Digital proofing color management trends
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First, Introduction
Due to the development of global digitalization, the emergence of digital technologies such as computer direct plate making (CTP) and computer direct manufacturing (CTF) in the printing industry has increased the demand for digital processes in the entire printing industry market to replace existing ones. Traditional production methods. The digital process involves a wide range of applications, and digital proofing is the most talked about and concerned. The industry is skeptical of the initial digital proofing, and it has gradually accepted and adopted today, with a transition period of several years. In theory, if the equipment is good, the stability of digital proofing should not be questionable. However, the factors affecting digital proofing are numerous, so until now, digital proofing, which seems to be no longer unfathomable, has been repeatedly explored and improved.
Second, is your digital position accurate?
◆The precision of the digital proofing machine and the ink system and proofing paper?
◆How to correct the color correction method and color gamut of the proofing software?
◆Evaluation focus: Interpretation correct, color, dot shape, resolution, angle?
◆ Test instrument and color management software?
◆ Prepress system manufacturers color knowledge background and color correction capabilities?
◆ Observing the environment and conditions (Metamerism)?
◆ Simulate printer characteristics and ink gamut?
◆ Different printing plants and different brand inks?
◆ Printing master?
◆What is the standard color?
◆What are the international standards?
The problems listed above should be some problems that the general practitioners will have. It should be written briefly. When doing digital proofing, the printer is a prerequisite. If the printer is not good, do color management. A drop point quality may not be ideal, relatively affecting the color gamut, so of course you have to choose a better printer, but some inks are not the same, for example, we are playing special colors At the time, the ink may not be the same. Different from the past, some of the inks that we are now popular, whether it is four or eight colors, are different. Therefore, the use of ink in different industries is selective and many special. The color is not printed by the printer. The only way to do this is to change the ink. The next step is the paper used. We have been talking about paper to determine the color. If the quality of the paper is not good, the performance of the color gamut or the color performance is more accurate, so there are three key points in this premise. It is necessary to have a printer, a good ink system and a proofing paper with good color rendering.
The way to proof the color software is also the color core. Can CMM achieve a perfect conversion, and the conversion limit of color gamut is very related to the paper used. It is better to use the coated paper and use the paper to make it out. It is not good, this is the congenital limitation, what paper is used to model what kind of color gamut, even if you use a good paper to simulate a poor color gamut, in fact, it can be done.
Then again is the correctness, color, dot shape, resolution, and angle of RIP interpretation. At present, the digital image can be used to achieve the above points. It is very close to the printing point. He can be more than the printing network. solid. Instruments, the accuracy of the instrument and the calculation method of color management, have been doing poor color management before, not their own skills are not good, but the software and the instrument is not good, so the good software is calculated to be more accurate, This is a problem with the instrument.
Observing the environment and conditional color (Metamerism), when you are doing color calibration, the observation environment is very important, because the color you see is difficult to match when viewed under a non-standard light source. Conditional equivalence, if two colors have different spectral distributions (spectral reflectance curve, or spectral transmittance curve. This curve can be measured by the spectrometer), when under certain lighting and observation conditions, the two colors will It looks the same, but if the lighting and observation conditions are changed, the two colors will look different because they do not have the same tristimulus value. This phenomenon is called conditional isochromatic phenomenon, or the same color. Heterogeneous phenomenon. Different printers use different presses and inks, as well as the masters of the printing house to print different effects, which is the most difficult problem to improve.
Third, the color correction criteria
Color scientist R.W.G.Hunt once said: People can feel more than 10 million different colors, we can't completely remember all the colors, and that one is not surprised. If you can measure color, you can control the color. In the early days, the measurement of the color engineering instrument and the mathematical equations were analyzed, and then the natural color was corrected by the qualitative analysis of visual psychology. The three elements that produce color are the specific light source, the matching object, and the standard observer. The International Lighting Commission CIE 1931 made a 2 degree viewing color experiment based on the color mixing principle. The human eye quantified the photoreaction, and three reaction curves were obtained for the response of red, green, and blue cells. We call this the 1931 Color Matching Function. In 1964, a color matching experiment was performed with a degree of 10 degrees.
