Color description sg babu
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Color Description in relation to CIE Value
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Color description sg babu
- 1. Color Description BY SG Babu
- 2. Nautre of Color Measurement • Subjective Phenomenon : Difficult to measure (Color, Taste and Smell) • Objective Phenomenon: Easy to measure. (Mass, Length, Width)
- 3. CIE System • French title of the international committee • Commission Internationale de l¢Eclairage • The system was set up in 1931 • The CIE system tell us – how a colour might be reproduced (by a mixture of three primary light sources) • The amounts of the three primaries required to match a particular colour provide a numerical specification of that colour. • 546.1nm is Green Color.
- 4. Factors that affect color • Light source used to illuminate its surface • Particular observer who views it. • Properties of the surface. • The nature of the surface is the most important factor. • Color is 3D (Hue, Chroma and Value)
- 5. Additive and Subtractive Mixing • Red + Blue = Purple • Red + Yellow = Orange • Yellow + Blue = Green • Red + Yellow + Blue in the correct proportions = Grey or Black.
- 6. Additive Mixing • Consider red and green lights shone onto a white screen. • The mixture of Red plus Green in the same appropriate proportions will reach our eye. • The colour seen is Yellow • Red Wavelength + Green Wavelength is what we see and hence we call it as additive mixing. • Multiple colors are produced by mixing R,B and G primaries.
- 7. CIE System of Color Specifications • Tristimulus Value: The amount of R,B and G Primaries that are required to match any color is called tristimulus value. (If RBG are primaries) • Any colors could be used as primary but Primary colors must not be possible to match by using a mixture of two other. • Tristimulus values can be positive or negative. • Negative values should be avoided.
- 8. CIE System of Color Specifications • There is no 3 perfect primaries which could produce all other colors, by a positive tristimulus value. (No set of real primaries can elimintate negative tristimulus value) • In the CIE system, imaginary primaries are indeed used so as to avoid negative values.
- 9. CIE System of Color Specifications • Impossibility to oserve color or judge color with unaided eyes at normal condition led to the addition of informations like standard observer. • It is possible to calculate tristimulus values (i.e. the amounts of three primaries [X,Y and Z] which, if additively mixed, would match a colour) of a samplespecified. • The CIE had to define standard primaries, standard light sources and a standard observer, together with standard observing and viewing conditions.
- 10. Standard primaries • SE = equal-energy stimulus, i.e. a stimulus having equal amounts of energy at all wavelengths through the visible spectrum. = Standard for Sensitivity of Eye to light of different wave length
- 11. Standard light sources and Standard illuminants • The appearance of color, depends on the light source. • Source is a physical emitter of light such as the sun or a lamp. • Illuminant refers to a specified spectral energy distribution. (Light is a form of Energy, in the spectrum of Electromagnetic radiation)
- 12. Fundamentals for Illuminants • Blackbody: A blackbody refers to an opaque object that emits thermal radiation. A perfect blackbody is one that absorbs all incoming light and does not reflect any. At room temperature, such an object would appear to be perfectly black (hence the term blackbody). • A blackbody is a theoretically ideal radiator and absorber of energy at all electromagnetic wavelengths.
- 14. Fundamentals for Illuminants • Black Body Radiation: The Energy emitted by a blackbody. Higher the Temperature shorter is the wave length.
- 16. Fundamental of Illuminants • Planckian Locus: In a CIE color space diagram, the plot of chromaticity coordinates of a blackbody radiator with temperatures from 1000 to 20,000 K is called Planckian locus. • Colors on this Locus between 2,000 to 20,000 are considered “white”, with 2,000 (warm white) being reddish white and 20,000 being blueish white. (Cool white)
- 17. Fundamental of Illuminants • Corelated Color Temepature (CCT): CCT describes the colour temperature of those white light sources (non blackbody emitters like LED and Fluorescent lamps) whose colours don’t fall exactly on the Planckian locus. • The CCT of a non-Planckian light source is the blackbody colour temperature that the source resembles most closely.
