How to correctly understand the main performance indicators of LED display

With the efforts of the LED Display Panel Committee (now the LED Display Panel Branch of China Optical Industry Association), the industry standard "LED Display Screen Test Method" (hereinafter referred to as the "Standard") is about to be released and implemented. This will correctly guide users to understand the performance indicators of LED displays, which is of great significance to regulating the LED display screen market. In order to better implement the standard, the following suggestions are put forward.

1. Maximum brightness

In the industry "standard" discussed at the end of last year, no clear characteristic requirements were given for the important performance of "maximum brightness", which is in line with GB/T1.2-2002. In "5.4.3 Values ​​determined by suppliers" of "Guidelines for Standardization Work Part 2: Methods for Determining the Content of Normative Technical Elements in Standards", it is mentioned that "if product diversity is allowed, some characteristic values ​​of products do not need to be specified (although these characteristics have a significant impact on product performance)". Because the use environment of LED display screens varies greatly, the illumination (that is, the ambient brightness as generally referred to by people) is different, so "for most complex products, as long as the corresponding test methods are specified in the standard, it is better for the supplier to provide a performance data (product information) list than the specific performance requirements given in the standard". These are all in line with international standards, but this has caused unrealistic comparisons in bidding, and users do not understand this, resulting in the "maximum brightness" required in many bids often being far higher than the actual needs. Therefore, it is recommended that in order to guide users to correctly understand the performance indicator of the "maximum brightness" of LED display screens, the industry needs to give a guide: in certain occasions, under different illumination environments, what value of the brightness of the LED display screen can meet the requirements.

2. Primary color dominant wavelength error

Changing the primary color dominant wavelength error indicator from "primary color wavelength error" to "primary color dominant wavelength error" can better explain what characteristics of the LED display this indicator reflects. The dominant wavelength of a color is equivalent to the hue of the color observed by the human eye. It is a psychological quantity and an attribute that distinguishes colors from each other. However, from the literal meaning of the performance requirements stipulated in this industry standard, users cannot understand that it is an indicator that reflects the color uniformity of the LED display. Therefore, should we guide users to understand this term first and then understand this indicator? Or should we first understand and understand the LED display from the customer's perspective, and then provide easy-to-understand performance characteristics that users can understand? Just like the aforementioned GB/T1.2-2002 (Guidelines for Standardization Work Part 2 Methods for Determining the Content of Normative Technical Elements in Standards), one of the principles for product standard formulation is the "performance principle": "Whenever possible, requirements should be expressed by performance characteristics rather than by design and description characteristics. This approach leaves the greatest room for technological development." "Primary color dominant wavelength error" is such a design requirement. If it is replaced by "color uniformity", there will be no limit on the wavelength of LED. For users, as long as you ensure that the color of the LED display is uniform, you don't have to consider what technical means you use to achieve it, leaving as much room as possible for technological development, which is greatly beneficial to the development of the industry.

3. Duty Cycle

Just like the "performance principle" mentioned above, "whenever possible, requirements should be expressed by performance characteristics rather than by design and description characteristics. This approach leaves the most room for technological development." We believe that "duty cycle" is purely a design technology requirement and should not be used as a performance indicator of LED display product standards. Everyone knows that no user cares about the driving duty cycle of the display. They care about the effect of the display, not our technical implementation. Why should we create this technical barrier ourselves and restrict the technological development of the industry?

4. Refresh rate

Judging from the measurement method of the Standard, it seems that the issues that users really care about are ignored. It also does not take into account the different driver ICs, driver circuits and methods used by various manufacturers, which makes testing difficult. For example, in the bidding for the full-color screen of Shenzhen Stadium, the test of this indicator brought many problems in the sample test of experts. The "refresh frequency" is the reciprocal of the time required to display a frame of the picture. If the display screen is regarded as a light source, it is the flickering frequency of the light source. We can use an instrument similar to a "light-sensitive frequency meter" to directly test the flickering frequency of the light source of the display screen to reflect this indicator. We have done tests in this regard and used an oscilloscope to measure the LED drive current waveform of any color to determine the "refresh frequency". The measured frequency was 200Hz under white field; under low grayscale levels such as 3-level grayscale, the measured frequency was as high as more than ten kilohertz, and when measured with a PR-650 spectrometer; whether in white field or at grayscale levels such as 200, 100, and 50, the measured light source flicker frequency was 200 Hz.

The above points are just a brief explanation of the characteristics of several LED display screens. There are many "working life" and "mean time between failures" encountered in bidding. There is no test method that can verify whether the LED display screen meets the requirements of stability, reliability or life in a short time; these requirements should not be stipulated. The producer can make a guarantee, but it cannot replace the requirements. It is a business concept and a contract concept, not a technical concept. The industry association should have a clear statement on this, which will be very beneficial to users, producers and the entire industry.

As for how to guide users to correctly understand a complex system product like LED display, the industry association should organize more LED display technology forums, analyze the product from the user's perspective based on reality, and guide users to correctly understand LED display.