D50 is the standard illuminator for CIE, and x-bar, y-bar, and z-bar are the 1931 standard observer color matching functions, respectively. The result of quantifying the light to the eye quantifies the tristimulus values in three data, XYZ. To get X, just add each wavelength (D50 value * x-bar * object) and then add X, Y and Z are the same algorithm. The tristimulus values are then converted to horseshoe-shaped chromaticity coordinates x and y, while the chromaticity coordinates of the D50 standard source are falling within the horseshoe-shaped white region. That is, the spectrum (380nm~730nm) into which the read color block is converted is conveniently calculated by converting the formula into XYZ or Lab. According to different colors, it can be verified from here that when the observer is unchanged, the reflection spectrum of the object is also unchanged, but the light source is changed, so the color data is also different. For example, when you take a print to a customer, you should look at the bottom of the standard light source. If it is not under the standard light source, the color may be different, because the color will not be the same as the light source.
There are some instruments with software that can be measured in the form of spectrum. Finally, it is decided to use the standard light source. It may be 6500 color temperature. If you can know the color temperature of the customer, you can substitute this color temperature. The ICC Profile made is more consistent there. That is to say, what is its reflectivity at every ten nm, and a spectrum can be converted out to convert a color, and the representation of CIE digital color is to calculate the tristimulus value (XYZ) and the color of CIE Lab. Spatial calculation. Therefore, the instrument will display the measured color data, which is consistent with what the eye sees, so when doing color measurement, the instrument is also calculated according to the human way. If you use CIE Lab, because Lab is probably hard to understand, it is to say how the color is, so it is converted into a color representation Lch. If Lch means color, CHROMA, hue angle, then you can use one of them. The color representation means that this color is probably where it is.
The true meaning of color data is the three data of the object's reflectivity, CIE standard light source, and human color matching function. Finally, a data XYZ is obtained by the equation product. There is a website (www.brucelindbloom.com), which specializes in color correction. It can input a lot of spectrum, it can help you convert to RGB, Lab, Lch, all kinds of ways can be converted without CMYK. For example, XYZ to convert RGB is how much, Lab can convert RGB, color space and Lab how to use this formula, as long as the input data can be, so this pair is very useful in color calculation,
Fourth, color measuring instruments
If you don't know how to use the instrument during color management, it is an obstacle to correct the color. Therefore, the Color Measuring Instrument is designed to measure the reflectance of the object. Transmittance ), CIE chromaticity values (such as XYZ tristimulus values) or visible spectra (380 nm - 730 nm) or radiance (Spectral Radiance) Various optical instruments such as color brightness. The colorimetric instrument mode is designed to measure the color of the source (Self-Luminance or Emission), such as Monitor. Measure the Surface Color, such as prints, printers. Transmitted Color such as Film.
Types of measuring instruments: Color Temperature meter, Exposure meter, Densitometer, Colorimeter, Sectrophotometer, Spectroradiometer It is non-contact A measuring instrument such as the PR650.
Color measurement instrument operation:
Single-point measurement:
X-rite DTP22, 528, GretagMachbeth SpectroEye
Strip Scanning: X-Rite DTP41, GretagMacbeth EyeOne
Automated XY Table: X-rite DTP70, GretagMacbeth SpectroScan, iCColor
Instrument operation is roughly divided into several types, one single point, one is Strip is a quantity, and the other is XY axis measurement, these are reflection instruments. Most of the instruments are 0/45°, 45/0° color measurement mechanism, which mainly measures the ratio of incident light and reflected light of the color block, and then obtains the concentration value and other derivative data through the formula. There are many methods for measuring, one is to use a polarizer, the other is to have a Filter. The concentration meter is roughly divided into reflection and transmission, which can measure the CMYK of printing, printing density, dot gain, dot area, hue error, gray scale and print contrast.
