The principle of density meter of printing quality control method and its application (2)

The Arabic numbers following the densitometer model are the serial numbers of the densitometer. There are two types of domestic densitometers, one is CMF, which stands for color reflection densitometer, and the other is CMT, which stands for transmission densitometer.

The measurement principle of the densitometer is very close to the visual inspection and identification principle of the print operator. The working principle of the optoelectronic densitometer is to form a light path through a light source and a light filter, and the receiver converts the light transmitted or reflected to light current of a corresponding intensity according to the intensity. After analog-to-digital conversion, it is displayed on the digital display. This gives the density value of transparencies or reflection prints.

2. The measurement principle of the reflection densitometer. The stable illumination light source is focused on the surface of the print through the lens, and some of the light is absorbed. The absorption depends on the thickness of the ink layer and the density of the colorant (Figure 9-4). The unabsorbed light is reflected by the surface of the printed paper, and the transmitted light collects reflected light that forms a 45° angle with the irradiated light. And sent to the receiver, the receiver converts the amount of light received into electricity, and the electronic system compares the measured current with a reference value (absolute white reflection). Based on the comparison value, the absorption characteristics of the measured ink layer are calculated, and the measurement result density unit is displayed on the screen.

(Figure 9-6 Measurement principle of reflection densitometer)

The relationship between reflectivity and density has been described in detail in the first chapter. However, for a more intuitive understanding of the relationship between reflectivity and density, see Figure 9-7.

(Figure 9-7 relationship between reflectance and density)

The application of 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 and then performing a full calibration. Make the densitometer's high-density value equal to the dark space density, so as to determine the range of the densitometer, such as the range of 0~3.00D or 0~3.50D.

In practical production, the usefulness of densitometers is numerous, but the main ones are as follows:

(1) The significance of measuring the ink characteristics and measuring the ink characteristics lies in providing reliable ink characteristics data, so that the plate making personnel compensates for the color setting before the image color processing.

(2) Manuscript scanning When scanning a manuscript with a scanner, the density of the manuscript can be measured with a densitometer. The density of general printed matter is 0.06-2.2. Reversal film density range up to 0.02 ~ 4.0. In addition, the black and white field calibration can also be determined by a density meter. Usually, the high optical density is 0 to 0.80 and the dark tone density range is 1.50 to 3.90.

(3) Imagesetter output, in order to ensure the quality of the PS version can be obtained, according to the film output screen must have sufficient density value. The density of a single dot should reach 3.0 or more, the density in the field should reach 4.0 or more, and the initial fog density should be less than 0.07.

(4) Proofing and printing, usually in proofing and printing, the density meter should be well used. When proofing with coated paper, the density range of each color is usually:
Y: 1.10~1.15 M: 1.40~1.50
C: 1.50~1.60 K: 1.70~1.80
In print production, a reflection densitometer is mainly used to determine the relevant parameters.