1. The measured geometric conditions. The color density measurement adopts the measurement geometric condition of 45 ° / 0 ° (that is, the relationship between the light beam and the measured plane is projected at an angle of 45 °, observed in the direction of 45 ° perpendicular to the measured surface), or 0 ° / 45 The geometric conditions of ° (ie vertical projection, observation at 45 °). This geometric condition is superior to other methods, because the light reflected from the first surface of the printed matter will not actually be emitted from the measurement direction, and the sensor cannot receive this part of light. The stronger the reflection of the first surface of the tested sample, the densitometer The higher the density measurement.

When measuring printed matter with a textured surface, the 45 ° / 0 ° geometric condition and ring lighting should be used. The 0 ° / 45 ° geometric condition can also be used to add the reflection ring to the receiving side (Figure 1-8). The measurement aperture angle should be ± 5 ° as much as possible, and should not be greater than ± 5 °.

2. The light source that illuminates the sample. The relative spectral distribution of the projection light source should meet the requirements of standard light source A, that is, 2856 ± 100K, and the spectral transmittance of the supporting optical components and infrared absorption filter should meet the requirements.

3. sensor. A photoelectric sensor element with sufficient sensitivity to optical radiation in the range of 380 to 720um can be used as a radiation receiver. At the short-wave end of the visible spectrum, the spectral sensitivities of the light source and the photoelectric sensor elements are in a sharp downward trend below 400um. But in the near-infrared part of the visible spectrum (greater than 720um), the silicon sensor element shows considerable sensitivity (Figure 1-9), which must be eliminated with an infrared absorption filter. When using a special kind of filter, if the filter itself already has a good infrared suppression effect, no additional infrared suppression filter is needed. In order to check whether the infrared radiation in the near infrared part has been properly suppressed, the instrument can be adjusted on the calibration board, and then the infrared suppression filter is inserted. If the measured value is higher than the original measurement range to 1.0 density unit, It shows that the sensor used has a sensitivity that cannot be ignored for radiation in the near-infrared part. The photoelectric sensing elements used in the density meter mainly include: photovoltaic cells. Photomultiplier tubes, semiconductor diodes, etc. Compared with the photomultiplier tube, the photodiode is greatly reduced in size and the power supply voltage is also very low, so the optical system can be designed to be very small and become the most commonly used sensor.

4. Measuring filter. In order to evaluate the characteristics of the standard yellow, magenta, and cyan inks used in printing and their printing color, the measurement filter should be a complementary color filter corresponding to yellow, magenta, and cyan ink, that is, red , Green and blue-violet filters. The spectral passbands of the three filters should be close to the main absorption bands of standard inks yellow, magenta, and cyan. The maximum pass position of the measurement filter is defined as follows according to the wavelength:

For basic yellow ink, use blue filter, λ = 430nm;
For basic magenta ink, use green filter, λ = 530um;
For basic cyan ink, use red filter, λ = 620um;
The position error of the maximum passband is ± 5nm.

The spectral characteristics of the measurement filter depend on the passband range (half-peak and full-peak width). In the measurement of color density, the effective spectral sensitivity of the densitometer on the optical path [it is equal to the relative spectral sensitivity of the sensor S (λ) r and the transmittance of the measurement filter, (λ)] and the relative spectral distribution function of the light source S (λ ) The product should meet the standard, where the spectral radiation of the light source is standard. According to the current technical conditions, the spectral sensitivity of the light sensor is not generally effective for various density measurements, so the characteristics of the filter must be determined according to the needs of the measurement. According to the passband range of the filter, the filters used for color density measurement are divided into wideband and narrowband. The former is mostly used in the densitometer used in plate making and printing process, and the latter is mostly used in the density meter used for adjustment and measurement. None of these filters can be used for color separation.

When performing visual color density measurement on non-color prints, the role of color filter and color filter is opposite to that of color density measurement, and its filter should reach the spectral brightness sensitivity of the human eye in the entire visible spectrum range V (λ) is close. Whether the filter is inserted in the illumination light path or in the measurement light path will cause a difference in the measurement results for the shiny test sample. This is caused by the ultraviolet component of the illumination light source, but the ultraviolet component is generally very small. It can be ignored. [next]

The sensitivity of the color response of the density meter can be changed by changing the filter on the optical path of the density meter.

