introductionDirect looking light emitting diode (LED) technology is the mainstream technology used to realize outdoor digital identification system, but the development of this technology also brings many interesting challenges to measure and evaluate contrast. When people choose suitable devices in a variety of display technologies or multiple display manufacturers for a specific purpose, like several other important display parameters, contrast is also a key consideration.
Most people in the display industry agree that few end-users can really understand the impact of image brightness, gamut breadth and fidelity on the contrast of the display. However, the fierce competition in comparison among liquid crystal display (LCD) consumer manufacturers with television as the main consumer goods has not slowed down at all. With the different test methods associated with the variability of test variables, the contrast index published by the manufacturer can appear almost any value. However, the space for the actual change of contrast is limited. At present, dynamic contrast ratio specifications have become more and more common and gradually accepted by people.
Not only the consumer display manufacturers, driven by the sales pressure of the consumer display market, the major LED display manufacturers are also facing great competitive pressure in terms of contrast. This is especially true for the outdoor LED display industry, because when selecting products, end users always first consider the contrast technical indicators of the product, even if they may not fully understand the significance of various variables in the test environment and real viewing environment of outdoor display.1. Components of large-size LCDIn order to better talk about the theme of this paper, it is necessary for us to understand the basic unit of large-size LED display screen - LED module. By arranging the modules in the prefabricated frames, then assembling these frames together, and finally coordinating the power consumption and signals between them, a display screen of any size and resolution can be formed.
For example, the display screen of the Royal Kansas baseball team is 85 feet wide and 105 feet high. It is composed of 36 parts and 5940 LED modules, with a resolution of 1584 1800 pixels, as shown in Figure 1.Figure 1 the HD display screen located in the major league baseball field is composed of about 6000 LED modulesThe mechanical parameters of the front surface of LED module are the key factors affecting the off state test of display screen. Although the design and manufacture of LED display screens of different companies will vary to some extent, most companies have invested a lot of energy to improve the "video black" state of the display screen from all aspects, as shown in Figure 2.
Figure 2 the LED module is equipped with a sunshade, which is often used to improve the contrast of outdoor identification systemIn most LED modules, the light shield is the last part to be installed. This component called "light shield" is a shutter assembly. It is fixed and installed only after the basic module is assembled. This component is very important for the measurement of brightness performance in off state. The name of the light shield comes from its ability to protect the LED panel from sunlight.Generally speaking, the less sunlight directly reflected by the display screen, the higher its total contrast. For most outdoor devices, they are exposed to the sun most of the day, so the manufacturer must strictly design the sunshade to ensure that the display can work normally. The design of the sun visor is also a job to find the best balance between the display contrast and the viewing angle.
In addition to the sunshade design of the LED module, the texturing approach of the sunshade surface will also affect how much ambient light will eventually be reflected into the eyes of the observer. Generally speaking, this is related to the processing method. The difference between rough surface and semi smooth surface is that it will more effectively reflect the ambient light in the way of diffuse reflection, which provides a darker off state for the audience, so as to improve the uniformity of the contrast of each viewing angle of the display screen. In contrast, the polishing coating treated in the last process is dim when viewed from the direction of light incidence angle, so it can theoretically enable the manufacturer to provide higher contrast. However, the coating will produce serious glare in the direction of reflection angle, which will affect the effect of side angle viewing.
The methods to realize the sunshade design and rough face mask mostly adopt the coating approach. Many but not all LED display manufacturers use anti reflection and anti ultraviolet (UV) black paint to treat the surface of LED module sunshade, trying to further reduce the reflection of light on the surface of LED module. External environmental conditions also have an important impact on the total contrast of the display screen. After the display screen is used for a long time, if it is not cleaned regularly, a large amount of dust and dirt will accumulate. After a certain time limit, it will begin to affect the total contrast of the display screen and gradually turn the "black" state into brown. At present, several methods to clean up dust and dirt have been introduced, and its actual effect remains to be verified. Another challenge for display manufacturers and users is to deal with the impact of ultraviolet light on the front surface of the display. The anti UV coating or plastic on the front surface of the display helps to minimize this impact.
Finally, each LED tube will be 1 3mm higher than the surface of the display screen, which will also reduce the contrast performance of the display screen.
