Do You Understand LCD Backlight?

Views: 239     Author: Reshine Display     Publish Time: 2023-11-02      Origin: Site

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Do You Understand LCD Backlight?

A backlight is a type of illumination that is used in LCDs. Unlike CRT, LCD does not generate light on its own. To produce an image, an LCD display requires illumination, also known as ambient light or backlight. There are currently two types of backlights on the market: CCFL and LED.


1. Introduction

TFT LCDs have traditionally used Cold Cathode Fluorescent Lamps as backlights. Before the introduction of LED backlights, it was the dominant player in the market. CCFL is a mature technology that produces a consistent color light source. However, the CCFL backlight is relatively large and inefficient. It is currently being phased out of the market.


LED: On the other hand, LED backlight is small, has low power consumption, high brightness, and a long service life. As a result, LED is displacing CCFL in the LCD backlight market.


LED Backlight of LCD Display LED, or Light Emitting Diode, is a semiconductor diode that emits light when an electric voltage is applied to it. LED backlights are classified into two types based on where they are installed.


Edge-lit: LED light bulbs are attached to the TFT LCD's edge. To direct light out the front, a light guide plate (an acrylic panel typically made of pure PMMA resin) is used. The backlight of an edge-lit LCD can be very thin. However, it is inefficient. The less efficient the edge-lit backlight, the thinner it is.


Bottom-lit: As the name implies, LED light bulbs are evenly distributed across the back of the TFT LCD. The uniformity and brightness of the bottom-lit backlight are advantages. However, the bottom-lit backlight is thicker than the edge-lit, and heating may be an issue.


Driving LED backlight LED is temperature sensitive. It also has a non-linear volt-ampere characteristic that is temperature-dependent.

LED is sensitive to temperature

As the temperature rises, the forward voltage drop decreases, and the forward current increases significantly.


LED light bulbs with constant current driving are connected serially. The LED temperature rises when a constant current is applied to the LED backlight. Because the supply current remains constant, the voltage applied to the LED backlight falls. Furthermore, the LED backlight will produce consistent brightness.


2. Driving with Constant Voltage

Because of the unique volt-ampere property of LEDs, higher temperature results in higher current when supplying constant voltage to serially connected LED backlights. A higher current generates more heat. For the serial circuit, this effect will be amplified. High current accelerates the degradation of LEDs and reduces their life span. Furthermore, because each LED has a slightly different volt-ampere property, each LED has a different pass-thru current. As a result, the LED backlight will be unevenly bright.


As a result, when using a TFT LCD with an LED backlight, the engineer should apply constant current to the LED backlight's power supply and control brightness with the PWM function.


3. Trends in LCD Backlighting

The LCD does not emit light. It relies on backlight to illuminate its pixels, which is especially important in low-light environments. Before we get into LCD backlight trends, let's go over some history.



4. History of LCD backlighting

The incandescent light bulb was used as a light source for LCD in the beginning. However, because of the heat it produces and the energy it consumes, it was quickly replaced.

The challengers were Cold Cathode Fluorescent Lamps (CCFL) and Electroluminescent Panels (ELP), which eliminate heat problems associated with incandescent light bulbs while consuming less power.

LED backlighting is now the dominant technology. It has a shorter ON/OFF time, less heat generated, and a higher brightness.


5. The new trends

The majority of today's LCDs have edge-lit backlighting. Several white LEDs are mounted behind one edge of the LCD screen, directing light into a complicated array of light guides, reflectors, and diffusers. To control the screen brightness, the LED backlight is typically driven by a pulse-width modulated signal.


Active matrix mini LED backlight, which was invented by LCD TV manufacturers, is now making its way into consumer and industrial LCD applications. Mini LEDs serve as backlight sources in the structure of a mini LED-backlit display and are arranged in arrays to form different local dimming zones that enable full brightness or darkness for the above LCD to achieve high brightness and contrast.


The use of mini LED as an LCD backlight provides a high contrast ratio, power conservation, and thinning benefits, allowing it to compete with OLED and demonstrate distinct performance in LCD applications.


6. What exactly is an LCD viewing angle?

The maximum viewing angle of an LCD is the angle from which a customer can see the screen. The term "viewing well" is commonly defined as having at least 10:1 contrast and no grayscale inversion.


To accommodate as many applications as possible, LCD manufacturers design LCD panels to be best viewed from an angle (called Bias) offset from the perpendicular by certain degrees. The viewing angle is the angle that encompasses both sides of the Bias angle while the LCD continues to "view well."

viewing

The term "viewing direction" will appear in the specifications of an LCD. The viewing direction is specified in the format of a clock. The Z axis is normal, the X axis is horizontal, and the Y axis is vertical, as shown in the figure below. The viewing angle of an LCD "above" the Z axis has a noon viewing direction. As a result, the 6 o'clock viewing direction LCD is best viewed from "below" the Z axis.

viewing angle

LCD manufacturers can theoretically produce LCDs with viewing directions of 3, 12, 9, and 6 o'clock. In practice, however, we usually view LCD from the 12 or 6 o'clock position.


Enhance LCD Viewing Angle

A simple TN-type TFT LCD, for example, has a viewing angle of 4565 degrees. The viewing angle of a TN-type LCD can be increased by adding extra wide polarizer film (EWP). An O-film enhancement polarizer, on the other hand, will increase the viewing angle to 75 degrees in each direction. However, these enhancements result in a reduction in contrast. LCD contrast must be adjusted. Another way to improve the LCD viewing angle is to use an advanced cell structure. Viewing angles on IPS and VA-type LCD panels are wider than on TN panels.


Conclusion

The use of a liquid crystal display with a proper viewing angle is critical to the success of your product. Also, keep in mind that optimized contrast is essential. Both parameters influence the visual appearance of the LCD as well as the overall appeal of your product. Cost-performance trade-offs drive decisions.


For example, if your project necessitates a 2.4" TFT LCD with a wide viewing angle. A TN TFT panel with an O-film solution may be preferable to an IPS LCD. Topway engineers are available to assist you in making design decisions.

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