Specification Requirements For LED Display Screen In DCI Cinema

- Jan 28, 2019-


DCI cinema LED display is one of the hottest topics in the industry. Compared with traditional projection display, LED display has many technical advantages, such as larger dynamic brightness range, better brightness and color uniformity, no graphic distortion, arbitrary size splicing, etc. The first step for the application of LED display in cinema is to meet the certification of DCI (Digital Cinema Initiatives). In view of the display pattern requirements, the technical specifications of LED suitable for this application are analyzed.——LED Display

Background and DCI standards

With the launch of the first DCI-certified LED display, the technology of LED display has opened up a huge market for cinema display. According to LED inside estimates, the Chinese cinema market currently has 40,000 display screens per month and is growing at a rate of more than 10% per year. According to the 4K resolution of the pixel point calculation, the whole market potential LED consumption will be 350kkk. However, the entry of LED display into the cinema market needs to meet the strict technical test CTP (Compliance Test Plan) requirements of DCI (Digital Cinema Initiatives) standards.——LED Display

DCI is jointly founded by the six largest film companies in the world to establish a unified standard for digital film broadcasting, protect the copyright of digital films and ensure the audiovisual experience of cinemas. DCI standard covers data form, encoding and decoding interface, transmission mode, security, cinema hardware system including server, projection, audio and control. The purpose of this paper is to understand the definition of LED specifications for DCI cinema display screen according to the standard requirements of the display screen itself.——LED Display

DCI's requirements for displaying images can be referred to Table 1. Screen resolution is 2K (2048 x 1080) or 4K (4096 x 2160), brightness is 48 + 10.2 cd/m2, contrast is at least 1200:1 (test condition is cinema ambient light, screen reflection is less than 0.03 cd/m2), white balance point is shown in figure 1, and certain errors are allowed. The main saturation wavelengths of standard R, G and B are 100% 621 nm, 89.5% 544 nm and 92.9% 465 nm, respectively.——LED Display

Table 1: DCI_DCSS_v12 Requirements for Display Images

Figure 1: White Balance Points and Gamut Requirements for DCI

II. Packaging Selection of LED

Because of the variety of screen sizes, DCI standard does not impose compulsory regulations on screen sizes. Then corresponding to the screen with fixed resolution of 4K (4096 x 2160), different screen sizes have different LED point spacing. For example, 2.5mm point spacing corresponds to the hall screen with 10.3M width, and 3.3mm point spacing corresponds to the hall screen with 13.5m width.——LED Display

For this point spacing range of display, the optional range of LED size can be 1010, 1616 to 2020, and a variety of packaging forms can be selected. Because of the movie settings in the cinema, even the first row of viewers can not distinguish the pixels. Based on this premise, we should choose the LED with the smallest package size (such as 1010), increase the proportion of the black panel or mask surface area in the whole screen area, and improve the contrast.——LED Display

3. Brightness Requirements of LED

According to the luminance requirement of 48cd/m 2, the distance between 2.5m M pixels and the driving form of 30 sweeps, the luminance of each LED is estimated to be 9mcd. According to the requirement of DCI white balance point, the ratio of three primary colors is R: G: B = 20.9: 72.2: 6.9. For single LED, the luminance of R, G, B is 1.9mcd, 6.5mcd and 0.6mcd. Such luminance specifications are far less luminous than the current mainstream 1010 size LED.——LED Display

It is true that the LED display control system can reduce the sales of high brightness LED display to 48 cd/m2 through gray level control, but it will bring many problems of low gray display, such as red color drift, uneven pixels.——LED Display

Limited by the minimum current specification of the driver chip, the driving current of the LED can not be reduced indefinitely, which requires the LED to reduce the luminance through its own design, if possible, further reduce the size of the chip, or enter more absorbent particles in the package. However, this is at the expense of photoelectric efficiency. If the minimum driving current of the driver chip can be further reduced or the number of scans supported can be further increased, the brightness of the LED display can be reduced without sacrificing the photoelectric efficiency. The LED itself, only by accurately selecting the proportion of R, G, B chips suitable for DCI white balance point color ratio, can meet the brightness requirements of driving chip current in the future, is the most perfect LED to meet the application requirements.——LED Display

IV. Color Requirements of LED

According to the DCI definition of DCI-P3 gamut, the R, G, B primary colors of display screen have strict color coordinate requirements. For the LED display screen, it is necessary for the LED to have strict color coordinate specifications. But at present, most of the LED suppliers use the main wavelength specifications to define color, while the main wavelength is defined from one dimension (hue), and the other dimension (saturation) information of color is missing. The color of the same main wavelength corresponds to a straight line passing through an equal energy point (0.333, 0.333) in the color space, rather than an exact color point. Therefore, in order to accurately meet the requirements of DCI-P3 gamut, first of all, we need to choose the color coordinates to define the color of the LED, rather than the inaccurate definition of the main wavelength.——LED Display