The colorimeter, which measures the screen, uses the RGB filter to find the tristimulus value. Spectral Concentration Meters, most of the concentration meters are now calculated using spectra, so the measured states are A, T, E, and I. For example, there are only two standards used by the printing factory. The two biggest differences between Status T and Status E are the yellow filters. If you use Status E to measure the yellow version of the printed matter, the concentration is 1.4-1.5 or higher, but you use Status. T is a general standard used in the United States. The measurement is only about 1 or so. So pay attention to which type of instrument it is used for, such as today's yellow version printed to 1.4-1.5, if using Status T to measure The words can't be printed. So be sure to switch to Status T or Status E. Spectrophotometer, measuring reflectance, such as DTP41, DTP22.
A spectrophotometer consisting of an integrating sphere spectrophotometer and a multi-angle spectrophotometer (a material that can be used to obtain specular reflection) to obtain a reflectance value for each small wavelength of light, called a spectral reflectance curve. A multi-angle spectrophotometer measures the color-changing ink on top of a thousand dollar bill. The integrating sphere spectrophotometer can measure such as translucent objects, paints, and metal objects. Most of the high-precision desktop integrating sphere spectrophotometers are used in the textile industry, and lithography is also used by a small number of people, such as SP60/62/64 CFS57CA/CF57CA, Color i5, and Premier 8200/8400.
Special measuring instruments, such as CCDOT, which use image processing to obtain the dot area, can not only measure the dot area of the plate, but also measure the flexographic letter.
Five, input and output color management system 3C
The focus of color management is at 3C, Calibration, Characterization, and Conversion. The most important technology in 3C is the conversion of color gamut. Calibration: Grayscale Calibration (Linearization) It can also be said that it is the tone curve of the adjustment equipment. Characterization: Character refers to each color input device or color output instrument (colour Output device), even colored materials (such as inks, dyed chemical phosphors on display screens, etc.) have a certain range of colors (colour Gamut) or color performance. The purpose of this step is to establish the color representation range of the color-developing device and mathematically record its character for color conversion. The CIE xyY mode represents the color range of an instrument and material; the human eye has the widest color range and the print has the smallest color range. The device profile (Profiling) defines the gamut space, which is the ICC. The generation of the profile.
Color conversion (conversion): color gamut conversion (Gamut Mapping), color conversion in color management is not to provide 100% of the same color, but to use the instrument or material to provide the best color, while allowing users to predict the results. For example, how to do color matching between printing and digital, it will involve the conversion of color gamut conversion.
Sixth, digital camera color management
The management of digital cameras is generally used, GretagMacbeth, ColorChecker or GretagMacbeth DC or SG, SG pays more attention to the performance of skin color. However, digital cameras have to face multiple light sources in nature, and the effects of multiple spectra, so metamerism is more serious. For these reasons, digital cameras have no way to be as accurate as scanners, which involves when they are in different sources. Underneath, it will shoot different colors, so the ICC of the digital camera will change due to the change of the light source.
Seven, the color management of Monitor
When you want to do color management on the screen, you must pay attention to the following points, and it is most important to determine the Luminance of Monitor, set the white point of Monitor, set the tone copy curve of Monitor, and set the black of Monitor.
◆Clean screen surface
◆ Warm up for 30 minutes
◆Environmental light source illumination is weakened
◆Determining Monitor's Luminance (foot-lamberts or cd/m2)
◆ Set the white point of Monitor (Kevlvins)
◆Set the tone copy curve of Monitor (Gamma)
◆ Set the monitor's black (foot-lamberts or cd/m2)
In the proofing of the screen, what is the brightness of the screen, and what is the Luminance required? The specification of s RGB is 80 cd/m2. If 80 cd/m2 is darker, it can be depending on the environment. Trial up to 85 Cd/m2 to 95 cd/m2. If it is a liquid crystal screen, it may need to be brighter. The next step is to decide the white point, which is actually the color temperature, and the characteristic curve of Gamma. Finally, there is a black setting for Moniter, which is the level of darkness. In the coloring, the black should be lit up to light black, so that it can be expressed on the dark part of the layer; but if you want to blacken the LCD screen, it is impossible to do it because the light tube of the LCD screen is always lit. Therefore, it is even more impossible to lighten the black color. Because the LCD screen does not represent the intensity of light and brightness in terms of the intensity of light, the LCD screen will be different.
