Color & Color Management

Overview

 I.

Color Perception  II.

Definition & characteristics of color Color Representation  III.

RGB, CMYK, XYZ, Lab Color Management Purpose, ICC profile, Calibration, Workflow

What color is an apple??

Color

 Color is a sensation formed by a combination of light source, modifier, and detector

Same object looks different in different light sources

Light as a Wave

 The physical form of light is both wave and particle(photon)  There are no colors in the electromagnetic wave but amplitude, wavelength and frequency.

 Some lights absorb to the surface, some reflect back.

 Color of an object is determine by the wavelength of a light reflected(emitted) from the surface of the object

Color Temperature

 A black body when heated to different temperatures emits certain colors.

 Color temperature is the correlated temperature of the black body.

Visual Perception

 Two types of photo receptors ( Rods and Cones ) are detecting chromaticity & luminance of an object.

Characteristics of Color

 Hue the real proper name for color the property or attribute of color(chroma) as it is perceived and determined by the wavelength of light

Characteristics of Color

 Saturation - the degree to which vividness ( or dullness of a color) a perception of a hue’s purity colorfulness of an area judged in proportion to its brightness

Characteristics of Color

 Brightness/Lightness intensity of light that range from a totally dark black or a luminous white.

the degree of brightness can be quantized with value or level highlight, mid ton, shadow ** Contrast is the range from the darkest regions of the image to the lightest regions.

** By using levels and histogram in Photoshop, you can adjust brightness and contrast.

Color Mixing

Color can be perceived either from a luminous object (TV, Monitor, Projector)or reflected from pigment or ink( Printouts).

 Additive approach Subtractive approach

Color Mixing

  Additive approach mixing lights creating secondary colors by mixing portions of 3 primary (red, green, blue) colored light.

Mixing equal amount of light in each primary color creates white light.

Subtractive approach Mixing pigment or paint.

3 primary color : Cyan, Magenta, Yellow Each ink absorbs or subtracts light of all colors except the ink’s own color which is reflected back to the viewer Practically, additional black color is need to complement pure blackness.

Color Representation

 Color Space(model) • A Color space( or model) is a way of representing colors and their relationship to each other. • • Different image processing systems use different color models ( color picture processing industry – CMY, computer monitor – RGB..) Most of color model is based on ‘tristimulus’ values ( based on 3 numbers ).

• Type : RGB, CMY, CMYK, HIS, HSV, XYZ, Lab, YCbCr, YUV, UVW….

• Color space is represented in a way a range of colors arranged in a 3D coordinate system. • • • Color gamut - the range of all the possible colors.

Device dependent - RGB, CMYK Device Independent - XYZ, Lab

Device dependent color models

 RGB and CMYK models are geared toward their use with hardware, such as computer monitor and printing presses.

 Colors in the same image in different devices look different because of the different physical attributes of the devices.

 There needs color management

RGB color model

 consists of the three additive primaries :Red, Green, and Blue  additive color model ( red + green = yellow)  used for computer monitor and many computer graphic system.

CMYK color model

 consists of the three subtractive color primaries ( cyan, magenta, yellow ) and black  used for printing purposes  subtractive color model ( red + green = brown)

Relationship between RGB and CMYK space

 CMYK space is a complement of RGB color space ( C,M,Y color are complementary colors of R, G, B color ) Cyan = Green + Blue = -Red Magenta = Red + Blue = - Green Yellow = Red + Green = -Blue BlacK = - (Red+Blue+Green) = - White = BLACK

Device Independent Color models

 XYZ Color Space  In 1931, the Commission Internationale de l’Eclairage (CIE) developed a light measurement standard.  international standard for device independent color model.

 any color can be precisely defined and matched.

 there are mutational color space sfrom this standard ( sRGB, Adobe RGB, Colormatch RGB, Bruce RGB..)

Device Independent Color models

 Lab color space  developed by the need of uniformed color space  L ( luminance ), a ( colors from red to green), b ( colors from blue to yellow )

Color Management

 Purpose of color management  All hardware devices reproduce color slightly differently, thus hardware must be calibrated and an accurate profile created.

 Color management is a task taken to get one device to accurately represent the color of another device.

Profile

 A profile is a set of mathematical measurements that defines the reproduction of colour for a particular device.

 Color management is the translation of an image file from one colour space to another using the profiles associated with the devices within that workflow.  'International Color Consortium' (ICC) profile is the industry standard for profile description.

Color Management System

 A software where you can calibrate and profiling your input/output devices.

 Monitor : Display calibrator(MAC), Adobe Gamma (PC)

Profile Tagging

 Embedding color profile in an image file to manage colors situated in different devices.

 Tagging monitor profile may degrade image quality as the file passes several systems.

 Thus tagging pre defined ‘working space’ RGB profile is desired.

 Monitor profile determines how the image displays on the screen, while the working color space determines the actual RGB color data of the image.

 sRGB for web graphics, Adobe RGB for most RGB & CMYK work, Wide Gamut RGB for most large range of colors.

Profile Tagging

Color Management Workflow

    Calibrate monitor Choose a working space Calibrating scanner Calibrating printer

References & Image Resources

 http://www.hf.faa.gov/Webtraining/VisualDisplays/Human VisSys2c5.htm

 http://www.adobe.co.jp/support/techguides/color/colormo dels/   Adobe CS online help Digital Imaging by Mark Galer and Les Horvat Focal Press © 2001  A simplified approach to Image Processing by Randy Crane, Prentice Hall 1997