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This article was written by Bruce L. Myers, Ph.D. and was provided by X-Rite, Inc.
The instructions below are based on an instrument in the default configuration. If you are not sure, you can reset factory defaults by pressing the up, down and "X" keys simultaneously. When released, choose "yes" when "load defaults" appears.
Caution: Resetting the instrument will bring the instrument to its new configuration, as such any standards, n-factors, and custom settings stored on the instrument will be erased.
SINGLE COLOR MODE:
To measure plates, it is important that the instrument be "locked in" to the image color of the plates that you are using. Most plates have green/blue image, therefore you would need to put your 500 series instrument in "single color" mode with "cyan" chosen. Other plates might have a reddish or dark gray image. If you are not sure, take a "density" reading of a 100% image area on the plate. The color selected by the Densitometer is the color that should be used for "single color" mode.
To set the instrument in "single color" mode, press the "X" key to go to the main menu. Press the down arrow until "Dot Area" is highlighted. Press the up arrow until "Options" is highlighted, and then press the "Enter" key. The words Color:Auto should be selected at the top of the "Dot Options" menu. Press the "Enter" key. Press the down arrow to select the color that you want to lock the instrument in on (typically, cyan). Press the "Enter" key. Press the "Previous" key (middle key) to return to "Dot Area" mode.
You are now set up to read dot in a single color mode, which is appropriate for plate measurement. When GATF tested these instruments to measure plates, this was the only adjustment that they made to the factory default options and the 518's performed exceedingly well. For the ultimate in accuracy, you might want to adjust the dot area equation using an "N" factor as described below.
Remember to follow the above steps to go back to "Auto" mode for press sheet and proof measurements.
50% CAL – "N" FACTOR:
For the ultimate in accuracy, you might want to adjust the dot area equation to read a known 50% dot, using something called an "N" factor. This method would require a reference plate from the manufacturer of the plates, or a recommended "N" factor from that manufacturer.
To set the "N" factor from a reference plate, in "Dot Area" mode press the "Up" arrow until "Options" is highlighted, and then press the "Enter" key. Press the "Down" arrow until "50% CAL" is highlighted, and then press the "Enter" key. You will now be in "50% CAL" mode, with "Paper" highlighted. Read the base (non-image) area of the reference plate. Now, "Solid" should be highlighted. Read the 100% image area of the reference plate. Now, "Tint" should be highlighted. Read the known 50% image on the reference plate. The "N" factor and "Save N" will be highlighted. Press the "Enter" key to save the "N" factor, and then press the "Previous" key to return to dot mode.
In some instances, a plate manufacturer may give you the "N" factor. In this case, from "Dot Area Options" press the down arrow until "n Factor" is selected. Press the "Enter" key, and use the "Enter", "Up" and "Down" keys to set the desired "N" factor.
Notice that below the numbers there is either the word "On" or "Off" above "Save & Exit". When you are using the "N" factor, you want this "On". When you don't want the "N" factor, turn this off. You can toggle these options by highlighting the word and pressing the "Enter" key. The instrument will retain your "N" factor for plates, use this menu to turn it on and off when appropriate (on for plates, off for press sheets and proofs). When you are finished, press the "Down" arrow until "Save & Exit" is highlighted, then press the "Enter" key to return to the "Dot Options" menu, then the "Previous" key to return to Dot mode.
To read plates in "Dot Area" mode:
In "Dot Area" mode, make sure "Paper" is highlighted and read the base of the plate. When done successfully, "Solid" should now be highlighted. Read a 100% image area on the plate. Now, "Tint" should be highlighted. Read the tint areas. Note that the instrument will give you the "density" of the tint area when you hold it down, when you lift it up it gives you the dot percentage reading.
Please note that your "Paper" (base of the plate) and "Solid" (100% plate image area) readings are critical. Do not read in an area with excessive gum buildup, processing artifacts, etc. To re-read these areas, press the "Up" arrow until "Paper" is highlighted, and read them again.
IMPORTANT:
Please make sure that you change the instrument to "Auto" mode with "N" factor turned off when reading sheets and proofs, and back on again for plates.
NOTE:
Although this document refers primarily to the 518 instrument, the procedure is the same for the 508, 520, 528 and 530 instruments.
BEST PRACTICES:
It is recommended that a linear reference plate of the type used be provided from the respective plate manufacturer. This reference plate should be imaged with the type of halftone dot and screen ruling being used by the shop. Readings taken from this plate should be noted, and this plate should be archived for further use. For example, if plate readings drift during standard process control readings the reference plate should be re-read and compared to the recorded values prior to adjusting process parameters. Keep in mind that although the instrument should be benchmarked to a known, linear reference plate, few computer to plate users should actually run linear plates as their standard. Factors such as proofing system utilized, pressroom variables, need to match archived work should be considered when setting up the raster image processor to image the plates. The linear reference plate is used to ensure proper utilization of the instrument: it stands to reason that if the instrument reads the linear reference plate consistently it will also read the determined target values of the production plates consistently, as well.
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