Because of the principle of display mixing, the initial design color gamut of the display screen can be equal to or greater than that of DCI-P3, that is, the initial design color coordinates of R, G and B three primary colors need not be directly the color coordinates specified by DCI-P3, but can be the region formed by the bilateral epitaxy lines and color space boundaries of each base point as shown in figure 2. Green and blue are a small triangle-like area respectively. Because of the color dispersion of LED, such a small color range can hardly be achieved in large-scale production of LED. For red, because the base point specified by DCI-P3 is located on the color space boundary (saturation 100%), there is no possibility that LED can satisfy in theory.——LED Display

Figure 2: LED color coordinate range meeting DCI color gamut requirements

Considering the actual production and testing errors, DCI standard defines the allowable error range of screen tri-primary color coordinates (see Table 1) in the application and authentication process, and rectangular color coordinate range in CIE 1931 color space as shown in Figure 3.——LED Display

Figure 3: DCI-P3 Tri-primary Error Range Allowed by DCI Standard (See Table 1 for Specific Parameters)

Combining with the principle of gamut mixing epitaxy mentioned above, the actual requirements of LED color coordinates can be extended to the areas shown in Fig. 4, 5 and 6. Red light is an area consisting of five coordinate points in Fig. 5 and color space boundary, green light is an area consisting of four coordinate points in Fig. 6 and blue light is an area consisting of five coordinate points and color space boundary in Fig. 7.——LED Display

So, is it possible to get a display that meets the requirements of DCI-P3 gamut by using LEDs with color defined according to these three areas? In practical application, the following two problems should be paid attention to:——LED Display

First, the color coordinates of the LED drift at different driving currents and operating temperatures. If the difference between the color point test current of the LED supplier and the working current of the LED in the design of the display screen is too large, it will probably cause the color point on the screen to drift out of the allowable area. Similarly, the slightly higher temperature of the LED than 25 C in practical application may also lead to color drift. To solve this problem, the best way is that the LED supplier knows the design current in advance, and then tests the color spot of the LED under the same current condition as possible. At the same time, the influence of temperature should be considered in advance.——LED Display

Secondly, the allowable color errors specified in DCI have been taken into account in the scope of the graphic area. In addition to the LED itself, the color errors on the actual LED display screen include the errors of the chip output driving current, the temperature errors in different areas of the screen, the errors of the correction system, and the errors of the color point measuring instrument. Therefore, the range of color dots defined by LED suppliers should be as small as possible in the graphic boundary area, leaving room for errors in display body design and subsequent testing.——LED Display

Figure 4: Red coordinate range of LED satisfying DCI gamut (including error) requirement

Fig. 5: LED green coordinate range meeting DCI gamut (including error) requirement

Figure 6: LED blue coordinate range meeting DCI gamut (including error) requirement

V. Possible Trends in Technical Requirements

Admittedly, the above DCI standards for display image requirements are actually based on the technical capabilities of projection system. For self-luminous LED display system, if the old projection standard is still used, it will undoubtedly greatly sacrifice the technical advantages of LED display. For the breakthroughs in cinema display technology brought about by LED, we have reason to believe that DCI standards will be updated in the near future. From the perspective of LED display, we suggest that we pay attention to three possible trends.——LED Display

First, the resolution is increased from 4K to 8K. 8K video broadcasting has appeared in the field of television broadcasting, but the cinema projection system is limited by DMD module, so it is difficult to improve the resolution. LED displays can easily double the spacing of points in the same size (e.g. 2.5mm to 1.25mm) and improve the resolution from 4K to 8K.——LED Display

Second, join HDR. HDR requires darker dark states and higher peak brightness (e.g. HDR 10 standard, which requires a dynamic range of at least 0.005 cd/m2 to 1000 cd/m2), both of which are very difficult to achieve in projection systems. Because of the advantages of natural brightness and contrast technology, LED display can bring HDR into the cinema system.——LED Display

Thirdly, the range of gamut is improved. The existing DCI-P3 gamut system is not the best gamut that LED can achieve. For example, the existing green and blue LED can theoretically provide higher saturation and further expand the display gamut. It is expected that future color requirements will be a new gamut between P3 and BT2020.——LED Display

(Author's brief introduction: Zhao Yun, Master of Engineering, Nanjing University, is currently Senior Application Engineer in Asia-Pacific Region of OSRAM Optoelectronic Semiconductor (China) Co., Ltd., responsible for technical support and market promotion of LED display applications.)——LED Display