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When measuring proofs or prints, why is a black backing better than a gray or white backing?
ANSI / ISO 5 / 4-1983 (R.1986)-ANSI PH 2.17-1985: Density Measurements Geometric Conditions for Reflection Densitometry (reflection density measurement geometric observation conditions) defines the geometric observation conditions of reflection density measurement . This standard material contains the following introduction:
"This International Standard specifies that the surface of the material on the back of the sample to be tested should be spectrally non-selective and diffusely reflective, and that the ISO reflection density value is 1.50 ± 0.20, which is very important. (Note: Spectral non-selective The surface is a gray surface; gray tones with a reflection density value of about 1.50 should show black.) Some reflection density standards usually specify that the backing material has a lower reflection density. "Annex A of the standard further explains: When determining the reflection density in a well-defined observation condition, it is very important to determine the characteristics of the material used behind the sample. "
According to the document, there are many reasons for the choice of black backing, including:
1. The black backing reduces the variable factors in the measurement process. Since many samples used in the printing and publishing process are usually not completely opaque, the pattern printed on the reverse side of the paper will affect the measurement results, and sometimes even greatly. Therefore, for operations that are aimed at measurement and process control, this Point is very important. According to a survey by the PSI Association, when measuring substrates with a transmission density equal to or less than 1.00, different backing materials and patterns printed on the reverse side of the material will affect the measurement results.
2. From the perspective of spectrum, density and physical requirements, the use of backing materials can greatly reduce the measurement data of low-density surfaces.
3. Using a black backing, the absorption ratio can be calculated directly from the density reading.
The standard also warns that samples in the printing industry are usually backed by two or more layers of the same backing material before observation. There is no reason why the ISO density value of the sample combined with the density value of the printing image and the printing material cannot be read together, although some precautions should be taken to determine the reading of the image and the printing material together, not the image layer itself Reading.
The change in measurement values ​​due to the absence of standard backing materials continues to intensify as the paper becomes thinner and the projected density value decreases. The initial investigation and research conducted by the PSI Association in this regard showed that for the printing industry paper, the delta density value of the paper under the conditions of black and white backing measured from 0.07 to 0.002, under the same circumstances, the transmission density value It will increase from 0.3 to 1.2. From the data, we can see that the difference between the transmission density of the printing material and the data measured by the same material under different black and white backings has an exponential relationship. The white backing used in this investigation is a non-selectively absorbable material coated with ceramic tiles on white steel, which is non-fluorescent and has a reflection coefficient of 89%. / ISO regulations. In order to actually observe the transmission density value of the paper, the PSI Association investigated several different papers and found that the transmission density value of the thinner paper used for multi-page publications and Bibles is about 0.30; 20 pounds white cloth offset The transmission density of the paper is about 0.60; the transmission density of 80 pounds of coated card material is about 0.90; and the transmission density of 120 pounds of coated card material is about 1.20. These investigations also show that the situation of photographic paper is similar, but with different curves.
Realizing that the black backing minimizes changes in the measurement results, it is recommended to use a standard black backing in the following situations:
Equipment installation;
Calibration of the instrument (Note: During the installation of the density meter, the calibration black backing used is very important, because this backing is defined in the ANSI / ISO density standard.);
Specification of job parameters (for example, printing a job to determine the target point), unless a different backing is specified;
Industry parameter specifications (for example, the type of printing of magazine advertisements and other products to determine the target point), unless a different backing is specifically specified;
Exchange measurement data between different organizations unless a different backing is specified.
For internal users, for example, an operator subjectively estimates an image and compares it with a proof or another model. The black backing will make the printed image look slightly different, which is not applicable at this time. . As suggested in the standard, in these cases the operator is likely to place one or more unprinted substrates under the paper and then evaluate it.
Many members of international organizations plan to develop a standard white backing. The standard of this backing must redefine the following parameters:
Fluorescence density of color brightness surface diffusion phenomenon
In order to ensure the information transmission of the measurement results, each factor mentioned above should be recorded when the white backing is used to ensure that the receiver of the measurement results can understand the operation of these measurement results.
From the two perspectives of measurement standards and user standpoints, the currently used standards have a certain stability, and the convenience of using black backing (existing materials, and common under various conditions, to ensure consistency with the current ANSI standard) And the challenge of defining a white backing shows that the development of a white backing standard will not help to reduce the variation in measurement results, nor will it reduce the user application of standard backing materials.
In general, the industry recommends a standard black backing for color measurement for measurement and process control purposes. This should be a standard method to ensure that these measurement operations are not affected by factors such as printing on the reverse side of the paper or the disproportionate image on the paper. The printing materials should not be stacked together during the measurement. First of all, if a black backing is not used during the density measurement, the user should communicate the information of the backing used, and also be aware that this measurement result is affected by additional changes. The degree of influence of these changing factors can be determined by comparing the measured readings of the same paper material with and without a standard black backing.
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