Factors Determining LED Display Quality and Application Fields

1.Factors Determining LED Display Quality

(1) Brightness and Viewing Angle

An LED display’s brightness is primarily determined by the luminous intensity and density of the LEDs. In recent years, new technologies in substrates, epitaxy, chips, and packaging—especially the stabilization of indium tin oxide (ITO) current spreading layer technology—have significantly increased the luminous intensity of LEDs.

Currently, top-tier small-power LEDs can achieve luminous intensities as high as 4000 mcd for green, 1500 mcd for red, and 1000 mcd for blue, with horizontal viewing angles of 110° and vertical angles of 50°. At a pixel pitch of 20 mm, an LED display’s brightness can exceed 10000 nit, allowing it to work around the clock in any environment.

For a single LED display, the brightness is highest when viewed directly from the front. As you move to the sides, the brightness gradually decreases. The angle at which the brightness drops to half of the frontal brightness is called the viewing angle. A wider viewing angle indicates a better side-viewing effect. LED displays typically have a horizontal and a vertical viewing angle. Generally, indoor screens should have a horizontal viewing angle of over 150°, while outdoor screens should be over 100°.

(2)Uniformity and Clarity

Inconsistent performance parameters among LEDs are the main reason for poor uniformity, which in turn is the primary factor limiting a display’s clarity, not pixel pitch. As LED display technology has evolved, uniformity has become the most important metric for judging quality. Phenomena like the “dust effect” and “mosaic phenomenon” are figurative descriptions of severe non-uniformity between pixels and modules.

The root causes of non-uniformity are:

• Inconsistent performance parameters of the LEDs.

• Insufficient assembly precision during manufacturing and installation.

• Inconsistent electrical parameters of other electronic components, or improper design of modules and PCBs.

The main culprit is “inconsistent LED performance parameters,” which includes variations in luminous intensity, optical axis, color coordinates, light intensity distribution curves for each primary color, and decay characteristics.

There are two primary technical approaches to solve this issue:

• Sub-binning: Further subdividing LED specification parameters to improve consistency.

• Post-correction: A method to improve uniformity after assembly, which has evolved from early module correction to today’s point-by-point correction. The correction technology has also expanded from simple luminous intensity correction to a combination of luminous intensity and color coordinate correction.

However, post-correction cannot completely eliminate all issues. Inconsistent optical axes, decay characteristics, poor assembly precision, and irregular design cannot be fixed by post-correction; in fact, the correction might even worsen these inconsistencies. Therefore, post-correction is only a “symptom” remedy, while LED parameter sub-binning is the fundamental solution and the future trend for the LED display industry.

It’s a common misconception in the industry to equate resolution with clarity. LED display clarity is a subjective perception that a human eye has of multiple factors, including resolution, uniformity (signal-to-noise ratio), brightness, and contrast. Simply reducing pixel pitch to increase resolution while ignoring uniformity is meaningless for improving clarity. An LED display with a severe “dust effect” or “mosaic phenomenon” cannot produce a clear image, no matter how small its pixel pitch or high its resolution. In this sense, uniformity, not pixel pitch, is the main factor currently limiting the improvement of LED display clarity.

(3)LED Display Pixel Out-of-Control Rate

The pixel out-of-control rate is the proportion of a display’s smallest imaging units (pixels) that are not working correctly. There are two types of pixel failures:

• Blind spots: Pixels that should be lit but are not.

• Bright spots: Pixels that should be off but remain lit.

An LED pixel is typically composed of 2R1G1B, 1R1G1B, or other combinations of red, green, and blue LEDs. If any single LED within a pixel fails, the entire pixel is considered out of control. For simplicity, the out-of-control rate is calculated for each primary color (red, green, blue) separately, and the maximum value is taken as the display’s overall pixel out-of-control rate.

The ratio of out-of-control pixels to the total number of pixels on the screen is called the whole-screen pixel out-of-control rate. Additionally, to prevent pixel failures from concentrating in one area, the regional pixel out-of-control rate is used. This is the ratio of out-of-control pixels to the total pixels within a 100×100 pixel area (10,000 pixels). This metric provides a quantifiable and intuitive way to enforce the standard that failed pixels should be “discretely distributed.”

Most domestic LED displays are aged before leaving the factory, and any failed pixels are repaired or replaced. The “whole-screen pixel out-of-control rate” is typically controlled to be within 1/104, and the “regional pixel out-of-control rate” is controlled to be within 3/104. Some manufacturers even have a corporate standard of zero failed pixels upon delivery. The acceptable out-of-control rate can vary based on the application. For video playback, a rate of 1/104 is generally acceptable and achievable. For simple character-based information displays, a rate of 12/104 is considered reasonable.

