Application Research of Response Method of Color Reflection Densitometer

First, the reflection density meter measurement principle

The reflection densitometer uses red, green, and blue color filters to obtain three colors of light, and by measuring the reflection of the ink on the three colors of light, color densities of cyan, magenta, and yellow are obtained. The color density measures the physical absorption characteristics of an ink for a certain color light, and reflects the relative value of a certain ink saturation. In practical applications, the standard is similar to the measurement purpose, and a standard broadband or narrowband filter is used to measure the density of different spectral ranges. Therefore, the measurement of reflection densitometers is divided into narrow-band measurement and wide-band measurement, which are two density measurement methods commonly used in the printing industry.

Because the broad band filter spectral curve is wide, the red, green, and blue bands cross each other, resulting in the passage of some other trichromatic light in the monochromatic color filter, and the narrow band filter has a stronger selectivity for the spectrum, so the narrow band The measured density value is higher than the broadband measurement.

The reflectivity of the remaining complementary color light after absorption by the ink, 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 subscripts represent different waveband ranges and correspond to red, green, and blue light in three different waveband ranges, corresponding to the ink density of cyan, magenta, and yellow inks. The upper footer k corresponds to different spectral energy distribution patterns in the same waveband range, corresponding to different Response measured density.

Second, the color density meter response method

All densitometers use a logarithmic relationship to determine the density. If the response methods in the respective measurement system (such as the color filter, photodetector, or built-in logarithmic relationship in the densitometer) are different, they are obtained. Density readings also vary. For this measurement, it is necessary to unify the response of the densitometer. The response method is defined as the result of designing, measuring, and calibrating according to the definition of the response of the standard. It provides a consistent density for all densitometers.

1 commonly used response method

In color density measurements, the densitometer displays measured values ​​depending on the spectral energy distribution Sik(λ) of the light source used in the densitometer, the spectral sensitivity rik(λ) of the detector, and the spectral transmittance τ(λ) of the color filter. Spectral Product, which is the response function of the densitometer measurement, denoted by the symbol ∏:

If the response function of the measurement is not strictly defined, the density measured by the red, green and blue beams is only specific to a specific density meter. For the same color sample, the reading provided by it cannot be directly related to another density. Measured readings are compared. In order to carry out international and inter-professional communication and standardization, these densitometers must be appropriately limited and unified as much as possible. Therefore, the International Organization for Standardization (ISO) and the American National Standards Institute (ANSI) for three filter density Response rules such as the T, G, E, and I response methods in the X-Rite 500 Series Densitometers were used.

The International Organization for Standardization ISO stipulates that if the logarithm of the response function of the whole measurement system of the densitometer reaches a specified value of a response, the data measured by the densitometer will be the response density. To meet the densitometer specified by the T response function, the measured data is the T response density.

The energy distribution requirements of red, green, and blue light used in the printing industry in China are in the band range. The corresponding response mode is the T-response mode. In the T-response mode, the spectral sensitivity rik(λ) of the photodetector is not affected by the CIE spectrum. The limitation of the tristimulus value does not include the conversion value of the visual and physiological factors and psychological factors of human eye color. 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 each wavelength of the ink, the color samples with the same reflectance are measured using different types of densitometers. Response densities should be equal to ensure consistency in density measurements.

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: 3.4mm. Measurement Geometry: 45°/0° (ANSI & ISO Standard); Spectral Sensor: DRS Technology, 24 Point Engine, 31 point report.

Samples: Print control strips C, M, Y solid blocks.

The fully calibrated X-Rite 518 is set to different response modes, and density measurements are performed on solid patches, respectively.

Color as a color vision, is a wide-band sensory phenomenon, so when used for gray balance, color correction measurement, you should use a broadband densitometer represented by the T-response method, while measuring ink thickness, overprint rate and dot area rate Use narrow-band densitometers to increase accuracy.

IV. Conclusion

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