Graphic Arts News Vol.2-No.1
Roles of measuring devices in color management
Roles of densitometers in calibration
Calibration is the procedure necessary to maintain the standardized conditions of a device. For instance, if a calibration function is included in an image setter, data for 50% toning will produce film with 50% tone.Calibration software is available pre-installed in photocopiers and RIPs. Densitometers are generally used for calibration, rather than spectrometers or color photometer, because coloring materials are not changed during calibration, and the spectral diffraction characteristics of colors are fixed. Three criteria to consider when selecting densitometers are replication accuracy, long-term stability, and measurement range.
The densitometer operates twice during calibration:once prior to calibration to create a test chart of the device to check the densities and then send the data to the calibration software, and once more after calibration to create another test chart to check that the densities match those targeted.
Spectrometers and color photometer in the color management system (CMS)
Calibration is sufficient to provide stable and standard functioning of a single device, but when more than one device is used, color management is required to obtain the color data to distinguish between the RGB in transmitted light and the CMYK in reflected light, and to manage the different color reproduction ranges of each device. In this case, a CMS with spectrophotometers and color photometer is used.Measuring devices perform two roles in the CMS. One role is to output accurate data to the CMS to produce a color reproduction profile of the device to record the colors the device is able to produce. The other role is to provide measurements as rapidly and as simply as possible to enhance user productivity.
Selection criteria of measuring devices used to create profiles
The accuracy of the measurement data is extremely important for the CMS. In fact it would be no exaggeration to say that the function of the CMS is totally dependent upon color measuring devices. High measurement accuracy and stability are particularly important to create profiles. Also note that long-term stability is much more important that stability in the short-term, because if the measuring capability is stable only for a month or two, and then begins to degrade, the device cannot make best use of the profile which was originally created. For this reason, measuring devices must be selected carefully.There are four important criteria required in spectrophotometers. One is replication accuracy, which can be tested by taking measurements of the same point several times and compared. The second point is long-term stability and durability, which can be difficult to judge, because data over half a year, or one year and longer must be monitored. However, this point can be judged to a certain extent by examining the structure of the spectrophotometer. Look for the existence of moving parts, and check that the sensor is completely sealed, because the outside air changes the characteristics of filters with the passage of time. Hardware durability is closely related to the structure of the hardware.
The third criterion is device variation error, which is the difference between devices. Although the same type of device used to create the profile is used during actual printing, device variation error would prevent accurate color management. The final point is that the spectrophotometer should conform to CIE/ISO standards, but this point is rarely a cause for concern, because so few spectrometers fall outside of these standards.
Measuring data in users' environments
Moving on now to problems actually experienced when using color management. First of all, the profiles provided by device manufacturers may be ineffective for actual color management. This is not a software problem, but is due to the fact that the environment in which the profile was created is different from that of the users. For instance, although manufacturers may use one of the best spectrometers, users may have lower standard color photometer, or devices with stability affected by changes in temperature and humidity, or changes in the colors produced by scanner CCDs or monitors.Measurement of more than several hundred patches of data is required to produce a profile for just one device, and there is no alternative to measuring each piece of data in the chart. The measuring method must be rapid and highly accurate. Automatic measuring systems and robot systems are used in some places.
Because slight differences in devices and hardware impact so greatly on the CMS, it is important that devices are stabilized using calibration, that measuring devices have long-term stability, and have a function to allow adjustments for changes imposed by the external environment, so that the CMS functions comprehensively from input to output.
Manufacturers should supply low-priced measuring devices to enable users to purchase them for everyday use, produce profiles considering compatibility, and establish technical support for customers by providing advice, etc.