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Densitometer's measurement principle is to determine the color density of printed products by measuring the amount of red, green and blue light reflected from the surface of the sample to be measured. The red, green, blue and visual filters of the densitometer correspond to the cyan, magenta, yellow and black inks, respectively. A spot color is a color other than the three primary colors and black, and the density meter does not have a color filter corresponding to the spot color measurement. Therefore, the density meter cannot be used to perform spot color hue verification. Conditional printers generally use colorimeters for spot color measurements. The companies that I worked for often printed spot colors. Because there was no colorimeter, when printing spot colors, in addition to relying on visual assessment, a density meter was also used to control the color depth and ink layer thickness of spot colors. This article talks about the experience of using density meters to control spot color printing quality, and the limitations of this method.
The color control of spot colors mainly depends on the ratio of each basic color ink during the ink transfer and the control of the ink layer thickness during printing. The appearance of the color at the time of inking must be measured with a colorimeter. Measurement with a densitometer is not advisable because the density measurement does not provide information about the hue, that is, the same density is used to measure the density of the secondary dispensed ink. Does not mean that their hue must be the same.
The color density of the ink printed on the paper is mainly determined by the ink's color characteristics, toner concentration, ink film thickness, and paper characteristics. For established inks and papers, the density mainly depends on the ink film thickness. As the ink film thickness increases, the density value also increases. Until the ink film thickness is extremely high, the density value does not increase with the increase of the ink film thickness, and eventually becomes stable.
Spot density is mainly used to control the ink film thickness. Spot color printing is generally printed in real-world color blocks, and the quality of the product is mainly reflected in whether the ink is thick and even. In order to achieve a thick and uniform ink visual effect, on the one hand, the ink color is required to be as uniform as possible; on the other hand, the ink layer is printed thick as much as possible without the occurrence of back stains. Whether or not a thick and uniform ink can be obtained, the key to mastering the ink balance is whether or not the ink film thickness can be accurately controlled. Therefore, it is a desirable method to measure the spot color density by using a density meter to control the ink film thickness. The problem that should pay attention to while using 1. Using the same densitometer density meter has many types and different performances. Different densitometers may have obvious differences in the density value obtained by measuring the same color block. Therefore, the same density meter must be used to control the ink film thickness of the same printing product. Ensure that the instrument's test head and sampling aperture are clean before starting the measurement. The power supply of the densitometer must be stable, and there must be a period of warm-up stabilization time before starting the measurement.
2. Choosing the appropriate spectral filter and the spectral density of the response state densitometer means the spectral energy distribution of the light source used in the densitometer, the spectral transmittance of the filter, the sensitivity of the receiver, and the spectral efficiency of the densitometer optical system. product. The density value measured by the densitometer is a function of the spectral characteristics of both the measured object and the spectrum product. On the one hand, the same color sample sheets are measured with different color filters of the same densitometer. There will be different density values; on the other hand, even if the same color channel of the same densitometer has different spectral product, some densitometers have A state, T state, and state. Several response states, such as I state, are available for selection. Different response states are used. The measured density values ​​will also be different. Theoretically, there is a one-to-one relationship between the response status, density value, and ink film thickness no matter which color channel is used. However, the higher the density value, the more pronounced the response to the subtle changes in ink film thickness. This change is detected and corrected, so the density meter should be tuned to a color channel that gives the highest reading of the measured spot color. The T state and the E state belong to the wide band filter, and the I state belongs to the narrow band filter. The main difference between them is the spectral distribution range of the transmitted light of the color filter, and the dark color is measured with the yellow color channel. Time. The density value measured with the state will be higher, and for the purpose of controlling the thickness of the ink film, it is more advantageous to use the narrow-band I state.
3. Paper with a black backing print is translucent or opaque. For translucent substrates, some light passes through the substrate, passes through the image printed on the back (assuming the image is printed on the back), and then from the tested sample The underlying substrate surface reflects off and enters the tester.
