Monitors and scanners work with light and add the components - namely red, green and blue - to form various colours. Each person's perception of these colours may be slightly different. Because we talk about the components a lot they are abbreviated to RGB. Now, printers actually separate a colour back out into its components and generally use cyan, magenta, yellow and black ie CMYK. The two methods do not have the same colour ranges. If we want to print something from a computer we need to convert from the RGB system to the CMYK system. So, there is a lot more goes on than simply transferring data. The problem comes in matching the screen and the printed output.
A scanner has no idea what the characteristics of your monitor are (ie brightness, contrast, colour temperature settings). Before you start making adjustments to the colours on the screen for an image you need to ensure your monitor is set up correctly by following the advice given in the monitors section of this site. Otherwise things are not going to look right on paper. Bear in mind that CRT monitors can display better levels of detail in shadows and highlights than a TFT screen.
If we employ a colour management system (CMS) we can calibrate each component so that what you see on screen is the same as on the printer - or at least very close. The system should warn you if you try to reproduce a colour that the printer can't cope with.
The International Colour Consortium (ICC) has produced a standard to describe the colour characteristics of any device whether it's a scanner, monitor, printer etc. The ICC profile plugs into management software which should make the relevant adjustments. The profile is installed with the appropriate device. If you have more than one printer you should be able to view on your monitor what the image will look like on a different printer. As far as the Windows operating system goes colour management is available under NT and Windows 98 or later.
Gamma relates to the number of colour values a monitor, scanner or printer can display. By contrast, the gamma of an image is a measure of its contrast and brightness. By altering the gamma of an image these two values can be changed together simultaneously. Graphics software with this option usually allows you to alter all the colour channels at once or each to have individual values. If an image appears too dark or too bright altering the gamma can bring a marked improvement. Again, if an image has a colour cast then altering the gamma for a particular colour (eg red) will allow you to reduce it. A small change in brightness on a monitor at a low brightness level is not equal to the same change at a high level. Gamma correction compensates for the inequality.
The idea is that the red, green and blue on screen is consistent from image to image and that it matches as closely as possible the printed image. The problem is that the phosphors on a screen do not excite equally so imbalances in the brightness and contrast as well as the colour balance can occur. So, to compensate decent graphics software allows you to create or alter an overall gamma setting for your display. So, there are two different gamma settings: that of the monitor and that of the image itself.
For a monitor there is an optimum setting whereby two grey squares appear near enough equal. In addition, the software may allow you to control red, yellow and green. Instructions are provided on screen and with a help file. Bear in mind you are lighting pixels. At the brightest levels the pixel will be white, at its darkest black. These equate to the highest and lowest colour values - however we choose to describe the image or its component pixels. Consequently each brightness level can be given a number and therefore represent a shade of colour. The more levels we use the more shades we can represent and therefore make our images more realistic. In practice an infinite number of levels cannot be perceived by the human eye so we stop at around 16 million colours.
Graphics Intro Scanning File Formats 1 File Formats (cont) More drawing
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