◆The first C of color management (Calibration correction)
When the brightness (brightnee) and contrast (contrast) settings are made, the brightness adjustment is actually adjusted to black, so you can turn the black to black and you can express the black level. If the brightness is in the middle When it is 50% brightness, adjust the contrast to adjust the intensity of white, that is, the brightness and intensity of white can be adjusted, so you can strengthen the intensity when adjusting the contrast. When measuring 80 cd/m2, it is actually adjusting the contrast. The contrast can be adjusted to the required illumination. If the situation is poor, it may be necessary to change the screen. Then introduce a corrected program Spyder Pro correction mode, adjust the gain control of the display to make the RGB three colors equal, and the brightness should be adjusted within the range of 85-95 cd/m2, and the error value can be adjusted to less than 0.5 and can be completed. In addition, the coordinates will be displayed. Generally, the color temperature of 6500 is set. This is its coordinate, so it can measure RGB three colors, then RGB to make a standard distribution, and make RGB three colors. Standard white, what we want is an axis.
◆The second C of color management (Characterization characterization)
Adjust the contrast (white of the screen), adjust the brightness (black of the screen), set the Monitor type: CRTorLCD, set Gamma=2.2 (PC, sRGB), set Gamma=1.8 (MAC), set the standard color temperature 6500k ( sRGB). When adjusting Gamma's replication curve to be set to 2.2 for PC or 1.8 for MAC, assuming that the RGB standard is followed, the MAC should be adjusted to 2.2, which may not be suitable; because APPLE was originally developed to check the black and white. Laser printer, so the copy curve is 1.8, so he also adjusted the screen to 1.8, but now black and white have used color laser printer, so it is recommended that the Gamma on the MAC is also adjusted to 2.2. After the above settings, the ICC can be generated. Gamma's convergence means that the RGB0 to 255 tone is adjusted, and the Spyder is divided into nine tones, and the RGB color curve is corrected to a standard gray, for example, the Gamma value is 2.2. Correct Gamma at coordinates every 30 The characteristic, to do a Gamma characteristic curve, is to put the screen on each RGB different point, trim it into a Gamma curve, then the correction is that RGB has three different curves, then you can generate A profile of an ICC.
On the LCD side, the LCD does not have a Gamma feature, so it needs to be simulated. Why is there no Gamma feature? Because its lamp is always lit, so the characteristics of Gamma are obtained because of the intensity of the light; To use the liquid crystal bias to simulate the Gamma color characteristics of the LCDD. Therefore, when the gamma color characteristic of the LCD is used, the time taken is longer than that of the CRT. The reason is that the CRT measures nine gray scale curves, but in the LCD, the RGB three colors are measured. The curve, so the data obtained by the measurement will be a little more.
◆The third C of color management (Conversion color conversion)
Color conversion refers to different color gamut comparisons, which are based on the Adobe gamut and Viewsonic If the color gamut of the G90f screen is used for comparison, there is no way to completely cover the color gamut of Adobe 1998 on the screen. Then again, the accuracy of the color on the screen is worse than that of the printer, and it is a illuminator. When the color is opposite to the real object, the error is often caused by the condition, so the screen needs to be calibrated once. Generally, the CRT is usually calibrated every week. Once, at least once a month, the LCD is also calibrated once a week, but because the LCD is used as a dye (dye to display color, longer than the CRT, it can be calibrated longer).
