Application Research on Response Mode of Color Reflection Densitometer
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First, the principle of reflection density meter measurement
The reflection densitometer obtains three-color light by using red, green, and blue color filters, and measures the reflection of the three-color light by the ink to obtain the color density of the cyan, magenta, and yellow decomposition colors. The color density measures the physical absorption characteristics of the ink for a certain color light, reflecting the relative value of a certain ink saturation. In practical applications, the density of different spectral ranges is measured using standard wideband or narrow-band color filters according to the measurement purposes. Therefore, the measurement of the reflection densitometer is divided into narrow-band measurement and wide-band measurement, which are two density measurement methods commonly used in the printing industry.
Because the wide-band color filter has a wide spectral curve, the red, green, and blue bands cross each other, causing some other three-color light to pass through the monochromatic color filter, and the narrow-band color filter has a strong selectivity to the spectrum, so the narrow band The measured density value is higher than the broadband measurement. The principle of the reflection densitometer is shown in Figure 1.
After the densitometer measures the spectral reflectance of the ink, the ink density can be determined by the formula (1). Where ρink is (1)
The reflectivity of the ink to the remaining complementary color after absorption, Sik(λ) is the spectral energy distribution of the illumination source, rik(λ) is the spectral sensitivity of the photodetector, and τ(λ) is the spectral transmittance of the color filter, P( λ) is the spectral reflectance of the ink for each wavelength. The lower foot mark represents different band ranges, corresponding to red, green and blue light in three different band ranges, corresponding to the ink density of cyan, magenta and yellow, and the upper foot mark k corresponds to different spectral energy distribution forms in the same band range, corresponding to different The density measured in response.
Second, the color density meter response method
All densitometers use a logarithmic relationship to determine the density. If the response in the respective measurement system (such as a color filter, photodetector, or built-in logarithmic relationship in a densitometer) is different, the resulting Density readings vary. For this measurement, it is necessary to unify the densitometer response mode. The response mode is designed to measure, calibrate and calibrate the standard response definition to provide consistent density for all densitometers.
1. Common response methods
In color density measurement, the density meter displays the measured value depending on the spectral energy distribution Sik (λ) of the source used in the densitometer, the spectral sensitivity rik(λ) of the detector, and the spectral transmittance τ(λ) of the color filter. The spectral product of the person, which is the response function of the densitometer measurement, denoted by the symbol ∏:
If the measured response function is not strictly specified, the density measured by the red, green, and blue beams is unique to a particular densitometer. For the same color sample, the reading provided cannot be directly related to another density. The measured readings are compared for comparison. For international and inter-industry communication and standardization, such densitometers must be appropriately limited and as uniform as possible. Therefore, the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI) have three filter color densities. Responsive regulations, such as T, G, E, I and other response methods in the X-Rite 500 series density meter.
The ISO standard stipulates that if the logarithm value lg∏ of the response function of the entire measurement system of the densitometer reaches a value specified by a response, the data measured by the densitometer is the response density. Table 2 shows the logarithmic value of the T response mode response function (only the logarithm value lg∏TG of the green light response function is listed in the table), and the density data specified by the T response function is satisfied. The measured data is the T response density.
Table 2 T response mode response function logarithmic value
The energy distribution requirements of red, green and blue light in the printing industry in China are in the range of T-response. In the T-response mode, the spectral sensitivity rik(λ) of the photodetector is not affected by CIE spectroscopy. The limitation of the tristimulus value does not include the conversion value of the visual and physiological factors of the human eye and the psychological factors. Since the response function in the standard T response mode is a standard quantity, and the unknown quantity is only the spectral reflectance ρ(λ) of the ink for each wavelength, the color samples having the same reflectance, the standard T measured by different types of densitometers The response density should be equal, ensuring consistency in density measurements.
2. Calculation of response density
According to the formulas (1) and (2), the response density of the magenta ink in the green region T can be expressed by the formula (3).
Where: Δλ-wavelength interval, can be taken as 10nm or 20nm as required.
The wavelength interval is 20 nm, wherein the value of ∏TG is obtained by the logarithm value ig∏TG of the green light response function of Table 2. The product of the broad-spectrum reflectance ρ(λ) of each wavelength of ∏TG and magenta ink is calculated by wavelength. The values are shown in Table 3.
Table 3 Product of ∏TG and spectral reflectance ρ(λ)
The T response density of the magenta ink can be obtained by substituting the values of ∏TG and ∏TG. ρ(λ) obtained in Table 3 into the formula (3). Other response methods are calculated similarly to T responses.
Third, the experimental results and analysis
Instrument: X-Rite 518 with polarizer (measurement response T, G, E, I).
Light source: standard light source A, color temperature 2856K; spectral range: 400nm-700nm. Measuring aperture: 3.4 mm. Measurement geometry: 45°/0° (ANSI & ISO standard); Spectral sensor: DRS technology, 24-point engine, 31-point report.
Samples: Printed control strips C, M, Y solid color blocks.
The fully calibrated X-Rite 518 is set to different response modes, and the density of the solid color blocks is measured separately. The measured data is shown in Table 2. The density curve of the commonly used response mode is shown in Table 2.
Table 4 Measurement data of common response methods
Tab. 2 measurement data of conventional response modes
Figure 2 Common response mode density curve
It can be seen from Fig. 2 that the DC, DM values measured by the T, G, and E response modes are basically the same, except that the yellow ink density increases from the T response to the I response, because the T, G, and E response modes use broadband red. Color filters and broadband blue filters, such as the commonly used Ryden 25 red filter, Ryden 47 blue filter. Since the G response mode is the broadband response of the X-Rite printing process, similar to the T-response method, it is sensitive to the thicker yellow ink, so the Dr value is larger, and the E-reaction mode uses the Ryden 47B narrow-band blue filter. The color patch, so the Dr measured for the yellow ink is larger than the G response. Response mode I is a narrowband response method developed by ANSI. Therefore, among the listed response modes, the I response has the highest density measured, especially the density value of yellow ink is 36.8% higher than the T response, so the thickness of the ink layer. The subtle changes are also magnified by 36.8%. This response is conducive to timely detecting and controlling changes in the thickness of the ink layer.
Color as a color vision is a broadband sensation phenomenon. Therefore, when used for gray balance and color correction measurement, a broadband density meter represented by T response mode should be selected, and when measuring ink thickness, overprint rate and dot area ratio, A narrow band densitometer is used to improve accuracy.
Fourth, the conclusion
In order to standardize the image reproduction, after the density meter is fully calibrated, the response mode must be correctly selected. Only in this way can the measurement data be consistent, which is the premise of standard data exchange and the standardization of image copy technology. The key is.