used • (b) Standard observer functions for an equal energy spectrum, with three imaginary primaries
Chromaticity Coordinates • To simplify , the coordinates X,Y,Z, could be divided by X+Y+Z to get CHROMATICIRY COORDINATES (x,y,z) which adds up to one. x = X / (X+Y+Z ) , b = y / (X+Y+Z) and z = Z/ ((X+Y+Z ) x + y + z = 1 Hence it is sufficient to know x and y.
TRISTIMULUS VALUES • When we know the Standard observer function,we can measure the Tristimulus values for any other light source with known Energy levels, for each wavelength. (normally in steps of 5nm)
TRISTIMULUS VALUES • Consider a simplest case of a monochromatic light. • Then if we measure the spectral energy( S L) for the particular wavelength as we know that Std.Observer function for this wave length is (xL) • Then X = SL* xL
TRISTIMULUS VALUES • Now if we consider a source which covers the spectrum, we have to add for the whole visible wave length range, giving us • Σ Sλ* xλ Or more mathematically X= ∫ Sλ* xλ * dλ
TRISTIMULUS VALUES • If we consider an object with reflectance Rλ Then similarly Σ Sλ* Rλ * xλ Or more mathematically X= ∫ Sλ* xλ *Rλ * dλ