No filter is needed in the optical path of the densitometer. In this case, the sensitivity of the blue region of the visual spectrum is higher than the sensitivity of the human eye. In fact, the brightness function or visual response of the eye is the green region (approximately 560nm) The sensitivity is slightly higher.

The use of visual filters allows the densitometer to simulate the visual response of the human eye, that is, the common effect of the spectral transmittance of the filter and the relative sensitivity of the sensor's spectrum is equal to the spectral brightness sensitivity V (λ) of the human eye. Rayden 106 filter is a visual filter with a slightly yellow color.

In order to check the correctness of the visual density measurement by the densitometer, a standard gray scale can be measured with the densitometer. The gray scale is marked with the correct density value of each step for comparison with the measured value of the densitometer. You can also use the chromaticity method to measure the brightness reference value Y of any achromatic sample under the light source A, and then divide the Y value by 100 to find the logarithm of the reciprocal of this quotient. This value should be measured on the sample with the densitometer The density values ​​are approximate.

The color density measured with red, green, and blue films does not have the same sensitivity as the human eye. The color measured with it is not like the color seen by the eye. The filter changes the spectral composition of the photocell through the lens, so it only A part of the visual spectrum was measured, but by measuring the amount of light passing through the filter, the relative amount of a certain pigment on the printed surface can be obtained. For example: a blue filter is used to measure the amount of blue light absorbed by yellow ink, a green filter is used to measure the amount of green light absorbed by magenta ink, etc. In this way, the density meter can be used to ensure that each color A control tool that keeps ink constant during the printing process.

The density reading through the filter can indicate the relative amount of a certain ink, but it cannot indicate the hue of a certain ink. The density meter is not a colorimeter, but the readings using three filters at the same time can approximately indicate the hue.

The properties of filters are related to the properties of standard inks. For example, filters that measure three primary colors of ink should have the properties listed in Table 1-1.

For printed matter printed with standard inks, the color density measured with the filters listed in Table 1-1 is the same as the narrowband color density measurement only in terms of the maximum absorption of the basic color, if measured with other color filters The obtained color density value and the narrow band measurement value are almost 0.1 color density units, then this densitometer does not meet the conditions specified in the standard.

If the maximum transmittance of the filter does not match the correct value or the light transmission range is too wide, it will result in a decrease in the color density value, especially when the blue filter is used to measure yellow ink, the effect is particularly obvious.

Due to the wide passband range of the color density measurement filter, if there is a difference in the spectral sensitivity of the sensor used by the densitometer, it will affect the measurement result. In this way, for the same sample, different densitometer measurements will result in different Density value.

Density meters for color density measurement in the printing industry generally use the so-called broadband filters listed in the table above. U.S. densitometers commonly use Layden No. 25 red filter, Layden No. 50 green filter and Layden No. 47 blue filter, but some densimeters use Layden 47B narrow-band response filters when measuring yellow ink. It can It gives a higher density reading than the broadband filter. Narrowband filters increase sensitivity to small changes in density, but also lose more color information. The measured colors are less like the colors seen by the eyes. Some manufacturers recommend using narrowband filters to improve evaluation Consistency between instruments when printing sheets. The red and green filters of the broadband and narrowband densitometers are usually the same, only the blue filters are different, except for the density meter used to control the inking. Figure 1-10 shows two blue filters with different bandwidths.

With the development of measurement and control technology, a so-called density meter for spectral color density measurement has appeared. It uses glass interference filters instead of color translucent filters. This is a narrow-band filter and uses this filter. The density meter has the largest response at the peak absorption of the ink, and is usually used for process control in the printing shop. Narrowband filters are obviously sensitive to yellow ink. When there is a slight change in the amount of ink, it can indicate a greater density change than the wide density meter. The solid density of the yellow ink will be 50% higher than that of the broadband densitometer.

Due to the narrow passband of the narrowband filter, even if there is a difference in the spectral sensitivity of the sensor, it does not actually affect the response of the instrument. In this way, when the same sample is measured with different densitometers, a more consistent measurement result can be obtained, and a more accurate evaluation can also be obtained for dot coverage and dot overlay printing. Table 1-2 lists the characteristic parameters of narrow-band filters.