The main cause of pixel failure is LED failure, with electrostatic discharge (ESD) being the biggest trigger. The failure mechanism of LEDs due to ESD is complex, as static electricity can come from equipment, tools, and even the human body. ESD events can range from hundreds to tens of thousands of volts and occur in nanoseconds. The failure of blue and green LEDs during manufacturing, installation, or use is often due to the ESD breakdown of the LED’s PN junction. The International Electrotechnical Commission (IEC) defines three grades of electrostatic discharge sensitivity (ESDS) for devices (human body model): Grade 1 (0-1999V), Grade 2 (2000-3999V), and Grade 3 (>4000V).

Due to the complex production process of LED displays, ESD is difficult to prevent. Therefore, LEDs with an ESDS of Grade 2 or higher are recommended (human body model), and comprehensive static protection measures must be implemented throughout the entire production, installation, and usage process.

(4)Grayscale Levels

Grayscale, also known as gray level or gray scale, refers to the degree of brightness change in a pixel. The grayscale for a single primary color typically ranges from 8 to 1024 levels. For example, if each of the two primary colors has 256 grayscale levels, the display can show $256\times 256=64$K colors, hence it’s called a 256-color display. For digital display technology, grayscale is the key factor determining the number of displayable colors. Generally, a higher grayscale level results in richer colors and a more detailed, delicate image.

Grayscale levels are primarily determined by the system’s A/D conversion bit depth, which requires corresponding support from the video processing chip, memory, and transmission system. Currently, most domestic LED displays use an 8-bit processing system, providing 256 (28) grayscale levels. This allows for 256 brightness variations from black to white. With the RGB primary colors, this creates $256\times 256\times 256=16,777,216$ colors, commonly referred to as 16 million colors. International brands, however, often use a 10-bit processing system, providing 1024 grayscale levels and enabling over 10.7 billion colors.

While grayscale is crucial for color count, an unlimited increase isn’t always better. The human eye’s resolution is limited, and increasing the system’s bit depth dramatically raises costs for video processing, storage, transmission, and scanning, leading to a decline in cost-effectiveness. Generally, an 8-bit system is sufficient for civil or commercial products, while a 10-bit system is appropriate for broadcast-grade products.

(5) Brightness Discrimination Level

The brightness discrimination level is the number of brightness levels a human eye can distinguish between the darkest and whitest parts of an image. Although a display might have very high grayscale levels (e.g., 256 or 1024), the human eye’s limited sensitivity cannot fully distinguish between all of them. Therefore, many adjacent grayscale levels may appear identical to the viewer. For a display, the more levels the eye can distinguish, the better, as it indicates a larger color space and greater potential for displaying rich colors. This level can be measured with specialized software, and a display that can achieve over 20 levels is considered good.

(6)Grayscale Nonlinear Transformation

Grayscale nonlinear transformation is the process of transforming grayscale data based on empirical or mathematical nonlinear relationships before it is sent to the display. Unlike traditional displays, LEDs have a linear display characteristic. To ensure the LED display’s output is compatible with traditional data sources without losing grayscale levels, a nonlinear transformation is typically performed in the post-processing stage of the LED display system. This transformation increases the bit depth of the data to prevent any loss of grayscale information. The “4096-level” or “16384-level” grayscale claimed by some control system suppliers refers to the grayscale levels after this nonlinear transformation. For example, 4096 levels often use an 8-bit source to a 12-bit space, while 16384 levels use an 8-bit source to a 16-bit space. Like grayscale itself, this parameter is not a “bigger is better” situation; a 12-bit transformation is generally sufficient.

(7)Lifespan

The lifespan of an LED display is determined by various factors. While the lifespan of other components (like power supplies) can be extended through replacement, the LED itself cannot be replaced in large quantities. Therefore, once the LEDs reach the end of their life, the entire display’s lifespan is effectively over. The lifespan of an LED is typically defined as the time it takes for its luminous intensity to decay to 50% of its initial value. Although an LED is often said to have a lifespan of 100,000 hours, this is an assessment under ideal conditions that is not achievable in real-world usage.

A simple experiment and formula can be used to estimate LED lifespan: An LED is run for 1,000 hours under conditions identical to its actual operating environment, and its initial and final luminous intensities are measured. The lifespan can then be calculated using a formula. For example, a well-known brand of blue LED operating at 50°C and 20mA for 1,000 hours might have its final luminous intensity measured at 88% of its initial value. Based on the formula, its lifespan in this environment would be 5,422 hours.

