Appunti per l'esame di Printing / Tecniche di Stampa - in inglese

Layout with CMYK elements and hotfoil stamping

Hot foil stamping techniques

• Hot foil in overprinting
• Hot foil as spot colors

Color management system

The goal is to match the color the designer wants with what is reproduced by the pre-press area, what the press gets, and what is visible on paper.

Color fundamentals

• The perception/interpretation of color is subjective and our viewing conditions can modify how we see a color.
• Measurement instrument:
  • Assigns a number to a specific color.
  • Most reliable one if we want to communicate which color it is.
• Color consists of three elements: hue, chroma (saturation), and value (lightness). With them, you can easily distinguish one color from another.
• Hue is how we perceive an object's color (each color of the color wheel is a hue).
• Chroma/Saturation describes how vivid (pure hue) or dull (close to grey) a color is.
• Value/Lightness describes the degree of lightness (is this color light or dark?).

Scales for measuring color

The most accurate way to measure color is through "color by numbers" techniques:

• Munsell scale:
  • Based on human perception.
  • Created before the availability of measuring and specifying color instruments.
  • Assigns numerical values to hue, chroma, and value.
  • Be careful with the light source (illuminant), object (sample), and observer/processor.
• CIE Color System:
  • International Commission on Illumination.
  • Responsible for international recommendations for photometry and colorimetry.
  • Uses three coordinates to locate a color in a color space: CIE XYZ, CIE L*a*b*, CIE L*C*h°.
  • Concept of a standard observer based on the average human response to wavelengths of light.
• CIE Color Systems - Standard Light Source (illuminants):
  • A mathematical representation of a theoretical light source.
  • Six types: CIE Standard Illuminant A, CIE Standard Illuminant D65, CIE Illuminant D50, CIE Illuminant F2 (CWF-2), CIE Illuminant F11 (NBF-11), Illuminant TL84.
• Color temperature:
  • A way to describe the light appearance provided by a light bulb.
  • Measured in Kelvin (K) on a scale from 1,000 to 10,000.
• Tristimulus values:
  • When the values are calculated, they’re converted into the tristimulus values of XYZ, providing the identification of the color numerically.

Expressing colors numerically

CIELAB: L* is lightness, a* is red/green value, b* is yellow/blue value.

From CIELAB to RGB and CMYK

• RGB color model:
  • An additive color model in which Red, Green, and Blue lights are added together in various ways to reproduce all the other shades of color.
  • Main purpose: for the sensing, representation, and display of images in electronic systems.
  • It's a device-dependent color model, therefore an RGB value does not define the same color across devices without some kind of color management.
• Additive synthesis:
  • It’s the mixing mechanism of primary colors to obtain other colors.
  • Based on the sum of light tending to white.
• Pigments/colored objects:
  • When struck by light, they only reflect the light corresponding to “their color” while absorbing all the remaining light.
• Subtractive synthesis:
  • It intervenes in the observation of color.
  • The coloration of things implies the existence of subtractive mechanisms, since it is based on their ability to absorb chromatic components of the light that illuminates, rather than emitting its own components.
  • The variation of the three primary colors (and in printing the black) percentage produces all the shades in between.
  • Remember that when we perceive the color, we are exposed to a determined source of light that can modify the color itself.
• CMYK numbers:
  • No precision in the identification of colors but a certain approximation.
  • RGB color method: same approximation as CMYK.
  • Every device displays RGB in its own way because RGB or CMYK values don’t describe the color from a qualitative but a quantitative point of view.

A color profile

What is:

• It’s a reference file or chart that each color device (camera, video, etc.) has.
• Made by L*a*b* representation that matches RGB or CMYK values and the corresponding Lab values (absolute).
• Takes its name from the ICC (International Color Consortium).
• It's the basis of color management technology.
• From www.colour.org: “The purpose of the ICC is to promote the use and adoption of open, vendor-neutral, cross-platform color management systems.”
• An ICC profile represents the chromatic response of a certain device in synthesis (RGB or CMYK) under specific use conditions.
• Any RGB or CMYK device can have a color profile.
• Some profiles have become recognized standards by the market or international institutions, for example:
  • sRGB: the most common RGB profile, used by non-professional cameras, smartphones, displays.
  • AdobeRGB: commonly used in the graphic and photographic workflows together with sRGB.
  • DCI-P3: Digital Cinema Initiatives.