- 18. Standard illuminants • Standard Illuminant A: It represents Black body radiator at an absolute temperature 2856K. • Source A can be realised by gas-filled coiled tungsten filament lamp operating at a correlated color temperature of 2856 K.
- 19. Standard illuminants • Standard Illuminant B and C: B Represents Direct Sunlight with CCT 4874 K. • C Represents Average Sunlight with CCT 6774 K. • Neither B or C represents real daylight in the near UV region. • The amount of light of any one wavelength reaching the eye is proportional to the energy of the source multiplied by the reflectance factor
- 20. Standard illuminants • Illuminant D65 is based on measurements of the total daylight (i.e. sun plus sky) in a number of countries. • Except for times near sunrise and sunset, the relative spectral energy distribution generally corresponds to correlated colour temperatures between 6000 and 7000 K. • D illuminants – D65 (for textiles) – D50 (for graphic arts)
- 21. Standard Observer • Appearance of Color varies as per the observer. • 20 Observer • 100 Observer
- 22. Standard Illumination and viewing Conditions • The original CIE recommendation was that the sample should be illuminated at 45° to the surface and the light viewed normally, i.e. at right angles to the surface. • This mode can be represented 45/0. • It was assumed that the opposite mode (0/45) would give the same result, but this is not the case if the incident light is polarized
- 23. Standard Illumination and viewing Conditions • Four possible sets of conditions: • These are 45/0, 0/45, d/0 and 0/d. • In the third case (d/0) the sample is illuminated by diffuse light • In the last case (0/d) it is the light reflected at all angles is collected (using an integrating sphere, as in many spectrophotometers).
- 24. Standard Reflectance Factor • Instrument manufacturers supply calibrated white tiles with their instruments. Using these, corrected R values are obtained automatically.
- 25. Tristimulus Value and Color • Y tristimulus value should roughly represent the lightness of a sample, i.e. the higher the Y value, the lighter the sample appears. • Y = 80, ;Sample will appear light • Y = 3, the sample will look dark.
- 26. CIE System • The CIE tristimulus values for a sample are related to the colour of the sample, but ignore other important features such as surface texture, gloss,sheen, etc. • Thus a gloss paint sample and a matt paint sample might have the same tristimulus values, but obviously will not look the same.
Notas del editor
- The CIE system basically attempts to tell us how a colour might be reproduced (by a mixture of three primary light sources) rather than described. The amounts of the three primaries required to match a particular colour provide a numerical specification of that colour. A green colour corresponding to 546.1 nm could be produced even more easily. A mercury lamp emits light at only four wavelengths in the visible region (404.7 nm, 435.8 nm, 546.1 nm and 577.8 nm). By filtering out the other three, the required green wavelength could be obtained.
- It is important to realise that the colour of an object depends on the light source used to illuminate its surface, the particular observer who views it, as well as the properties of the surface itself. The nature of the surface is the most important factor.
- Note that there is no way in which the two colours interact with each other. Red and Green are single wavelengths, both the wavelengths reach our eye and are not interfered with in any way by each other.
- The only restriction in the choice of primaries being that it must not be possible to match any one of the primaries using a mixture of the other two. However, there is no set of real primary colours that can be used to match all colours using positive amounts of the primaries, i.e. there is no set of real primaries that will eliminate negative tristimulus values entirely.
- The only restriction in the choice of primaries being that it must not be possible to match any one of the primaries using a mixture of the other two. However, there is no set of real primary colours that can be used to match all colours using positive amounts of the primaries, i.e. there is no set of real primaries that will eliminate negative tristimulus values entirely.
- It is possible to calculate tristimulus values (i.e. the amounts of three primaries which, if additively mixed, would match a colour) of a sample specified. The CIE had to define standard primaries, standard light sources and a standard observer, together with standard observing and viewing conditions.
