Introduction to Polarization Effect & White Screen Case Study

Brief Introduction to the Polarization (Dark Edge) Phenomenon

Previously, we encountered a phenomenon where, before the LCD fully powers on, a noticeable dark or black border appears around the edges of the screen.

We generally refer to this as a polarization phenomenon. It appears similar to the illustration below.

Characteristics of This Phenomenon

• Occurs only during initial power-on or cold start
• Most noticeable under full white or light-colored backgrounds
• Gradually disappears as the panel temperature rises during operation
• Does not affect display functionality
• Does not affect touch performance
• Does not impact panel lifetime

The dark border typically fades within a few seconds to several tens of seconds.

This is a normal optical characteristic of LCD panels, not a defect and not a quality issue.

Root Causes of the Dark Edge Phenomenon

When the LCD is first powered on, the liquid crystal molecules, voltage driving system, and backlight system have not yet reached a stable operating state. This causes slightly reduced brightness in edge areas, forming a visible dark border.

1. Initial Polarization of Liquid Crystal

At power-on:

• Liquid crystal molecular alignment is not fully established
• Vcom (common voltage) waveform is not yet fully stabilized
• Positive/negative polarity switching has not fully entered steady-state

The edge area is more sensitive to voltage variations due to:

• Seal glue (frame seal)
• Black Matrix (BM) limitations

Under a white background, the edges therefore appear darker than the center.

2. Low Temperature Effects

When the LCD temperature is low:

• Liquid crystal viscosity is higher
• Response time is slower

Edge areas warm up more slowly due to structural shielding, so brightness recovery is delayed — making the dark border more visible.

3. Backlight Startup Instability

At initial startup:

• Backlight LED brightness has not fully stabilized
• Light guide plate edge illumination is not yet uniform

This results in lower brightness at the edges compared to the center.

4. Structural Light Absorption

LCD edges inherently include several unavoidable structures:

• Seal glue (Seal)
• FPC circuit area
• Black Matrix masking area (BM)
• Backlight frame edges

These structures absorb more light during startup, amplifying the dark edge effect.

Solutions / Mitigation Methods

1. Software Optimization (Highly Recommended)

• Avoid displaying high-gray or pure white images during the first few seconds after power-on
• Use dark backgrounds (black / dark blue / dark gray) during startup
  → This almost completely hides the phenomenon
• Delay full-white display by 1–3 seconds
• Apply gradual backlight ramp-up (soft start) to improve cold-start edge uniformity

2. Hardware Optimization (If Further Improvement Is Required)

• Improve panel preheating strategy
  For example, briefly drive low gray levels at startup to accelerate stabilization
• Fine-tune Vcom voltage (must be verified per panel specification)
  Helps reduce initial polarization deviation
• Optimize backlight startup curve
  Allows the light guide plate to reach uniform brightness more quickly

3. Usage Recommendations

• Avoid displaying pure white high-brightness images immediately under very low temperature conditions
• At low temperatures, liquid crystal viscosity increases, making dark edges more noticeable

USB-C Sample White Screen Issue – Analysis & Improvement

1. Observed White Screen Condition

During testing, another issue was observed:

After testing, the display remained connected to the computer. The computer was powered off, but the power supply remained connected. Under this condition, the display panel turned completely white.

The factory initially suggested this was a polarization phenomenon. However, we believe it is more likely related to power supply, backlight control, or display driver logic.

When the laptop lid is closed, the system may enter sleep or standby mode. Although power remains connected, there is theoretically no video signal. In this condition:

• The display should go dark
• The backlight should turn off

2. First Improvement Verification

To resolve the issue, the engineer routed out a video signal detection pin to determine whether a valid video signal was present.

The logic implemented:

• When video signal is detected → turn on display power
• When no signal → cut off display power

Testing showed this method was effective:

• When the video signal was removed, the white screen did not appear
• However, during signal restoration, a brief white transition was visible

Since this was only a functional verification, further optimization will be implemented via software:

• When signal is detected, apply a short delay before powering on the display
• This avoids the visible white transition

3. Second Improvement Verification

Based on the first solution, further hardware and software optimizations were implemented.

An external MCU was used to control:

• Signal arrival detection
• Display power enable
• Backlight enable

Only after confirming valid signal arrival does the MCU power on the display and backlight.

Results:

• When the computer is shut down, the display shows no image
• The backlight is fully turned off
• The white screen issue is completely resolved

4. Minor Startup Backlight Flash Issue

Another minor issue was observed:

When the computer boots up and USB-C first supplies power to the display, the display circuitry is still in reset state. During this moment, the backlight briefly flashes.

Solution:

• Add a delay circuit stage
• Stagger the power timing between system reset and backlight enable

This eliminates the brief backlight flash.

The adjusted startup time is approximately 1–2 seconds.

If you have any questions, please contact our engineering.