The main purpose of calibrating is to set white and black points, contrast, brightness, and gamma (mid tone density). Since monitors differ from one to the next (even same brand and models), no two will respond in exactly the same way. The older your monitor is, the more likely it will lessen in both brightness and clarity. For color critical work, most monitors are dependable up to only two years. Some are better. Some are worse.
You will have to be the judge. Calibrating your monitor is very important for color critical work. Do this a minimum of once a week. Even high-end soft proof workstations require frequent re-calibrations. You will need software to calibrate your monitor. Adobe Gamma (supplied with the Windows version of Photoshop) and Monitor Calibrator (Mac OS only) are simple to use. Both programs have "wizards" that can guide you, step by step, through the process. There are also a variety of more sophisticated software that can be purchased from third party developers, as well as high-end software that is included with the purchase of a monitor that is specially designed for color critical applications.
If you are unsure about the accuracy of colors as seen on your screen or on your desktop printer versus what will come off the press, please request a hardcopy color-proof from your us. This may be an added expense, but will give you a better representation on how the colors may print on press.
Please remember that all color is relative to the computer screen you are using, the lighting conditions, the display settings on your computer, the paper you are printing on, etc.
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