BASICS OF COLOR SEPARATION
Converting continuous tone color into printing cylinders, printing plates- is called
COLOR SEPARATION
E.g. gravure printing press- prints one color and desired density in one printing unit
Innumerable hues, variations in color- converted into 4 color separations- later to printing cylinders-to print by primary ink pigments.
Four transparent printing inks- primary.
Overlapped- create secondary and overprint colors.
Yellow, magenta, cyan, black - for them must be made color separations.
Color separation: Traditional camera techniques- using filters to separate colors;
Electronic color scanner- filtered photomultiplier tubes.
Results are quantified-for future reference.
Current technology- high standards.

COLOR THEORY
Human perception- customer- subjective judgement.
All colors- contained in white light source- reflected from substrate.
4 variables in color reproduction: light source, colored object, human eye, human brain.

STANDARD WHITE LIGHT SOURCE
Light – mixture of all colors; variation in source can make the same subject appear different
Incident (projected); or reflected.
Ideal = average daylight; not available at different locations at different time of day.
Light- combination of all wavelengths of energy.
Light- travels at speed of 186,000 miles/sec.
Passing the light through glass prism- different wavelengths refract at different angles- rainbow of colors visible to eye is created.
Unit to measure the wavelength- nm nanometer 10-9 m.
Visible spectrum- 400-700 nm violet (blue)- to  red.
Light sources for viewing the color- white light sources- inbalance of hues.
White light – standardized- spectral energy distribution- balanced amount of red, blue and green light.
Industry standard- controlling spectral energy distribution = color rendering index = 90
2 different light sources conform to this standard- allow any copy to be viewed under the same conditions.
Color temperature- measured in degrees of Kelvin.
0oK= - 273.1 oC absolute zero - (temperature at which radiator would emit no light).
Many different light sources available - imperative to view or evaluate color reproduction under standard source.
Industry standard light source: 5000 oK: average daylight 10:00 am- 2:00 pm northern hemisphere.
Some gravure plants- Roto News- at 7500 oK- due to color absorption factor of substrate.
Current industry trend- use 5000 oK for all color reproduction work.
Quality control: GATF/RHEM indicator- patch with 2 alternating magenta strips- metameric in nature.When placed under standard 5000 oK light strips appear as 1 color- comprised of different mixes of color.
Non-standard illumination- 2 magenta strips appear as different colors.
Commercially available viewing booths- standard illumination.

COLORED PIGMENTATION
Colored substance- second variable ( original camera copy, color photograph, transparency)
Printed reproduction- ink pigment- way how it interacts with light.
Hue- appearance- comes from pigment absorption of white light.
Printing inks- 3 process colors- yellow, magenta, cyan - each absorb  1/3 of visible light spectrum.
Allowing residual 2/3 to be reflected
E.g.: yellow ink- absorbs blue, reflects red and green light- combine- create yellow to the eye
3 subtractive primary colors overlap- create black- total absorption- no reflection.
Black-referred to as a color- is not a color- rather : total absence of reflected light.
Why printers use black: 3 primary are not ideal- not able to absorb 100 % of light.
To overprint 3 process colors- over one another- produces muddy black-
Fourth color – black required- pure black- more economical, accurate.
 
ADDITIVE COLOR
Creation of color separation based not on behavior of pigments - but on behavior of light itself.
3 primary colors - red, blue and green - additive in value.
3 additive colors of light combined in equal % - form white light.

COLOR CORRECTION FOR COLOR IMPURITIES
Ink pigment- not perfect in absorbing 1/3 of visible spectrum , and perfectly reflect other 2/3.
Adjustment during separation-color correction.
Understanding ink deficiencies.
Yellow- best and purest pigment on the market.
Yellow would almost totally absorb the blue 1/3 of spectrum, reflect red and green.
Second purest- magenta- impurity is often yellow.
Least pure- cyan- appears to have magenta impurity- would have also yellow.
Major impurity- magenta, minor yellow.
Color being impure- pigment can still be chemically pure individual.
Color impurities in pigments cannot be removed- purpose of color correction- to reduce the amount of ink being printed in those regions of each separation where they overlap.
Yellow separation receives most color correction- yellow is already contained in magenta and cyan.
Process inks- vary from one to other manufacturer.
Color separators must know the ink/pigment impurities for each client before performing color correction.

THE HUMAN EYE
3rd  variable  in color sensation- reaction of human eye.
Retina of human eye-2 types of sensors- rods and cones.
Rods- reduced illumination – existing colors are transmitted as tonal gradation of achromatic vision.
Sensitivity of rods- at dusk- colored objects are silhouetted or gray.
Limited illumination restricts cones from sensing color.
Every individual perceives color differently- based on differences in 6 million cones in eye.
Cones- selective receptors of red, green or blue light or chromatic color.
Color-blindness- 8 % males, 1 % females; Green-red portion of the spectrum.

MENTAL ASPECTS OF COLOR THEORY
Physiological- psychological aspects –subjective part of color evaluation.
Human brain is not capable of remembering color or tone on absolute basis.
Densitometry, colorimetry- evaluation of color- verification of standards.
Person’s ability to view the color is affected by:
Angle of viewing (90 deg- best);
Angle of illumination- 45 deg- standard- or commercial viewing booth;
Distance of viewing- 14-18 in;
Texture of surface being viewed;
Glossy color images vs. flat color;
Size and shape of colored image;
Health, mental attitude.

