What Is Colour A

Lesson 3 What Is Colour

Color enriches our lives as a natural, visual experience. Understanding color can help you use it more effectively.

To see color, three essential elements must be present: light, an illuminated object, and an observer.

The colors we see are affected by the intensity of light and by its spectral content. At low levels of illumination, objects are less colorful. In bright daylight, we see more color, contrast, and saturation.

The color spectrum shows the range of wavelengths of energy that are visible to the human eye. Variation in wavelengths alters the colors we see.

As Isaac Newton showed with his prisms, white light is a mixture of all the colors of the visible spectrum.

Actually, light sources we think of as "white" differ in spectral distribution. Skylight is a bluish white. Tungsten light bulbs are yellowish white. For critical color evaluation, special standardized light sources are used to avoid distorting colors.

To avoid distortions, standard viewing conditions employing special light sources are widely used to evaluate color proofs in the printing and publishing industry.

Light sources vary in color temperature. When you set up a computer monitor, you can adjust the "white point," which is its color temperature. If a television monitor will be used to display the images you create on the computer, you would choose a white point of 6500. For printed reproduction, a white point of 5000 will match standard viewing conditions.

Before light reaches our eyes, it is modified by colorants in the objects we see. Colorants used in reproducing images are pigments and dyes

When light strikes an object, wavelengths may be reflected, absorbed or transmitted. Colorants can be mixed to control the wavelengths and colors we see.

Densitometers measure the amount of light reflected or transmitted. They are useful quality control tools for consistent color reproduction.

Light waves that reach the eye stimulate a visual process so complex it's not yet fully understood. Within the retina, cones respond to color hues and brightness. Rods sense only brightness. Three types of cones respond to wavelengths in ways that produce all the colors we see.

The human eye is an excellent judge of color in side-by-side comparisons. We can see differences that are difficult to measure especially among lighter colors.

The eye comes equipped with an automatic color balance feature called "chromatic adaptation." It adjusts to overall color shifts, like those produced by different light sources

We can demonstrate chromatic adaptation right before your eyes. Watch the yellow towel. Let's cover the towel with a cyan filter and see what happens.

The filter has a rough edge so you can see it. When covered by cyan, the towel appears green, as you might expect. Now, lets cover the entire scene with that same cyan filter

Now, the towel looks yellow again! The eye tends to cancel out overall color shifts using white as the strongest reference point

Color perception is also influenced by tones and colors surrounding an image. The color patches on the left and right are the same.

Observers may also suffer from deficiencies in color vision. Six to eight percent of men and a few women have some degree of color deficiency.

Generally, we human observers outperform any instrument in judging color ... because we see color in context through the unique psycho-physical process called perception.

Color can be defined by three properties: hue, saturation, and lightness or brightness.

When we call an object "red," we are referring to its hue. Hue is determined by the dominant wavelength

The saturation of a color ranges from neutral to brilliant. The circle on the right is a more vivid red than the circle on the left although both have the same hue

Lightness or brightness refers to the amount of light the color reflects or transmits

Color ordering systems, such as the Munsell System, use the three properties of color to identify unique colors. Notice that colors are distributed in three dimensions.

We commonly see colors arrayed in two dimensions. This is a useful, but incomplete representation.

Colors actually occupy a three-dimensional space.

Lightness is the third dimension that is not shown in color wheels often used in image editing software.

Scientific color values were established earlier this century by the CIE group. CIE models for defining color space all rely on the same basic numbers.

CIE values were determined by testing human observers. These perception-based values can be used to measure and compare colors produced by different methods of reproduction.

The CIE chromaticity diagram is one way to plot the colors the human eye can see.

A color can be specified by its colorimetric values. A colorimeter is an instrument that measures color using numbers derived from CIE values.

A spectrophotometer is another instrument for measuring color. It samples wavelengths across the color spectrum.

Measuring color allows us to compare the color gamut, or range of colors produced by different methods. We find that color transparency film produces a wide range of colors including some a monitor cannot display.

Digital color printers and printing presses have different color gamuts. They can never capture all the colors in an original transparency, but they can simulate the appearance very successfully if color reproduction is understood and controlled. Color management software, described later, helps transform color from one gamut to another.

About the different color modes

Color models describe colors numerically. There are different methods of describing colors numerically, and a color mode determines which method or set of numbers to use to display and print an image. Photoshop bases its color modes on the color models that are useful for images used in publishing. You can choose from RGB (red, green, blue); CMYK (cyan, magenta, yellow, black); Lab Color (based on CIE L*a*b*), and Grayscale. Photoshop also includes modes for specialized color output such as Indexed Color and Duotone.

Note: ImageReady only uses the RGB mode to work with images, since its documents are primarily intended for the Web. For more information about color models

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.

People see only a small part of the energy spectrum. 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.

For example, the RGB color model has many RGB color spaces, such as ColorMatch, Adobe RGB, sRGB, and ProPhoto RGB. You can take the same RGB values (R=220, B=230, and G=5) and assign profiles that describe different RGB color spaces. The color will look different in each color space, but the numeric values and model will still be the same.