The lifespan of the LED determines the lifespan of the display, but they are not the same. An LED display doesn’t have every LED running at full capacity at all times. When a display is playing normal video content, its lifespan is typically 6 to 10 times that of a single LED. If the LEDs are used at lower currents, their lifespan can be even longer. Therefore, a display using the aforementioned brand of LED might have a lifespan of around 50,000 hours.

LED lifespan can be extended through both device manufacturing and application practices:

• Manufacturing: Choosing high-quality epitaxial materials, increasing chip area to reduce current density, balancing current density, reducing thermal resistance, and using high-performance, UV-resistant packaging materials.

• Application: Prioritizing heat dissipation in module design, engineering implementation, and maintenance; reducing LED operating currents; and properly configuring LEDs so that the decay rates of all primary colors are synchronized.

(8)Energy Consumption and Efficiency

Improving LED luminous efficiency and reducing the energy consumption of LED displays is a key development direction with several positive implications:

• Lower investment in power capacity expansion, power equipment, and cooling systems.

• Energy savings, reduced emissions, environmental protection, and lower operating costs.

• Reduced display temperature rise, slower LED decay, extended display lifespan, and improved system reliability.

• Minimized temperature drift of optoelectronic parameters, leading to more stable image quality.

(9) LED Quality Assurance Technology

Producing high-quality LED displays requires attention to the following aspects:

• Anti-static measures: During assembly, proper anti-static measures are essential. These include dedicated anti-static grounding, flooring, soldering irons, mats, wrist straps, clothing, humidity control, and equipment grounding.

• Driver circuit layout and wiring design: The layout of driver ICs on the circuit board can affect LED brightness. Poor design with long transmission paths can cause excessive voltage drop, reducing LED brightness.

• Current value design: The nominal current for an LED is 20mA, but the maximum operating current should not exceed 80% of this value. For small pixel pitch displays with poor heat dissipation, the current should be even lower.

• LED mixing (“混灯”): LEDs of the same color but different brightness must be mixed or arranged according to a designed pattern to ensure consistent brightness across the entire screen.

• LED perpendicularity: For through-hole LEDs, manufacturing processes must ensure the LEDs are mounted perpendicular to the PCB. Any deviation can affect the pre-set brightness consistency.

• Wave soldering temperature and time control: Wave soldering parameters must be strictly controlled, with a preheat temperature of $100℃\pm 5℃$ (not exceeding 120℃) and a soldering temperature of $245℃\pm 5℃$ for no more than 3 seconds. After soldering, the LEDs must not be vibrated or impacted until they return to room temperature.

• Open solder joint control: More than 50% of the time, an unlit LED is caused by some form of open solder joint. This requires strict process control and enhanced quality inspection, with vibration testing being an effective method before shipping.

• Heat dissipation design: LEDs generate heat during operation, and excessive temperatures can negatively impact their decay rate and stability. Therefore, the heat dissipation design of the PCB and cabinet is crucial for extending LED lifespan.

2.Application Fields of LED Displays

LED displays are widely used in a variety of fields:

• Securities trading and financial information displays: This sector once accounted for over 50% of the domestic demand for LED displays and remains a major market.

• Airport and station dynamic information displays: LED displays are the preferred choice for providing real-time information to travelers.

• Port and station passenger guidance systems: LED displays are a key part of automated systems that integrate with broadcasting, arrival/departure systems, and ticketing information.

• Stadium information displays: LED displays have replaced traditional incandescent and CRT screens as the main method for displaying information in major sports venues.

• Road traffic information displays: Used as variable message signs and speed limit signs in urban traffic and on highways.

• Dispatch and command centers: High-density LED displays are increasingly being adopted for power dispatch, vehicle tracking, and fleet management.

• Business promotion and information display in service sectors: Used in post offices, telecommunications, and shopping malls for advertising and information dissemination.

• Advertising media: In addition to large indoor and outdoor LED billboards, clustered LED display advertising systems are becoming more common in major cities.

• Performances and public gatherings: Large LED displays are widely used for live video broadcasts for public and political events, enhancing their impact.

• Exhibitions and rentals: Large LED displays are a crucial service provided by exhibition organizers to exhibitors. Professional rental companies also specialize in this service.

• Schools and hospitals: Used in schools for announcements and educational content, and in hospitals for publicizing information about medicines and health knowledge.