Application Research on Response Mode of Color Reflectance Density Meter

1. Measuring principle of reflection density meter

The reflection densitometer uses red, green, and blue filters to obtain three-color light. By measuring the reflection of the three-color light by the ink, the color density of the cyan, magenta, and yellow decomposition colors is obtained. Color density measures the physical absorption characteristics of an ink to a certain color light, and reflects the relative value of the saturation of an ink. In practical applications, according to the similarities and differences of measurement purposes, standard broadband or narrow-band color filters are used to measure the density of different spectral ranges Therefore, the measurement of the reflection densitometer is divided into narrow-band measurement and broadband measurement, which are two kinds of density measurement methods commonly used in the printing industry.

Because the broadband filter has a wider spectral curve, and the red, green, and blue bands cross each other, causing some other three-color light to pass through the monochromatic filter, and the narrow-band filter has a strong selectivity for the spectrum, so the narrow band The measured density value is higher than the broadband measurement value.

The reflectivity of the remaining complementary color light after absorption by the ink, Sik (λ) is the spectral energy distribution of the illuminating light source, rik (λ) is the spectral sensitivity of the photodetector, τ (λ) is the spectral transmittance of the color filter, P ( λ) is the spectral reflectance of the ink for each wavelength. The lower subscripts represent different band ranges, corresponding to three different bands of red, green, and blue, corresponding to the cyan, magenta, and yellow ink densities. The upper subscript k corresponds to different spectral energy distribution forms in the same band, corresponding to different Density measured in response mode.

Second, the response method of the color density meter

All densitometers use a logarithmic relationship to determine the density. If the response methods in their respective measurement systems (such as color filters, photodetectors, or the built-in logarithmic relationship in the densitometer) are different, the resulting Density readings also vary. For this purpose, it is necessary to unify the response mode of the densitometer. The provisions of the response mode are the results of design, measurement and calibration according to the standard response definition, which is to provide a consistent density for all densitometers.

1. Common response methods

In color density measurement, the measured value displayed by the densitometer depends on the spectral energy distribution Sik (λ) of the light source used in the densitometer, the spectral sensitivity of the detector rik (λ) and the spectral transmittance of the color filter τ (λ). The spectral product of the tester, which is the response function measured by the densitometer, is represented by the symbol ∏:

If the measured response function is not strictly specified, the density measured with the red, green, and blue beams is unique to a specific densitometer. For the same color sample, the readings provided by it cannot be directly related to the other density Compare the measured readings. In order to carry out international and inter-plant communication and standardization, such density meters must be properly restricted and unified as much as possible. Therefore, the International Standardization Organization (ISO) and the American National Standards Institute (ANSI) The response mode is specified, such as T, G, E, I and other response modes in X-Rite 500 series density meters.

The International Organization for Standardization ISO stipulates that if the logarithm lg∏ of the response function of the entire measuring system of the density meter reaches the value specified by a certain response, the data measured by the density meter is the response density. A density meter that satisfies the T response function specification, the measured data is the T response density.

The energy distribution requirements of red, green, and blue light used in China's printing industry in its band range, the corresponding response mode is the T response mode, in the T response mode, the spectral sensitivity of the photodetector rik (λ) is not subject to the CIE spectrum The limit of three stimulus values ​​does not include the conversion value of human eye color visual physiological factors and psychological factors. Since the response function in the standard T response method is a standard quantity, and the unknown quantity is only the spectral reflectance ρ (λ) of each wavelength of the ink, therefore, for color samples with the same reflectance, the standard T measured by different types of densitometers The response density should be equal to ensure the consistency of the density measurement.

3. Experimental results and analysis

Instrument: X-Rite 518 with polarizer (measurement response mode T, G, E, I).

Light source: standard light source A, color temperature 2856K; spectral range: 400nm-700nm. Measuring aperture: 3.4mm. Measurement geometry: 45 ° / 0 ° (ANSI & ISO standard); Spectral sensor: DRS technology, 24-point engine, 31-point report.

Proof sheet: Printed measurement and control strip C, M, Y solid color patch.

Set the fully calibrated X-Rite 518 to different response methods, and measure the density of the solid patches separately.

Color as a color vision is a broadband perception phenomenon, so when used for gray balance and color correction measurement, a broadband density meter represented by T response method should be selected, and when measuring ink thickness, overprint rate and dot area ratio Use a narrow-band density meter to improve accuracy.

4. Conclusion

In order to achieve the standardization of image copying, the density meter must be properly selected for the response method after full calibration. Only in this way can the consistency of the measurement data be achieved. This is the prerequisite for standard data exchange and the standardization of image copying technology The key is.

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