Analysis on the Principle and Design Method of Chromaticity Correction of LED Display Screen

With the technical progress of point-to-point correction, customers have higher and higher requirements for the display quality of LED screen, from only pursuing the brightness and white balance index to gradually improving the requirements for display uniformity and color fidelity. The use of point-to-point correction technology to greatly improve the uniformity of the display screen is currently in the process of rapid popularization and application, and the demand for chromaticity correction has gradually surfaced, which has attracted more and more attention in the industry. This paper will briefly introduce the principle, application, implementation methods and skills of LED display chromaticity correction.

1. Basic concepts of chromaticity correctionCIE XYZ 1931 standard chromaticity system shall be used for chromaticity measurement and calculation of LED display screen. In order to introduce the calculation formula of chromaticity correction later, first, briefly sort out the basic concepts related to chromaticity correction:1.1 tristimulus value

According to Glassman's color matching principle, three primary colors are selected. Any one of the three primary colors cannot be obtained by adding and mixing the other two primary colors. For example, RGB primary colors, the relative brightness unit of the three primary colors is determined by selecting a specific white light as the standard, and the light of other colors can be regarded as a mixture of different amounts of three primary colors, The number of three primary colors required is the tristimulus value.CIE XYZ 1931 chromaticity system uses three imaginary primary colors, [x], [y], [Z] to replace RGB primary colors, and determines the units of three primary colors by matching isoenergetic white light. When quantitatively expressing the brightness and chromaticity of a light source, the chromaticity equation can be expressed as follows:C [C] = x [x] y [y] Z [Z] (equation 1)

In the formula, x, y and Z are the three stimulus values, and the three stimulus values of the mixed color are the sum of the three stimulus values of each component color.Note that among the three primary colors, only [y] primary color represents both chromaticity and brightness, [x] and [Z] only represent chromaticity.1.2 color coordinates

The relationship between the color coordinates x, y, Z and the tristimulus value XYZ in CIE XYZ 1931 chromaticity system is as follows:It can be seen that x, y and Z are not independent, and X y Z = 1. Therefore, only x and Y color coordinates can uniquely express the chromaticity.With three stimulus values, we can calculate the color coordinates x and y. Conversely, with the color coordinates x, y, and y, the tristimulus value XYZ can also be calculated, as shown in the following formula:

Tristimulus value XYZ is the basis of mixed color superposition calculation, and mixed color superposition calculation is the theoretical basis of chromaticity correction.1.3 gamut spaceColor gamut refers to the range area composed of the number of colors that can be expressed by a surface color mode, and also refers to the color range that can be expressed by specific media such as screen display, digital output and printing reproduction.

(Figure 1)Note: the large blue and white triangles in the figure are the assumed original color gamut space of the two LED screens, and the small black triangles inside are the set target standard color gamut space sRGB. The color gamut triangle is completely included by the two original color gamut triangles, so it is the color gamut space that can be realized by correction for both displays.For LED display screen, corresponding to CIE 1931 chromaticity diagram (see Figure 1), it is the color gamut triangle composed of three primary color coordinate lines and the set white point. The three primary color gamut triangle determines the color that the LED display can represent. The white dot defines the required ratio of three primary colors, that is, the unit quantity. After the production of a display screen is completed, its color gamut triangle is determined, and the adjustment of white balance can be realized by changing the ratio of RGB by adjusting resistance and other methods.

The colors inside the three primary color gamut triangle are all the colors that can be realized by the mixing of the three primary colors.Therefore, the original color gamut space definition of an LED display screen shall include the following parameters:RxRy GxGy BxBy WxWy

The above four groups of color coordinates are the color coordinates of red, green, blue and white when the display screen displays (r255, G0, B0), (R0, g255, B0), (R0, G0, B255) and (r255, g255, B255).White is formed by mixing RGB three primary colors. Therefore, if the luminance values ry, Gy and by of RGB three colors are given, the respective tristimulus values of RGB three colors can be calculated.The white color coordinates Wx, WY and white brightness value WY can be calculated by the superposition of XYZ three stimulus values of RGB three colors:

On the contrary, given the color coordinates and brightness value of white, the required brightness value of RGB three primary colors can also be calculated.2. Application field of chromaticity correction2.1 improve color fidelity

The most original application of LED screen chromaticity correction serves to improve the color fidelity of the display, make the displayed image consistent with the color of the source image, and restore the natural color more truly. To put it more popularly, it is to make the displayed color more "positive".

The display content on the LED screen is generally from the TV camera, camera, or computer. The color gamut space of TV and computer monitor is inconsistent with that of LED screen, which leads to the phenomenon of display color distortion. For example, the common color gamut space standards pal and NTSC of TV and the color gamut space standard sRGB of computer monitor are inconsistent with the inherent color gamut space of LED display screen. The LED color gamut space is large, the color performance is usually over saturated, and the visual feeling is more gorgeous and exaggerated, so it is distorted.

One of the objectives of chromaticity correction is to correct the color gamut space of the display screen to the color gamut space of the video or image source, or as close as possible, so as to improve the color fidelity of the display.2.2 mixing of different batches of boxesRental screen owners often encounter this situation: batch boxes purchased by time periods are expected to be mixed together to facilitate undertaking larger performance projects. Engineers sometimes encounter the requirements of customers, hoping to expand the area of an original display screen, and make a new part of the box and the old screen into a large screen.

