Understanding Colour Colour Models Dr Jimmy Lam Tutorial from Adobe Photoshop CS

Using color models to describe color (1) We see only a small part of the electromagnetic spectrum. This small part is often called the visible spectrum. We see all light because light is defined as being that part of the electromagnetic spectrum that we can see. Color models attempt to describe the colors we see and work with. Each color model represents a different method for describing and classifying color. All color models use numeric values to represent the visible spectrum of color

Using color models to describe color (2) The range of colors that can be produced using a particular color model, such as RGB or CMYK, is a color space. Other color models are HSL, HSB, Lab, and XYZ. A color model determines the relationship between values, and the color space defines the absolute meaning of those values as colors. Some color models have a fixed color space (such as Lab and XYZ) because they relate directly to the way humans perceive color. These models are described as being device independent. Other color models (RGB, HSL, HSB, CMYK, and so forth) can have many different color spaces. Because these models vary with each associated color space or device, they are described as being device dependent.

Model 1: HSB model HSB is a color model that is somewhat analogous to Munsell's system of hue, value, and chroma in that it uses three similar axes to define a color. HSB is derived from RGB color spaces and is a device dependent color space. In HSB, the three fundamental characteristics of color are: Hue is the color reflected from or transmitted through an object. It is measured as a location on the standard color wheel, expressed as a degree between 0° and 360°. In common use, hue is identified by the name of the color such as red, orange, or green. To view the color wheel, see Using the color wheel.Using the color wheel Saturation, sometimes called chroma, is the strength or purity of the color. Saturation represents the amount of gray in proportion to the hue, measured as a percentage from 0% (gray) to 100% (fully saturated). On the standard color wheel, saturation increases from the center to the edge. Brightness is the relative lightness or darkness of the color, usually measured as a percentage from 0% (black) to 100% (white

Model 2: RGB model A large percentage of the visible spectrum can be represented by mixing red, green, and blue colored light in various proportions and intensities. Where the colors overlap, they create secondary colors--cyan, magenta, yellow--and white. RGB colors are also called additive colors. Additive colors are created by mixing spectral light in varying combinations. Adding all colors together creates white--that is, all visible wavelengths are transmitted back to the eye. Additive colors are used for lighting, video, and monitors. Your monitor, for example, creates color by emitting light through red, green, and blue phosphors.

Model 3: CMYK model The CMYK model is based on the light- absorbing quality of ink printed on paper. As white light strikes translucent inks, certain visible wavelengths are absorbed (subtracted), while others are reflected back to your eyes. For this reason, these colors are called subtractive colors. In theory, pure cyan (C), magenta (M), and yellow (Y) pigments should combine to absorb all light and produce black. Because all printing inks contain some impurities, however, these three inks actually produce a muddy brown. For this reason, black ink (K) is used in addition to the cyan, magenta, and yellow inks in four-color printing. (K is used for black instead of B to avoid confusion with blue.)

Model 4: Lab model The CIE L*a*b* color model (Lab) is based on the human perception of color. It is one of several color models produced by the Commission Internationale d'Eclairage (CIE), an organization dedicated to creating standards for all aspects of light. The numeric values in Lab describe all the colors that a person with normal vision sees. Because Lab describes how a color looks rather than how much of a particular colorant is needed for a device (such as a monitor, desktop printer, or digital camera) to produce colors, Lab is considered to be a device-independent color model. Color management systems use Lab as a color reference to predictably transform a color from one color space to another color space. Lab describes color in terms of luminance or its lightness component (L) and two chromatic components: the a component (green and red) and the b component (blue and yellow).

Why colors sometimes don't match The colors in an image change as you bring the image in from a scanner or digital camera and view it on your computer monitor. The colors change again when you send the image out to be printed on your desktop printer or printing press. This is because every device--a digital camera, scanner, computer monitor, desktop printer, printing press--operates within a different color space. Incorrect color transformation or lack of color transformation from one color space to another causes color inconsistency. Correctly transforming the color values creates color consistency.