The color of the pattern on the back of the specimen and the surface of the material at the bottom of the specimen. Performance affects the accuracy of density measurements. The thinner the paper, the higher the transparency, and the more obvious this effect is if the image is printed on the back. The solution to eliminate this effect is to use a black underlay, that is, place the specimen on a black lining for measurement, and the light that reaches the black underlay is absorbed by the black underlay. If the substrate is a cardboard, this effect can be ignored.
4. Correct calibration and measurement Before using densitometer measurements, the densitometer should be calibrated. For the purpose of controlling the thickness of the ink layer, blanks of the currently printed paper should be used for calibration. After calibration, the measured data is subtracted. The data after the paper's color density excludes the effect of the paper on the density of the ink layer.
Whether it is a calibration or an official measurement, it is necessary to ensure that the sample is clean, flat and crease free. No finger scratches, etc., and place it on a black backing, keeping the bottom of the instrument and the sample in the same plane.
5. Control the difference between dry and wet density When the light source of the densitometer is incident on the image part of the printed product at an incident angle of 45 degrees, the reflected light includes two parts, and part of the light reflected from the surface of the ink film is called the first layer surface reflected light: the other part is worn Through the ink film, the light reflected by the paper surface. When the ink layer is in a wet state, the ink has a good leveling property, and the ink film surface is more prone to specular reflection. The specular reflection light is emitted at a 45 degree angle from the opposite direction of the incident light, and the density meter receiver is at 0 degree. In the position, the reflected light from the surface of the first layer of the mirror reflection will not be received and the measured density value is high.
During the ink drying process, a part of the binder in the ink penetrates into the fibers of the paper. The surface of the dried ink film will become flatter than the original. The light reflected from the surface of the first layer tends to be diffusely reflected, and the surface of the diffusely reflected first layer reflects light. Some will be received by the receiver at position 0' and the measured density will be lower. Because the production site needs to timely adjust the change of ink color depth and ink layer thickness according to the density measurement value, the wet color density should be adopted uniformly when the density detection is used to control the ink layer thickness, and the wet color density and the dry color density cannot be performed. Compare.
6. The limitation of density detection is that the curve with the density value as the abscissa and the Munsell value as the ordinate is drawn. The slope of the curve with the smaller density is larger, and the slope of the curve with the larger density is smaller. The discrimination of the light and shade is more sensitive in the less dense part. In relatively high density, it is relatively dull.
When printing light-colored spot colors (such as beige). On the one hand, the overall density of this color ink is low, and the change in ink layer thickness does not cause a significant change in density; on the other hand, the human eye is more sensitive to low-density areas, so when printing shallow spot colors, use a density meter. Controlling the thickness of the ink film is not as sensitive as visual assessment, and the use of a density meter is of little significance.
In conclusion, in the production practice, it is effective to use the same reflection density meter to control deep spot colors. Since there are quite a few packaging products that need to be reprinted, if we only print a small piece of color, each time we consume a small amount of ink, we will deploy more spot color inks at one time. We will record the wetness at the time of the first printing. The color density value and the color channel and the response state will be as close to the previous density value as possible when printing.
If printing a large area of ​​the background color, we will commission a professional ink transfer company with ink, so that the ink has good hue stability. Control and record the wet color density each time you print. Do this. On the one hand, the consistency of the color of the packaged product is higher; on the other hand, under the requirement of printing as thick as possible, the quality problem of back-scratching is avoided.
Correct application of reflective densitometer in spot color printing
Reflective density measurement is widely used for color dot printing quality control and production process control. In the printing process, using the reflection densitometer (hereinafter referred to as densitometer) to measure the additional control strips on the printed sheets, it is possible to detect the solid density, dot gain, relative contrast and overprint of the yellow, magenta, cyan and black colors. Rate, etc., use these data to conveniently and effectively control the tone and color reproduction in the printing process.