Print quality management with control bar and density meter (5)

Densitometers are one of the most important quality control tools in prepress and print production. The accuracy, consistency, and quality control of color reproduction in most processes depend on densitometer measurements. Practice has proved that the effective use of densitometers is a powerful tool for implementing standardization, standardization, and data quality management of printing and copying projects. Therefore, quality management should be replaced by densitometer as soon as possible to replace the old experience management, so as to push the company's quality management to a new height.

1. The type of density meter

Densitometer is divided into two kinds of transmission and reflection, transmission densitometer is mainly used to measure the density of the transmission of the original, the output density of the photo film and dot percentage, and for the linearization of the photosetter; reflection densitometer is mainly used to measure the reflectivity of the original density And proofing, print proofs of various colors of the field density and dot area, dot increase, overprint, print contrast, color deviation, and measurement, calculation of the ink's three major characteristics, namely, color cast, with gray, efficiency. With the rapid development of science and technology, densitometers are constantly updated. At present, there are many kinds and types of densitometers manufactured in foreign countries. For example, the X-Rite 500 series reflectance densitometer from X-Rite, USA, which adds many functions, can not only measure the density but also measure the color of different color spaces. Degree, color difference value, dot area, K value, overprint rate, color error and gray scale, and can also be online with the computer, the measurement data processing. As shown in Figure 6.

In general, the first English letter of the densitometer model indicates the characteristics of the densitometer. For example, T stands for transmission, R stands for reflection, D (M) stands for densitometer, O stands for dot area tester, and TR stands for transmission and reflection. OT stands for dot area transmission densitometer, TD stands for transmission densitometer, and RD stands for reflection densitometer. Arabic numerals behind the densitometer type are the serial numbers of the densitometer.
There are two types of domestic densitometers: the first one is CMF, which represents a color reflection densitometer, and the second is CMT, which represents a transmission densitometer.

2 Densitometer measurement principle

Densitometer's measurement principle is very close to the visual inspection principle of the printer. The principle of optoelectronic densitometer is to form a light path through the light source and filter, so as to transmit or reflect the light quantity of the tested sample to the receiver. The receiver converts the transmitted or reflected light into light current of corresponding intensity according to different intensity. After the analog-digital conversion, it is displayed on the digital display to obtain the density value of the transparencies or reflection prints.

(1) Measurement principle of reflection densitometer

The stable irradiation light source is focused on the surface of the printed product through the lens. A part of the light is absorbed. The absorption condition depends on the thickness of the ink layer and the density of the colorant. The unabsorbed light is reflected by the surface of the printing paper. The lens collects reflected light at an angle of 45° with the illuminating light and transmits it to the receiver. The receiver converts the amount of light received into the power and the electronic system compares this measured current with the reference value (absolute white reflectance). This comparison value calculates the absorption characteristics of the measured ink layer and the measurement results are displayed on the screen in density units.

(2) The relationship between reflectance and density

The reflection densitometer measures the reflectance, and then uses the following formula to calculate the density value: reflectance = reflected light/incident light, density = lg (1/reflectance).

3. Calibration of the density meter

Before using the densitometer, it is necessary to calibrate the zero adjustment on a randomly equipped standard whiteboard. A standard whiteboard is an ideal, completely white, magnesium sulfate reflective surface. Zeroing the densitometer means adjusting the densitometer to a standard low density value.

After the densitometer is zeroed, it is sometimes necessary to adjust the slope or high density of the densitometer, also known as full calibration. When fully calibrated, the densitometer needs to measure black or dark space so that the density is equal to the density of the black or dark space. The calibration of high and low density determines the range of the output value of the densitometer, such as 0 to 3.00D or 0 to 3.50D.

Densitometer calibration cycle can be based on the use of the situation, generally should be corrected once a day. Note Before calibration, make sure that the calibration white plate is clean and clean. After calibration, place the calibration white plate in a dry, dust-free place and avoid sunlight.

4. The density meter measures the ink characteristics

The use of reflective densitometers to measure and calculate the three major characteristics of ink, namely color shift, ash and efficiency, has great practicality. The first is to test the quality and stability of the purchased ink. Due to the current variety of inks, the quality of the ink produced in each batch is not stable, and there are phenomena such as color shift and inconsistent ash components, resulting in inaccurate printing colors. The second is to provide accurate data for pre-press image color processing. For example, most of the magenta inks used in today's printing shops are magenta inks, and the inks are yellowish. Using them as large reds (M100%+Y100%), they tend to be yellowish because the magenta inks are more than Y50%. If you change to M100%+Y90%, you can reproduce the big red color correctly. Another example is to set the purple (C100% + M100%), the resulting purple print will become black, because the magenta ink is yellow, blue ink is red, if the formula ratio is changed to C90% + M75% or C90% + M80%, Purple is bright.

The author's experience is that before performing image color processing, it is necessary to first understand the color shift, ash and other characteristics of the ink used in the product, and compensate when setting the color.

Measurement and calculation methods:

Yellow, magenta, and blue are the three primary colors of ink, which are the complementary colors of the three primary colors of blue, green, and red. The ideal three-primary inks should absorb one-third of the spectrum of the visible spectrum and reflect another two-thirds of the spectrum. However, in reality, this ideal ink does not exist. The absorption of the two parts of the ink is inconsistent. The result is a grayscale; moreover, the two reflected lights are no longer equal and the hue changes, which is the color deviation of the ink. As for ash, it can be understood as the black ink component contained in the color ink. Color shift and ash can help monitor the purity of the ink.

The efficiency refers to the purity of the ink, that is, the degree of color shift of the primary ink. With less ash content, the purity of the ink is high and the efficiency is high. Generally, the color shift of the Y ink is the least, and the efficiency is the highest.
Calculation formula: color cast = [(ML)/(HL)] × 100%; with gray = (L/H) × 100%; efficiency = [1-(L+M)/(2 × H)] × 100 %. Among them, L is the minimum density measured in the R, G, and B color filters, M is the median density measured in the R, G, and B color filters, and H is the highest density measured in the R, G, and B color filters.

5. The advantages of densitometers for color measurement

For decades, the author has used densitometers for data-based color management, benefiting a great deal and realizing that there are five major advantages.

(1) Color is greatly affected by people's subjective perception. Everyone has a different sense of color. Densitometry can provide an objective analysis to overcome individual differences and thus unify the criteria for judgment of the depth of ink.

(2) Light source and environment have a great influence on visual color measurement. In the window and outside the window to see the color is not the same, in the printing shop and office to see the color is not the same, but measured with a density meter, then from the environment.

(3) Guaranteed proofing and printing production quality. Densitometer measurement is very important in proofing and printing. In the production process, the depth of the color density is affected by various factors. With the density standard, the depth change of density can be effectively controlled through measurement, thereby ensuring the consistency of the shade of ink and stability.
(4) Proofing and printing quality standards, using density meter for data management, can provide printing operators in different regions and different manufacturers to monitor the production process, to achieve the same effect of darkness and lightness.

(5) Color file. Data visualization has become the premise of print quality management. Standard color data can be recorded and saved for use in the next batch of prints. This prevents the proofs from being faded after a certain period of time. Causes inconsistent print colors in each batch.


Source: Ke Yin Media - "Printing Technology"