SCREEN-TO-PRINT matching presents a special challenge to monitors. The industry-standard color temperature for lighting used to view printed pieces is 5,000 degrees Kelvin, known as D50. The challenge lies in the fact that D50 is not a native color temperature for most monitors -- that lies somewhere between 6,500 and 9,300 degrees Kelvin.
All monitor calibrators work basically the same way: They reduce the brightness of one or more of the electron guns in the monitor (usually the blue gun) to bring the color temperature down to D50. In doing so, they make the monitor lose brightness. The hard part in using a monitor at D50 is maintaining reasonable brightness. If the display isn't bright enough, white appears as a dingy yellow.
There are two approaches to achieving a D50 color temperature. The less common but superior way is to directly control the red, green, and blue electron guns in the monitor through voltage amplifiers. This technique is used in the RasterOps SuperScan Mc 21 and Mc 21HR -- both monitors have D50 as a preset color temperature. The Radius PressView monitors also have voltage amplifiers, but control is via a serial cable and the ProSense calibrator.
The more common way vendors achieve D50 is via the use of a lookup table that's stored inside the video circuitry's DAC (digital-to-analog converter). To reach D50, the vendor turns the blue channel down so that it has fewer possible shades. For our testing, we used Radius ThunderColor 30/1600 PCI graphics cards, which have three 10-bit DACs. The calibrator can bring the 10-bit DAC for the blue channel down while still delivering a full range of 256 shades. However, we found that only the Radius ProSense calibrator took advantage of the 10-bit DAC.
When we relied solely on the calibration kits provided by the vendors, only the two Radius monitors provided a truly accurate screen representation of the Matchprint proofs. The other systems all gave an overly optimistic view of what happens to a deep blue when it's printed, failing to show the purplish hue that results. This effect is caused by the routines Photoshop performs for color matching. When Photoshop displays CMYK files, it looks to its own separation tables to get device-independent Lab color equivalents for the CMYK colors and then it looks at the monitor's color gamut to figure out the RGB values that would represent those colors accurately on the monitor.
Thus, effective monitor calibration is only one of three components necessary for a good match between printed output and on-screen images when you're using Photoshop. You also need a way to report the monitor's color gamut to Photoshop's Monitor Setup preferences and an accurate separation table for converting the CMYK data to an accurate RGB representation for the monitor.
Of all the vendors whose products we tested, Radius was the only one that provided all three components of the color-calibration equation with Photoshop. We created several custom Photoshop separation tables, using a variety of third-party utilities, and came quite close to matching the Matchprint proofs on-screen. But we found that Radius' separation tables gave us a closer match, so we used them in our evaluations of screen-to-print matching to ensure that the only variable in our juried image-quality testing was the monitor calibration.
The ProSense calibrator is offered as a stand-alone product for $799 and comes with a set of preconfigured separation tables for monitors other than the Radius PressViews. Specific tables for various makes and models of monitors are available; you can also use the ProSense to create one.
The Radius ProSense calibrator, teamed with a Radius ThunderColor 30/1600 PCI graphics card, can vastly improve your color monitor's images at a D50 color temperature.
