Densitometry - Offset Print Quality Control
Densitometry, colorimetry, and spectrophotometry are the three methods used to measure color in the graphic arts. The densitometer measures optical density. It is essentially colorblind, measuring only the level of reflectance of the transmittance of a film. Density varies with ink film thickness, but data can also be used to determine dot area. Colorimeters define color numerically in various color spaces, but they do not measure density or dot area. Colorimeters are useful to determine color difference in an accurate metric called △E (delta E). Spectrophotometry yields the highest level of color in formation; it is the most accurate and versatile. Spectral analysis yields color curves and permits calculation of density, dot area, and colorimetry.
Of the three, the densitometer is the most widely used device in the pressroom for measuring and evaluating color testing and control images. Its primary use in the pressroom is to analyze test forms and color bars to determine how the press is performing and to monitor the pressrun, providing feedback on a number of factors including ink density, dot gain, trapping, and print contrast. The densitometer can also be used to inspect the quality of supplied off-press proofs and press proofs and determine their conformance to standards and specifications. It can also be used to analyze the incoming materials, such as ink and paper, used in the pressroom. An example of digital color transmission/reflection densitometer is shown in Figure 3-10.
A densitometer indirectly determines the amount of light absorbed by a surface by measuring the proportion of light reflected from or transmitted through a measured surface. It is assumed that-for practical purposes-the amount of light absorbed is equal to the amount of incident light (the supplied light) minus the amount of light reflected. The densitometer then calculates density using an equation. (A reflectometer records or reads the percent reflectance of a beam of light compared to the standard beam, while a densitometer converts the numbers into the familiar logarithmic density units.) Transmission densitometers compare transmitted light, and reflection densitometers compare reflected light. Because different makes of densitometer as the standard and use it throughout the company.
Color filter. The densitometer can measure the process colors of yellow, magenta, and cyan with the aid of color filters. On many modern digital densitometers, the filtration of the colors happens automatically, without the press operator's intervention. However, older densitometer models require the press operator to dial in the proper filter for the density measurement.
Densitometers are usually equipped with three color filters-red, green, and blue. The blue filter measures the reflected light in a range between about 400 nm and 500 nm and reads it as the blue density. similarly, the green filter measures the reflected light in a range between about 500 nm and 600 nm, and the red filter measures between about 600 nm and 700 nm. The exact range of wavelengths measured depends on the specifications of the filters used with the densitometers. For example, so-called wide-band filters typically measure a range of 50~70 nm, while the "narrow-band" filters generally measure a range of about 20 nm. With both types of filters, the density reading is an average of the densities over the range of wavelengths measured, not the density at a specific wavelength.
The purpose of each filter is to render the process color as if it were black. Yellow when viewed through a blue filter will appear to be black. The green filter will render the color magenta as black, and the red filter will render the color cyan as black. Using the correct filter allows the densitometer to single out a reading for each for process color, even if the colored ink films are overlapping.
Scanning densitometer. A scanning densitometer is a computerized quality control device that measures and analyzes printed color bars using a densitometer. There are inline scanning densitometers, which are mounted directly on the press to read density information on the fly. More commonly in use are off-line scanning densitometers, which require the printed sheet to be manually positioned for scanning.
The densitometer provides an indication of printed ink film thickness by measuring the ink densities on color bars printed on the sheet. the ink patches to be read are spaced within each ink key zone of the printed sheet. The results are compared with desired densities, and a computer monitor displays the degree of variation. Ink fountain keys and fountain rollers can be then be adjusted if necessary. Scanning densitometers may also provide information on dot gain, print contrast, trapping, hue error, grayness, slurring, and doubling.
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