The intensity of light reflected from each LED has a certain relationship with the LED size and the color of the LED epoxy shell. Each LED contains a reflector for focusing light, and this reflector will also reflect various ambient light entering the LED. Therefore, each LED itself may reduce the overall contrast of the display screen. Fortunately, however, LED manufacturers have adopted the optimized design of only one red LED, one green LED and one blue LED pixel by improving power efficiency and brightness. For the LED display screen produced in the past, according to the design of louvers and the surface texture treatment at that time, each pixel must contain 12, 8, 6 or 4 LEDs to provide sufficient light intensity (as shown in Figure 3). However, the continuous progress of LED technology has greatly improved the contrast of the display screen.
Recently, a new operation method in the LED video industry is to use the so-called black LED package to improve the contrast of outdoor light LED video display. Most red, green and blue outdoor light LEDs add more red, green and blue dyes to epoxy resin packaging materials to effectively improve color discrimination and display contrast. The so-called "black encapsulated" LEDs, in short, just add some more dyes to color them, making them slightly darker rather than blackening. Because more coloring dyes are added, the black LED package does maximize the overall display contrast, but at the cost of increasing power consumption and sacrificing display brightness. The difference between standard led and black LED can be seen by comparison in Figure 4.
Figure 4 difference between standard LED (upper row) and black LED (lower row)
It should be added that all contrast enhancement methods should not only be coordinated in a unified way in the production process of a batch of modules, but also coordinate all modules within an appropriate time range. Some consumer groups will regularly require the coordination and calibration of the module after the display has been used for 5 10 years. The normal working life of LED video display screen is generally 75000 100000 hours. At the end of the service life, the company needs to provide 11 years of support to ensure that the contrast remains unchanged, which needs to be planned at the beginning of each project.
2 variables affecting contrastUnderstanding the design of outdoor LED display and how it affects the contrast is of great help to deeply understand the variables that will affect the contrast test of this kind of products. As mentioned earlier, the contrast of outdoor large-size LED video display screen greatly depends on the construction method of the module described previously, including the design of louvers, texturing, coating and led density.
In order to make the contrast test meaningful and repeatable, it is necessary to prepare a completely dark darkroom to eliminate the interference of external light. In contrast test, LED module, test instrument and light source are the three key factors of interaction. Most LED display manufacturers will only do a basic on-off test process, and its off state corresponds to video black. However, there is no unified test technical standard in the whole industry. Although there are some legal standards for measuring contrast for some display technologies, such as VESA FPDM (Video Electronic Standards Association flat panel display test), it is not fully applicable to the LED industry. Different from flat panel display measurement, the ambient light source (sun) and measurement tools (viewer) are changing every moment of the day, and the pedestrians driving or walking are also changing every minute. This is difficult to repeat in the test and explain clearly in a product manual.
It should also be noted that under the test conditions, compared with the large-size complete LED display screen, it can only be tested on a single module or a collection of modules.In the measured environment, the ambient light is an artificial light source to reproduce the real outdoor viewing environment. Obviously, adjusting the brightness of the light source in the test environment (in Lux) will greatly affect the test results, which can be used to enhance the advantages of product publicity. Since the sunlight on the surface of the display screen itself is changeable, any result can be considered reasonable.At present, in the instructions published by the display manufacturer, the ambient brightness range is 40 50000 lux.
The contrast test results can also be improved by adjusting the angle of the light source relative to the display module. As mentioned earlier, the LED module will have horizontal shutters, namely sunshades, distributed between each row of LEDs or each row of pixel points. Increasing the angle of the light source relative to the LED can create a larger shadow area on the module surface, so as to finally improve the black field state of the display screen.Finally, as mentioned earlier, the display manufacturer can also provide convenience for controlling the final contrast parameters by adjusting the angle of the test instrument relative to the display surface. It should be noted again that this may also be the result of the interaction between the light source and the horizontal sunshade.These three basic variable factors - light source intensity, light source angle and test instrument angle - jointly determine the final published LED display contrast. Unless the tests of different displays are unified, it is not appropriate to compare the test results of different products. Therefore, the contrast specification published by the display manufacturer does not have much practical significance (as shown in Figure 5).
In the real viewing environment and test environment, the three variables of light source intensity, light source angle and test angle still have a certain correspondence. The condition of the sky has a great influence on the intensity of sunlight on the surface of the display screen. The angle of sunlight on the surface of the display screen at different times of the year and at different times of the day