I was looking up information about monitor calibration and came across the Spyder 3 Express, Pro, and Elite.
How does the Spyder 3 work to calibrate your monitor correctly?
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These hardware calibration devices work by running a piece of software on the computer that displays a series of colors and grey levels. By placing the spyder reading device on the computer monitor, it is able to "see" what the computer is displaying. By taking a series of measurements, a profile of the total system including video drivers and monitor quirks can be assessed. Once this profile is built, it is usually loaded into the OS so that the monitor then displays calibrated image colors.
The differences between the product line usually covers how many monitors can be calibrated for each computer to accommodate multi-screen setups.