Photoshop uses color modes (similar to a color model) that let you work with an image in a specific color space. Photoshop keeps track of an image's color space and will indicate in the title bar if the working space and the document's color space don't match.

Photoshop accommodates the color models that are most suitable for photographic images or graphics. Color models used in Photoshop are CMYK, HSB, RGB, and Lab.

RGB Color mode

Photoshop's RGB Color mode uses the RGB model, assigning an intensity value to each pixel ranging from 0 (black) to 255 (white) for each of the RGB (red, green, blue) components in a color image. For example, a bright red color might have an R value of 246, a G value of 20, and a B value of 50. When the values of all three components are equal, the result is a shade of neutral gray. When the value of all components are 255, the result is pure white; when the values are 0, pure black.

RGB images use three colors, or channels, to reproduce colors on-screen. The three channels translate to 24 (8 bits x 3 channels) bits of color information per pixel. With 24-bit images, up to 16.7 million colors can be reproduced. With 48-bit images (16 bits per channel), even more colors can be reproduced. In addition to being the default mode for new Photoshop images, the RGB model is used by computer monitors to display colors. This means that when working in color modes other than RGB, such as CMYK, Photoshop interpolates the CMYK image to RGB for display on-screen.

Although RGB is a standard color model, the exact range of colors represented can vary, depending on the application or display device. Photoshop's RGB Color mode varies according to the working space setting that you have specified in the Color Settings dialog box.

About working spaces

A working space is the default profile for an image while you're editing it in Photoshop. It defines the color space in which you are editing an image.

Lab

Cannot be selected as a working space in the Color Settings dialog box. If you choose to edit a document in Lab mode, the document is converted to your monitor's color space prior to viewing and you will have limited editing capabilities. Because Lab encompasses the complete range of visible colors, for practical reasons it is better to work in color spaces (such as RGB and CMYK) that are smaller and more in line with the color spaces of devices that capture, display, or output images.

RGB working spaces

Are based on the RGB color model. Some RGB working space options are device-dependent (such as a monitor-profile-based working space) and some are device-independent (such as Adobe RGB, Apple RGB, and sRGB). It's probably best to use a device-independent working space for most image editing.

CMYK working spaces

Are device-dependent. While an RGB working space can be device-independent, CMYK working spaces are based on actual combinations of ink and paper. It's often best to use an RGB working space for editing your image in Photoshop and then use the appropriate CMYK profile to convert your RGB image to CMYK in preparation for printing. The data in the CMYK working space is the data that will be used to convert the image to CMYK. For more information on RGB and CMYK working spaces, see Choosing an RGB working space and Choosing a CMYK working space. Photoshop gives you the option of embedding the profile of your working space when you save an image. Embedding a profile is recommended because it tells Photoshop and other color-management-savvy applications the color meaning of the numeric values in the document.

Adobe RGB and sRGB are smaller color spaces than Lab and provide a practical working space to edit and output images. A. Adobe RGB (1998) B. sRGB

In addition to determining the number of colors that can be displayed in an image, color modes affect the number of channels and the file size of an image. CMYK Color mode

In Photoshop's CMYK mode, each pixel is assigned a percentage value for each of the process inks. The lightest (highlight) colors are assigned small percentages of process ink colors, the darker (shadow) colors higher percentages. For example, a bright red might contain 2% cyan, 93% magenta, 90% yellow, and 0% black. In CMYK images, pure white is generated when all four components have values of 0%.

Use the CMYK mode when preparing an image to be printed using process colors. Converting an RGB image into CMYK creates a color separation. If you start with an RGB image, it's best to edit first in RGB and then convert to CMYK at the end of your process. In RGB mode, you can use the Proof Setup commands to simulate the effects of a CMYK conversion without changing the actual image data. You can also use CMYK mode to work directly with CMYK images scanned or imported from high-end systems. Although CMYK is a standard color model, the exact range of colors represented can vary, depending on the press and printing conditions. Photoshop's CMYK Color mode varies according to the working space setting that you have specified in the Color Settings dialog box.

Lab Color mode

In Photoshop, the Lab Color mode has a lightness component (L) that can range from 0 to 100. In the Adobe Color Picker, the a component (green-red axis) and the b component (blue-yellow axis) can range from +127 to -128. In the Color palette, the a component and the b component can range from +120 to -120.

You can use Lab mode to work with Photo CD images, edit the luminance and the color values in an image independently, move images between systems, and print to PostScript Level 2 and Level 3 printers. To print Lab images to other color PostScript devices, convert to CMYK first.

Lab images can be saved in Photoshop, Photoshop EPS, Large Document Format (PSB), PDF, Photoshop Raw, TIFF, Photoshop DCS 1.0, or Photoshop DCS 2.0 formats. 48-bit (16 bits per channel) Lab images can be saved in Photoshop, Large Document Format (PSB), Photoshop PDF, Photoshop Raw, or TIFF formats. Note: The DCS 1.0 and DCS 2.0 formats convert the file to CMYK when opened.

Lab color is the intermediate color model Photoshop uses when converting from one color mode to another.

Bitmap mode

This mode uses one of two color values (black or white) to represent the pixels in an image. Images in Bitmap mode are called bitmapped 1-bit images because they have a bit depth of 1.