Application of Standard Light Sources in Color Management (I)

The world we live in seems to be a colourful world because of the presence of light and visual nerves. The photoreceptor cells in our eyes feel the light of different wavelengths outside the world and make our world look colorful and colorful. People always like beautiful things. They hope that beautiful colors can be kept around or can be shared with more people. Then they begin to reproduce colors, and then produce paintings, photography, printing, printing, and so on. Related industries. However, color is an elusive thing, not so easy to grasp, for the reproduction of color is not an easy thing, so people in the printing field introduced the concept of color management system. In order to be able to get the same printing effect as the original, color management has been introduced into every process of production: from the input of the original—electrical scanning or digital photography, to the image editing on the display, to the tradition of the proofs or digital Proofing until the comparison of printed products and proofs, adding scientific color management at each link to maximize the accuracy of color reproduction. In China, color management can be said to be an emerging concept, but it is rapidly being accepted and used by color-related industries in the country, especially the printing industry. More and more companies engaged in color management solutions have entered this market, and more and more companies have applied color management systems to their own production processes.

So what is color management? The color management that people are talking about generally refers to the management of the transfer of color space (scanners, computer screens, printers, and printers, etc.), and it should also include color consistency, repeatability, or stability management. Color management is very important to the current printing industry because pre-production of many current printing businesses is done by advertising companies, and many businesses do not always have to print in one company. Printing companies also need to have Communication, for such an open system, requires more coordination, a lot of time and manpower for color modification, to eliminate color deviations from each other due to their different environments. Color management has more meanings. If each enterprise has a perfect color management application, such a process can be greatly simplified. The printing factory can clearly predict and reproduce the color effect of advertising design, and the printing factories can easily achieve the same. The color of the prints. Therefore, the true role of color management is that it is a bridge of color communication, and everyone can benefit from it.

The color management system is not a simple software system. This is a misunderstanding of many people's understanding of color management. The color management system should be a hardware and software interaction system. It should include the following elements:

The first of course is the hardware system. Computers, monitors, scanners or digital cameras, printers, printers, color measuring instruments, etc. These are the hardware elements for color management. How to use instruments and software to coordinate and manage the color transformation in these spaces is us One of the ultimate goals of color management.

However, the color characteristics of each device are different, and even the characteristics of two machines of the same model are different. Therefore, the second important element for scientific color management is the Device Profile. This document is to measure the color sample of the printing equipment through the spectrophotometer, then calculate and produce the ICC Profile file in the professional software, it can describe the color characteristic of the printing equipment accurately, have determined the pros and cons of printing the color management effect. The exact ICC profile depends on the accuracy of the measurement system used and on the algorithm and control parameters that generate the ICC Profile software. Therefore, the precise feature file of each device is one of the essential elements of color management.

The second is the profile of the simulated object. Color management is intended to use limited devices to make color more pre-determined. It uses a device-independent color profile to compare the space between devices and the standard color profile to compare color changes between devices. Color characterization The file is characterized by the device's color reproduction feasibility. Color management module (CMM) is used to perform color conversion. By using color profiles for each device and color management module, it can provide a wide range of applications and reduce copying. The time and cost.

Finally, there is one point that is often overlooked. It is to observe the environment of color, that is, the light source environment. The ultimate judge of color management results is still the human eye. No matter how the device is debugged, the color image is different or not. The accuracy and inaccuracy are determined by the observer through his own eyes, because a series of values ​​can only give an abstract color concept, just like what we usually say. In fact, people always believe in their own eyes. But the eyes will deceive you, or the light is deceiving you, sometimes different colors under the same light will make you look the same, the same color looks different under different light sources. Therefore, in color management, we must create a good and stable environment for observing colors and see as much color as possible. Of course, this ideal environment is the environment under the natural light in the sunny morning. This is the best observation environment, but it is a difficult environment for actual operation, because we always perform color management indoors, so we It is necessary to use artificial light sources to create an environment that can imitate ideal natural light as much as possible to observe colors. This artificial light source is a standard light source. Only in such an environment can we correctly conduct color management before we can accurately evaluate the results of color management.

So why does the color look the same at times and sometimes it looks different? Why does color management require a standard light source to create an observation environment while other light sources do not? Let's find out why.

As mentioned above, the ultimate judgement of color management is to observe through our eyes. Let’s first understand why our eyes see different colors. Let's first understand why our eyes can see different colors. Color is the response of the optic nerve to light, a kind of visual nerve stimulation, no visual nerve or light, no color. Note: Light does not equal color. The wavelength of light that one can see is generally between 380 nm and 780 nm. The following is the wavelength of visible light.

Light color wavelength λ(nm) represents wavelength red (Red) 780-630 700
Orange 630~600 620
Yellow 600 to 570 580
Green 570-500 550
Cyan 500-470 500
Blue 470-420 470
Violet 420~380 420

From an application point of view, the 600nm to 780nm band is red (R), the 570nm to 600nm is yellow (Y), the 500nm to 570nm band is green (G), and the 470nm to 500nm band is cyan (C) The 380nm to 470nm band is blue light (B). We can find that R, that is, the wavelength of the red light is 180nm, the wavelength of the G is 70nm, and the wavelength of the B is 90nm, which is obviously larger than the 30nm of the Y and C. They respectively occupy a larger proportion in the visible spectrum. Constitute what we call RGB shades. The generation of white light is a complete transmission or reflection of all light. We can see the color because we have three cone-shaped cones in our eyeball that are responsible for sensing colored light, pyramidal cells ργβ and rod-like cells responsible for sensing light and shade. Among them, the ones responsible for the red light are the pyramidal cells, the ones responsible for the green light are the y-cones, the blue cells responsible for the blue light are the beta cones, and together with the rod cells, they constitute the main receiving light for our eyes. Device. Then it passes through the nerves to the brain and forms our sense of color. So we can see objects of different colors because the light is shining on the color objects. The color objects absorb part of the light waves while reflecting some of the light waves. This part of the reflected light acts on our eyes again. So we see this world. It's colorful.

In other words, color is the result of the brain's perception of the effects of light on the three stimuli of R, G, and B formed by cells on the retina of the eye. In other words, if the tristimulus values ​​of R, G, and B are equal, X, Y, and Z are the same, then even if the spectral distribution of the two color objects is not the same, we feel that their colors are still the same.

Although the spectral curves of the color object 1 and the color object 2 are not the same, the XYZ values ​​respectively calculated by the illuminator C are the same, that is, the color object 1 and the color object 2 are under the illuminator C for the eyes. The tristimulus values ​​of the retina are equal, so the brain's feeling is that they are the same color. We call this the illuminant conditional color phenomenon, or the same color phenomenon. If we replace the lighting body C with the lighting body A.

Under illumination C, the XYZ values ​​calculated by the color object 1 and the color object 2 are not the same. That is, the tristimulus value for the eye retina is not the same, so our brains feel that it is two different. color. This is why different colors appear to be the same under the same light source. Similarly, if the spectrum of the color object is the same, it is possible to produce two different tristimulus values ​​under the illumination of two different light sources, and the color will look different.

Through this we can see that the lighting body is one of the important factors affecting color. If there is no standard lighting body or light source that can play a role in our color management, then the results of our color management will be Great discount. This is why the standard light source of D50 or D65 is used for color communication in the printing field.

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