Different densitometers will display different readings due to differences in filter, photodiode, and light source characteristics. Most current densitometer manufacturers say that their densitometers have a "T" status response. This is because the "T" status response has an important feature: all densimeters manufactured according to standards can be mutually calibrated after proper calibration. In order to ensure consistent measurement values, it is necessary to use the "T" state density meter to ensure consistency between instruments. The "T" state response is similar to the broadband response. In principle, the characteristics of the "T" state response are: the distribution of the logarithmic value of the spectral product of the light source, filter, and sensor must meet the standard requirements; the spectral product describes the total response of a densitometer at a given wavelength .

There is also a densitometer with positive color response or positive color sensitivity, which is better than the use of visual filters when used to detect the density of positive color copies. For a long time, in order to calculate the exposure of positive color films, density meters with visual filters have been used. [next]

Density meter is a very precise instrument, but only if it is properly zeroed on a standard fixed bridge, can accurate measurement results be obtained. Zeroing the density meter means adjusting the density meter to the correct low density value. The calibration board is an ideal completely white diffuse reflection amount, but the completely white diffuse reflection surface does not exist. When measuring with a density meter on a standard whiteboard, the low density value of the standard whiteboard should be selected as the zero value. This is similar to when weighing, if you do not adjust the scale to zero, your weight will be inaccurate. When the densitometer completes the calibration and makes the actual measurement, it is essentially assumed that the displayed reading is due to the light absorption of the sample.

After the density meter is zeroed, the slope or high density value of the density meter is adjusted so that its high density is exactly equal to the density of the standard blackboard. Adjusting this high density point is like stretching a rubber ruler until its length is equal to the length of a standard ruler. The high and low density calibration determines the output range of the densitometer, for example, in the range of 0 to 3.0.

Changing the zero adjustment of the density meter will shift the response graph of the density meter up or down, but will not change its shape. That is to say, if the density meter is zeroed on the paper after calibration, the density of the paper is automatically subtracted from all the measured densities, which are in bright, middle and dark tones with the measured density Regardless of region, the slope of the density meter's response graph is constant. However, if the high-scale end of the density meter is readjusted, the graph will rotate up or down around the zero point, and the slope of the line will change, thereby changing the response of the density meter.

A density meter calibrated with a standard calibration plate will be in the same state as another density meter calibrated. Under normal circumstances, the operator no longer adjusts the slope of the graph.

Some density meters do zero adjustment first, and then use the visual filter on the density meter to calibrate the high-scale end. After that, the slope of the graph line cannot be adjusted, but it can be adjusted to zero. After the high scale is calibrated through the visual color filter, the zero calibration is performed through the red, green, and blue-violet filters. Most new density meters have an independent high-scale end adjustment link.

Whether the density meter should be zeroed on the calibration board or on white paper depends on what the purpose of the density data is. After the densitometer is correctly calibrated on a standard whiteboard, the reflection density can be measured, but if the purpose of the measurement is to evaluate the ink using the method recommended by the American Printing Technology Association or to optimize the tone reproduction using the method of the Rochester Institute of Technology At that time, it should be zeroed on the paper to exclude the influence of the paper. In this way, the reading of the measured value has subtracted the density of the paper, and the slope of the graph is constant. If it is to evaluate the joint effect of ink and paper, for example, when calculating with the Newburger equation, the value calibrated on the calibration whiteboard should be retained. When many people in a workshop use a density meter, calibration on a standard whiteboard can avoid adjusting the density meter for each paper. Once a standard sample is approved, it is convenient to measure and record the necessary density value, and use it as a reference value to control the operation of the printing press.

When the transmission density meter is calibrated, there is air on the optical path, that is, zero on the air. Density meter manufacturers provide standard transmission calibration references to ensure that users perform high and low calibration.

The linearity of the output of the density meter should be maintained. In order to check the linearization, the value of each level of the standard hairpin can be measured with a visual filter. The measured value is close to the standard value. The deviation value should not exceed ± 0.02, or must not deviate from the 45 ° line ± 0.04 °. Using the same method, repeat the inspection process with red, green, and blue-violet filters.

For density meters equipped with polarizing filters, a special multi-step standard is required to check linearization, with corresponding standard specifications attached.

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