Research on the Chromaticity Drift Characteristics of LED Display Screens

In recent years, flat panel display technology represented by LED display screens has developed rapidly and has been widely used in various fields of production and life. In particular, the emergence and continuous development of new technologies such as point-by-point correction technology and color gamut correction technology have greatly improved the consistency of brightness and color of display screens , and obtained better image display quality. Although LED has the outstanding advantage of high stability compared with other light sources , it will inevitably experience attenuation of light intensity and drift of color coordinates when working for a long time. In order to further understand the attenuation and drift of LED full-color screens working for a long time, this paper conducted a preliminary monitoring experiment. 2. Color consistency of LED display screen

Colorimetry is a comprehensive subject developed in the last century based on the fields of physical optics , visual physiology and visual psychology. It goes beyond the scope of physics in the usual sense, but it does belong to the category of physics. There are many phenomena in scientific research, production and life that are often associated with this subject. These phenomena are a mixture of physical and physiological processes [2].

The ultimate goal of LED full-color screen is to present a good visual effect to people, and the consistency of its brightness and chromaticity is a relatively important aspect. Within the chromaticity range, there are two aspects: color uniformity and color fidelity. Color uniformity refers to the consistency of color difference when the same color is displayed between pixels, modules, and modules on the same screen; color fidelity refers to the degree of consistency between the color reproduction of the image on the display and the source image or source scene. With the continuous advancement of technology, the mainstream LED full-color screens on the market have achieved higher color fidelity with their 14-bit or even higher color performance capabilities, but at the same time, the problem of low color consistency caused by low color uniformity has become more and more obvious.

By compensating for this difference in chromaticity through point-by-point correction technology, better visual effects can be achieved. However, as time goes by, the LED full-color screen will have varying degrees of brightness uniformity and chromaticity differences. In order to understand the trend of this difference, guide the determination of relevant parameter indicators of the LED full-color screen, and provide reliable correction data for secondary correction in the later maintenance stage, it is necessary to conduct in-depth research on the changes in the brightness and chromaticity related parameters of the LED full-color screen over time, and more accurately understand its attenuation after long-term work. There are many factors that evaluate the chromaticity consistency of the LED full-color screen. Here, this article focuses on the study of the drift of the base color coordinates.

3. Test model

3.1 Experimental plan

In this paper, a 64*32 pixel indoor LED full-color screen with a P7.62 dot pitch produced by Changchun Xida Electronic Technology Co., Ltd. was selected as the test object. In order to simulate the actual use of the screen as much as possible, a 50% brightness white field was used for uninterrupted testing 24 hours a day during the daily attenuation process. In order to simulate the attenuation of the display screen after it was delivered to the user, the indoor LED full-color screen used in the test module was fully aged and used as the starting point of the attenuation test time. Under dark room conditions, the test screen was measured every 6 hours (12 hours at night) according to the module. The changes in its chromaticity parameters were collected, and the measuring instrument was the BM-7 color brightness meter of Yuanfang Optoelectronic Information Co., Ltd.

Figure 1 is a schematic diagram of the system for this experiment.

Figure 1 Schematic diagram of LED full-color screen attenuation and color coordinate drift test system

The first module of the screen is lit up, and the data acquisition system is used to collect relevant data; the computer extracts and saves the data; the control system then lights up the next module of the screen for measurement, and the chromaticity parameters of the screen at that moment are obtained module by module.

3.2 Experimental error analysis

The main reasons that may cause abnormal test data are:

1. Measurement error;

2. The influence of external stray light;

3. Fluctuation of working current.

4. Changes in ambient temperature;

In darkroom conditions, the full-color LED screen has the characteristics of high brightness, and the influence of external stray light becomes negligible; and the working current of the LED is kept at 20mA by the constant current drive control circuit and is relatively stable. Although the influence of working current fluctuation exists, it will not affect the overall trend of the entire attenuation process; the change of ambient temperature during the test has more long-term and macroscopic effects on the LED color coordinates; the influence of measurement error can be explained by Table 1:

Table 1 Color coordinate data measured under different measurement conditions

As can be seen from Table 1, even if the reference measurement position is artificially deviated by 10°, the measured color coordinate data does not change significantly. Therefore, apart from the measurement error caused by the inherent accuracy of the instrument itself, the measurement error caused by human operation of the instrument can be basically ignored.

4. Analysis of test results

The time-varying state of the color coordinate data of the three primary colors RGB is analyzed, and Figures 2, 3 and 4 are obtained respectively.

a)

b)

Figure 2 a) Changes of the x-value of the primary red color coordinate over time b) Changes of the y-value of the primary red color coordinate over time

a)

b)

Figure 3 a) Changes of the green primary color coordinate x value over time b) Changes of the green primary color coordinate y value over time

a)

b)

Figure 4 a) Changes of the x-value of the blue primary color coordinate over time b) Changes of the y-value of the blue primary color coordinate over time

From the above results, it can be seen that after sufficient aging, the LED full-color screen will enter a stable period, and the color coordinates of the three primary colors will change irregularly within a small range. The range of change is limited, which can be obtained from Table 2:

Table 2 Peak data of the color coordinate changes of the three primary colors

In order to more intuitively obtain the changing trend of the LED full-color screen during this attenuation process, the color coordinate changes under the CIE1976 system are further analyzed here, as shown in Figures 5, 6 and 7.

a)

b)

Figure 5 a) Changes of the red primary color coordinate u value over time b) Changes of the red primary color coordinate v value over time

a)

b)

Figure 6 a) Changes of the green primary color coordinate u value over time b) Changes of the green primary color coordinate v value over time

a)

b)

Figure 7 a) Changes of the blue primary color coordinate u value over time b) Changes of the blue primary color coordinate v value over time

At the same time, this paper further analyzes the changes in the wavelengths of the three primary colors during the attenuation process, as shown in Table 3.

Table 3 Changes in the wavelengths of the three primary colors during the attenuation process

It can be seen from Table 3 that in this attenuation process, the wavelength offset of each primary color is small, which has little effect on the display quality of the display screen; at the same time, the wavelength offset of each primary color shows certain differences.

After the LED full-color screen is fully aged, its color coordinates become relatively stable within a certain period of time, and only show irregular small oscillations within a small range. During this period, the drift of color coordinates has little effect on the display effect of the screen image quality. In the later maintenance and secondary correction of the LED display, the brightness range should be the main focus. At the same time, in a short period of time, although the drift of its various primary color coordinates is small, it also shows different degrees of difference. After a period of attenuation, the screen will again show its chromaticity inconsistency. In this case, it is necessary to maintain and correct the brightness and chromaticity of the LED display at the same time. Of course, just a few hundred hours of monitoring is far from enough. To fully grasp the regularity of the color coordinate drift of the LED full-color screen during the attenuation process, a lot of experiments and long-term unremitting efforts are still needed.