- Real colours can be matched using positive amounts of the chosen primaries (X), (Y) and (Z). 700 nm = Red 546.1 nm = Green 435.8 = Blue
- The amount of light reflected, and hence the appearance, depends on the light source. In practice, we use many different light sources, particularly various phases of daylight, and various types of fluorescent tube and tungsten light. Thus an illuminant can readily be specified, but may not be realisable in practice. In calculating tristimulus values from reflectance values, the tabulated energy distribution is used, but may be different from the actual distribution of the light source in the spectrophotometer.)
- This takes the form of an electromagnetic field having an intensity-versus-wavelength relation whose graph looks like a skewed, bell-shaped statistical curve. The maximum point on the curve shows the wavelength at which the radiation intensity is greatest. This wavelength depends on the thermodynamic temperature , in kelvin s, of the object. The higher the temperature, the shorter the wavelength at which the radiation is most intense
- This takes the form of an electromagnetic field having an intensity-versus-wavelength relation whose graph looks like a skewed, bell-shaped statistical curve. The maximum point on the curve shows the wavelength at which the radiation intensity is greatest. This wavelength depends on the thermodynamic temperature , in kelvin s, of the object. The higher the temperature, the shorter the wavelength at which the radiation is most intense
- In a CIE colour space diagram, the plot of the chromaticity coordinates of a blackbody radiator with temperatures from 1,000 to 20,000 Kelvin is called the Planckian locus. Colours on this locus in the range from about 2,000 to 20,000 K are considered to be “white”, with 2,000 K being reddish white (“warm white”) and 20,000 K being bluish white (“cool white”).
- Other, more energy efficient light sources – such as fluorescent or discharge lamps, or LEDs – are not blackbody or incandescent sources. Taking one example, LEDs emit light by a process called electroluminescence. v The chromaticity coordinates of the white light emitted by an LED will not necessarily fall directly on the Planckian locus in the colour space diagram. For those light sources, we should refer to them as having a correlated colour temperature (CCT) . CCT describes the colour temperature of those white light sources (non blackbody emitters) whose colours don’t fall exactly on the Planckian locus. The CCT of a non-Planckian light source is the blackbody colour temperature that the source resembles most closely. Correlated colour temperature is also reported in units of Kelvin (K).
- The amount of light reflected, and hence the appearance, depends on the light source. In practice, we use many different light sources, particularly various phases of daylight, and various types of fluorescent tube and tungsten light. Thus an illuminant can readily be specified, but may not be realisable in practice. In calculating tristimulus values from reflectance values, the tabulated energy distribution is used, but may be different from the actual distribution of the light source in the spectrophotometer.)
- Wright10 and Guild11 used visual tristimulus colourimeters in which onehalf of the field of view consisted of a mixture of (R), (G) and (B) primaries, while the colour in the other half was light of a single wavelength. To produce a match experimentally, it was necessary to add some of (R), (G) or (B) to the wavelength to be matched. Each used a somewhat different technique, and in particular different primaries were used. Both considered each wavelength throughout the visible spectrum and averaged results from a number of observers. The results differed from one observer to another (as expected), but when the average results from the two experiments were converted to a common set of primaries, the agreement was considered to be satisfactory. The results were expressed as the tristimulus values for an equal energy spectrum, i.e. using primaries (R), (G) and (B) the results were expressed as the amounts r¯, g¯ and b¯ required to match one unit of energy of each wavelength throughout the visible region. Since (R), (G) and (B) were real primaries, some of the values were negative. The CIE adopted three unreal primaries (X), (Y) and (Z) and the colour matching functions in terms of these primaries are denoted by x¯, y¯ and Z¯ and are always positive. This ensures that tristimulus values for all real colours are always positive.
- The original CIE recommendation was that the sample should be illuminated at 45° to the surface and the light viewed normally, i.e. at right angles to the surface. This mode can be represented 45/0. It was assumed that the opposite mode (0/45) would give the same result, but this is not the case if the incident light is polarized17. Four possible sets of conditions are now recommended. These are 45/0, 0/45, d/0 and 0/d. In the third case the sample is illuminated by diffuse light while in the last case the light reflected at all angles is collected (using an integrating sphere, as in many spectrophotometers).