Understanding color space and gamut A color space is a range of colors in the visible spectrum. A color space can also be a variant of a color model. Adobe RGB, Apple RGB, and sRGB are examples of different color spaces based on the same color model The gamuts of different color spaces A. Lab color space encompasses all visible colors B. RGB color space C. CMYK color space

Understanding color space and gamut (II) The range of color encompassed by a color space is called a gamut. The different devices (computer monitor, scanner, desktop printer, printing press, digital camera) throughout your workflow operate within different color spaces and each with different gamuts. Some colors within the gamut of your computer monitor are not within the gamut of your inkjet printer, and vice versa. When a color cannot be produced on a device, it's considered to be outside the color space of that particular device. In other words, the color is out of gamut.

Understanding color space and gamut (III) The color gamuts of various devices and documents A.Lab color space (entire visible spectrum) B. Documents (working space) C. Devices

Why each device has a different color space The devices throughout your workflow use different methods to produce color. These methods are often based on different color models and as a result, the devices operate in different color spaces. Your computer monitor produces color with light and operates in an RGB color space. Your desktop printer produces colors with inks or dyes that work in a CMYK color space. Some colors can be viewed on a computer monitor that cannot be printed on desktop printer or printing press. Likewise, some colors can be produced with inks that cannot be displayed on a monitor. Even when two devices use the same color model to produce color, they will have different color spaces. For example, a CRT monitor and an LCD monitor both use RGB to produce colors. However, because both use different means to display color, the range of color (color space) of both monitors would be different. The CRT monitor would display a specific red value differently than the LCD monitor. Even two monitors of the same brand and model frequently display the same color differently. It's nearly impossible for two devices to be completely identical, given the limitations of manufacturing and materials. Go to an electronics store and look at a wall of television monitors. Each monitor displays colors differently.

Device-dependent color models RGB and CMYK are device-dependent color models. This is because the colors produced using these models depend on the device that is producing the color. For example, the RGB value of 255,0,0 is a formula for red. However, the red displayed using this formula on one computer monitor will be different than the red displayed on another monitor. Each monitor interprets the formula for red according to its own color space

Device-independent color models Device-independent color models describe absolute colors instead of formulas whose resulting colors depend on a device's interpretation of its numeric values. Lab is a device-independent color model and is used by many color management systems as a color reference. It enables the color management system to translate colors from the color space of one device to another with the best possible accuracy

How color management works A color management system reconciles the differences between the color spaces of different devices. It translates the RGB or CMYK values in a document so that the colors will be represented as consistently as possible on different devices.

How color management works (2) Managing color with profiles: A. Profiles describe the color spaces of the input device and the document. B. Using the profiles' descriptions, the color management system identifies the document's actual colors. C. The monitor's profile tells the color management system how to translate the numeric values to the monitor's color space. D. Using the output device's profile, the color management system translates the document's numeric values to the color space of the output device so the actual colors are printed

Profiles Describe the color spaces of the different devices and also the color space of a document. A color management system needs separate profiles for each device and a profile embedded in a document. A device profile does not change the color values in a document, it tells the color management system how a device interprets the color values in a document. A document's embedded profile defines the documents' working space if you preserve the profile when opening it in Photoshop. Photoshop also lets you convert or discard the embedded profile, which uses the working space specified in the color settings. The working space tells the color management system what the RGB or CMYK values mean as you work on the document in Photoshop

Calibrating and creating profiles The color management system uses profiles to know how a device produces color and what the actual colors in a document are. Device profiles are often installed when a device is added to your system. The accuracy of these profiles (often called generic profiles or canned profiles) varies from manufacturer to manufacturer. Third-party software and hardware can also create device profiles (often called custom profiles).

Calibrating and creating profiles (3) Color management system using profiles to correctly transform color from one color space to another A.Profile describing the meaning of the RGB values in the document B.Color management system using a color reference (Lab) to identify the actual colors C.Output profile (destination profile) describing the device's color space so the RGB values are translated to maintain consistent colors when printed in CMYK

Discussion Discuss the FOUR colour models in CAD system What is Colour Management System?