Eight, Matchprint Virtual Proofing System
This is a very accurate Virtual Proofing developed by KPG. System, the error can be within △ E1, of course, it is necessary to proofread the screen and printed matter in a standard observation environment, the proofreading method is to follow a standard of ICC, and to do in a very strict environment For color, the most important thing is that the remote screen, local screen, proofing or print can be consistent. Matchprint Virtual Proofing System also goes with RealTime Proof, this RealTime Proof is a technology that uses image cutting. It is not limited to any file size and type. It can see the desired manuscript in the shortest time. In addition, there is also technology for image cutting. When you know the resolution of the user's screen, you can return the resolution of a screen, so if you transfer it over the network, it can be seen in a short time. Even zooming in and out can immediately see what you want. The result is that when it comes to remote proofing, you can see the color of the customer directly on the screen. That's not just about providing this technology, but also the technology that provides the modification. For example, if the color does not look very accurate and is far from the actual object, you can modify it with the location where the color is not accurate, that is, at the customer. The printing process can be done directly on the screen, so the printing industry has to be borderless. This is a very useful system software.
Nine, color management of the printer
Choosing a good printer (larger color gamut) is half the success. The general inkjet and laser printers are all RGB color modes, so the purpose of the ICC Profile is the RGB gamut space. Most printers controlled by RIP (Raster Processor) are in CMYK mode, so the purpose of ICC Profile is the gamut space of CMYK.
(1) Color management first C (Calibration correction)
The first is to choose whether the quality of the inkjet paper used is good, as well as the cleaning status of the nozzle, vertical and horizontal calibration, gray balance and resolution. After these pre-questions are completed, it is necessary to calibrate each tone, and each level should be clearly defined. For example, the printer prints four-color guides to measure with a spectrometer, while calculating and then To fix, so we need 50 is 50, this is a special function of RIP; but if you want to print directly, without RIP, there is no way to do calibration (Calibration), gray balance ( Base Linearization) action.
(2) Color management second C (Characterization characterization)
This means measuring the color space printed on the paper by the printer, and then reading the color through the instrument. Although different instruments have different guides, the number of patches will vary depending on the distribution of the instrument. Different arrangements but the final result will be the same. The device profile defines the gamut space of the printer, such as the print guide (TC918).RGB, 918 color blocks). In fact, characterization means that the printer prints a guide using a spectrometer to read, and then calculates the color into a space and interprets it in RIP to know what color the prediction can print. Generate an ICC using the color management software (ProfileMaker) The Profile calculates an ICC Profile based on the reference data and the measured data.
(3) Color management third C (Conversion color conversion)
When doing color gamut conversion, each software should set the correct source (input) ICC Profile, such as digital camera, scanner, printer (RGB, CMYK), sRGB, display. Purpose (output) ICC Profile, such as printer (RGB, CMYK), display. Rendering Intents: Absolute Color (Absolute) Colorimetric), Relative Colorimetric, Preceptual, Saturation.
Source, purpose and color gamut conversion, which are compressed. If you compress, you will lose some colors. How to achieve perfect compression? Now there is another new way to return to the most primitive state. Just do the correspondence table of colors. In general, the color gamut conversion provided by ICC is these four functions, absolute chromaticity, relative chromaticity, perceptuality, and chroma. Color gamut conversion is the ICC of the screen Profile, the ICC Profile of the printer, through the gamut conversion action, you can see consistent results, this is RIP to do color management work. There are two ways to print the printer guide, one is printed IT8 or more accurate use ECI2002 guide, the printing machine also prints the same guide, and then measure, after the amount is put into the RIP for color conversion.
Ten, Photoshop color management settings
How to accurately simulate the final color of the screen, there will be a color gamut in use in Photoshop's color setting, which is the gamut space of the work. There will be a CMYK on the other side, which is the ink of the printing machine. Use the ISO standard to download from the eci.org association ISO Target, then put Target into CMYK, it is recommended to use the reserved description file, the color gamut conversion uses relative color. Just put the ICC of the press Profile input, no matter which printer is placed on the screen, you can see the color printed by the printer.
The specified description file is not the same as the conversion description file. The specified description file refers to an ICC Profile. The conversion description file has two ICC profiles. If the RGB image file uses the specified description file, the CMYK will never be seen. Color description, only the color description of RGB can be seen. If the RGB image taken by the digital camera is used to convert the description file during the color gamut conversion, the two color modes of RGB or CMYK can be converted and displayed. Photoshop provides a view that can be customized in the proofreading settings. That is to say, when RGB has not been converted, it is possible to predict what kind of result will be printed when printing.