FATIGUE AND STRAIN
Nerve cells- fatigued- ability to view the color- impaired.
Eye fatigue- long time looking at bright source or color hue.
Fatigue influences next color to be viewed.

SUBJECTIVE MEMORY COLORS
Memory color - key elements in image- blue sky, green grass, red apples-
Each individual recalls those.
Color separations may be rejected due to differences in remembered colors.
Geographical differences, individual background- play important role in the perception of color reproduction.

TRANSMITTED COLORS
Transparent copy- preferred medium for color separation.
Transparent dye, pigment.
Transparent copy- 35mm transparency, or larger transparency.
Transparency- first generation original- sharpest image resolution.
4x5, 5x7 or 8x10 in. transparencies- preferred (enlargement, retouching imperfections).

DYES AND TRANSPARENCIES
Use the same manufacturer’s duplicating film as the original transparency.
Emulsions sensitive to specific memory colors- color may shift at different manufacturer’s film.
Transparent retouching dyes recommended by film manufacturer (match by eye, but produces different spectral sensitivity in the camera).

REFLECTED COLOR
Reflection copy- most popular form of black and white copy.
Water color, line drawing,  color print, oil painting.
Reflection copy- less popular with color separators- often second generation copy- reproduction.
Original generation of color photograph- color negative.
Each new generation - shifts in color, increased grain, loss of image resolution.
Dye transfer = color photograph made from 3 dyed layers contacted in register.
Expensive, excellent reflection copy.

INK AND SUBSTRATE
All colors- on white paper.
Inks filter subtracting pure color from white light- combining the rest.
Each primary color - absorbs its complementary color from white light.
Paper - color cast - also absorbs color from incoming light and reflects the reminder.
White paper - balance of white reflection - warm whites, blue whites- in viewing booth.
The whiteness of paper measured by the degree of equal red, green, blue reflectance under standard illumination.
Brightness - reflectivity of substrate at 457 nm.

MEASUREMENT OF COLOR
Three dimensional phenomenon:
Hue, Saturation (Chroma), Lightness
Hue – actual color (adequately describes the difference between red, orange, blue, yellow..
Saturation – purity of a given hue- its freedom from gray (engine red- brick red = red hue)
Lightness – how light or dark the value is – degree of density

COLOR MEASUREMENT MODELS
Efforts- physical color models relied on human perception.
DIN, Ostwald, Munsell systems.
Most common colorimetric measurement is CIE
(Commision Internationale de l’Eclairage; International Commission on Illumination)
CIE defines standard light source, and average observer (average human sensitivity to red, green, blue)
Tristimulus values red = X, green = Y, blue = Z, changed  into chromaticity coordinates

x(hue) = X/(X+Y+Z)  y (chroma) = Y(green)/X+Y+Z

x, and y used to plot specific color on the chromaticity diagram

Three instruments for measuring the color: Spectrophotometer, Colorimeter, Densitometer

SPECTROPHOTOMETER
Reflected or transmitted light- measures along the electromagnetic spectrum
Instrument produces data as a spectral energy distribution curve –spectrometric curve
Measures: color of the ink
Used also for color matching- metameric color match – different light source- they look different
Spectrophotometer- identifies different spectrophotometric curves in metameric inks
- Illuminates the sample with essentially monochromatic light;
- Measures all the diffusely reflected light at each wavelength;
- Expresses the amount of light reflected as % of diffuse reflectance at the same wavelength by white reference standard.

COLORIMETER
Measurement of color that uses tristimulus reading similar to that of the human eye
Parallers the way the human eye uses red, green, blue sensitive cones to translate color vision

INK and COLOR
First-source of light.
Without light – there is no color to be seen.
Distinct interaction with light produce different color.
Color results from interaction of light and pigment.
Light missing – result-black.
Pigment is missing -  result is white.
Color and Light present: How they will be viewed?
Color receptors of eye accept the light reflected by broccoli, cranberry juice, orange.
Transform it to electrical impulses. Brain – accept it as color.

Color and Light
Light from sun appears to be white- composed of several colors –when in equal portions- appear white.
Light – radiant energy –travels in waves.
400-700 nm, wavelength determines the color.
E.g. beam of 430-450 nm – bright blue light.
630-650 nm - engine red.
Two beams overlap-result-lighter color (light being added to light).
Green, blue and red light combined:
Area of blue and red combination - magenta (Green missing)
Green and red combination -yellow (Blue missing)
Blue and green - cyan (Red missing )
Any color can be produced by combination of green, red and blue light: Additive primary colors

Pigments and Light
Subtractive system- description of interaction of light and pigment.
Chemical composition of pigments causes partial absorbtion of light.
Selective absorption causes that white light is disturbed and color is produced
All light is absorbed – black sensation.
All of the light is reflected - white sensation.
Pigment absorbs only blue and red: green is reflected – visual sensation is green.
Primary colors of the light – red, blue, and green.
Combination of equal amounts of two primaries - secondary color in additive system: Cyan, magenta, yellow.