However, due to the difference between the original brightness and the original color gamut space, different batches of boxes go their own way and are incompatible.At this time, chromaticity correction can correct the original color gamut space of boxes in different batches to a coincident target color gamut space, so as to realize the mixing of boxes in different batches and the splicing of new and old screens.2.3 chromaticity uniformity correction

The purpose of chromaticity uniformity correction is to improve the color difference between pixels of the display screen. At this time, each pixel and the combination of a group of RGB lamps can be regarded as a color gamut space. The task of chromaticity correction is to correct the color gamut space of all pixels on the display screen to the same color gamut space.At present, the application of chromaticity uniformity is very limited due to the color separation accuracy of light and color splitter and effective lamp mixing technology. Because the color difference resolution of human eyes to pixel level is about 4nm, and the color separation accuracy of light and color splitter can generally reach 1nm. Chromaticity uniformity correction is necessary unless a very large number of batches and a small number of inventory LED lamps are used for the same screen.3. Chromaticity correction principle

After clarifying the concept of chromaticity and the application field of chromaticity correction, let's take a look at the principle and specific implementation method of chromaticity correction of LED display screen.The principle of chromaticity correction is gamut space transformation. The inherent wide gamut space of LED screen is transformed into a target gamut space set by the user, which can be either a standard gamut space or a user-defined gamut space.For LED screen, in order to ensure the display quality, we must ensure the uniformity of brightness while correcting chromaticity. Therefore, the bright color correction must be completed synchronously. Therefore, the corrected target brightness value shall be given at the same time.

3.1 calculation of gamut space conversion coefficient matrixColor gamut space conversion first needs to determine the original color gamut space tristimulus value matrix [XYZ_ Original] and target gamut space tristimulus matrix [XYZ_ Target], so as to calculate the conversion coefficient matrix [conversion]_ coefficientorder

According to the needs of color gamut space transformation, there are:conversion_ coefficient *XYZ_ original= XYZ_ Target] (equation 9)From the above formula:

conversion_ coefficient= XYZ_ target* XYZ_ Original] - 1 (equation 10)In the conversion coefficient matrix of equation 8,RR is the luminance coefficient of the red light when the display source signal is red;

RG is the brightness coefficient of green light when the display source signal is red;RB is the luminance coefficient of the blue lamp when the display source signal is red;GR is the luminance coefficient of red light when the display source signal is green;

GG is the brightness coefficient of the green light when the display source signal is green;GB is the luminance coefficient of blue lamp when the display source signal is green;BR is the brightness coefficient of red light when the display source signal is blue;

BG is the brightness coefficient of green light when the display source signal is blue;BB is the brightness coefficient of the blue lamp when the display source signal is blue;3.2 application of gamut space conversion coefficient matrix

After obtaining the pixel by pixel color gamut space conversion coefficient matrix, the control system can adjust the color gamut space of the display screen to the target color gamut space by performing real-time pixel by pixel operation on the display source signal.Suppose the conversion coefficient matrix of a pixel is as follows:When the display signal is (r255, G128, B64), the actual lighting of the three lamps of the pixel is as follows:

That is, the source signal (r255, G128, B64) is actually displayed as (R238, G128, B71) on this pixel.It should be noted that the above operation is based on linear brightness. In practical application, the coefficient application and linear operation should be carried out after gamma correction.4. Chromaticity correction skills of LED screen

The calculation method and application method are very clear. However, there are still some matters needing attention and skills in the calibration practice of LED screen.4.1 target gamut space settingThe reasonable setting of the target color gamut space is very important. Otherwise, it may not be achieved, or the white balance may not be achieved, or the brightness uniformity may be damaged.

1) The three primary color coordinates of the target color gamut space must all be within the original color gamut triangle. The color outside the original color gamut triangle cannot be realized by color mixing on this display screen. In the Sv-1 correction system of zhongkeweiyou, CIE1931 color map is provided. The program will draw the original color gamut triangle and the target color gamut triangle, and give a graphical prompt that the target three primary colors are already within the original color gamut triangle to avoid setting errors.

2) Because the maximum brightness of the three primary colors of the display screen is limited, if the white point coordinates and brightness values of the target color gamut space are set unreasonably, a large number of pixels on the display screen will not reach the predetermined target value. In the Sv-1 correction system, the number, proportion and position of pixels that cannot reach the target value on the display screen will be calculated according to the setting parameters of the target color gamut space, The simulation diagram is displayed to help users reasonably set the target white point and brightness.

4.2 requirements for display screen and control systemFrom the example chromaticity correction data in 3.2, it can be seen that the value of the complementary color coefficient in the conversion correction coefficient is small and changes greatly, sometimes only one thousandth, and sometimes even 1 / 3. Therefore, the application of conversion correction coefficient puts forward higher requirements for display screen and control system.1) The display screen must be able to realize gray scale of more than 12 bits;

2) The control system shall be able to read the conversion correction 9 coefficient data with accuracy of at least 12 bits and conduct real-time operation;Only when the above two conditions are met can the display screen ensure the accuracy of chromaticity correction and the brightness uniformity after correction.4.3 most applications of chromaticity correction are gamut space correction.

The original three primary color coordinates required for the correction of color gamut space can be obtained by measuring the regional average color coordinates with a conventional color luminance meter. However, because the RGB brightness ratio in each pixel is inconsistent, it is necessary to ensure the calibration