Why do LCD screens have bright spot?
A "bright spot " (also known as a "bad spot " or "bright spot defect ") on a LCD screen refers to a pixel (or sub-pixel) on the screen that remains constantly lit when it should display black
A "bright spot" (also known as a "bad spot" or "bright spot defect") on a LCD screen refers to a pixel (or sub-pixel) on the screen that remains constantly lit when it should display black. It will appear as small dots in red, green, blue, or white, especially glaring against dark backgrounds.
The fundamental reason is that the physical structure of the pixel has malfunctioned, losing its normal "switching" ability. To understand this, it is necessary to first understand the basic working principle of LCD pixels:
Core principle: How pixels work
Each LCD pixel is like a tiny 'light valve', and its basic structure is:
Liquid crystal layer: a special liquid whose molecular arrangement can be changed by an electric field.
Up and down polarizing film: Like a fence, it only allows light from specific directions to pass through.
Color filter: divided into three sub pixels of red, green, and blue, and mixed to produce various colors.
TFT thin film transistor: Each sub-pixel corresponds to a TFT, which serves as an independent electronic switch.
Backlight: located at the back of the screen, providing a uniform white light source.
Normal workflow: When a current signal is applied to the liquid crystal through the TFT "switch", the liquid crystal molecules deflect, thereby changing the polarization direction of light and ultimately controlling the amount of light passing through. By precisely controlling the opening and closing degree of the red, green, and blue sub pixels, millions of colors can be mixed. When black is required to be displayed, the three sub pixels should be completely turned off to prevent any backlight from passing through.
Specific reasons for the emergence of highlights
The highlight is that this "light valve" is stuck in the "open" position. The specific fault points may occur in the following stages:
TFT transistor malfunction (most common cause)
The switch is stuck in the on state: the TFT transistor controlling the pixel cannot be turned off due to manufacturing defects or later damage. This is like a broken faucet that cannot be turned off, causing current to continue to be applied to the liquid crystal. The liquid crystal molecules are always in a state that allows light to pass through, and the sub-pixel stays on.
If all the TFTs of the red, green, and blue sub pixels are "stuck", the pixel will be displayed as a white bright spot.
If only one or two sub pixels of the TFT are "stuck", it will be displayed as colored highlights such as red, green, blue, cyan, magenta, yellow, etc.
Defects in the LCD itself
During the production process, if the liquid crystal material at the pixel position is injected unevenly, contains impurities, or the gap (cell thickness) between the upper and lower substrates is abnormal, it may also cause the liquid crystal molecules at that point to be unable to respond to electrical signals and remain in a transparent state.
External physical damage
Screen compression, impact, or puncture: may cause permanent damage to the TFT circuit or liquid crystal structure of the pixel, resulting in bright spots.
Electrostatic breakdown: Strong electrostatic pulses may penetrate fragile TFT transistors, rendering them ineffective.
Why can't it be avoided? The production process is crucial
The majority of highlights are generated during the manufacturing process of screen panels and are determined by the limits of the manufacturing process.
Dust in the microscopic world: In complex semiconductors and micro scale processes, even in cleanrooms, extremely small dust particles are inevitably present. A dust particle smaller than a pixel falling on a key thin film layer may cause the TFT circuit of that pixel to break or short-circuit, forming bright or dark spots.
Process fluctuations: In hundreds of processes such as deposition, etching, and packaging, any small fluctuations in temperature, humidity, material concentration, or equipment accuracy can cause individual "defective products" in millions or even millions of pixels.
Yield rate and industry standards: Due to the above reasons, the cost of producing a 100% perfect and defect free screen is extremely high and almost impossible. Therefore, the industry has established standards for accepting bad pixels (such as Class I. Class II, etc. in ISO 9241-307 standard), allowing a small number of bad pixels to exist in specific areas of the screen. As long as the number of bad defects does not exceed the standard, it is considered a qualified product leaving the factory.
Highlight vs. Dark Spot
For a more comprehensive understanding, here is a comparison:
Highlight: TFT switch cannot be turned off, backlight can always pass through.
Dark spot: TFT switch "cannot be turned on", LCD blockage, or color filter defect, causing the backlight to be completely unable to pass through, appearing as a small black dot in any color.
Project | Instructions |
Essence | The physical hardware failure of a pixel is usually due to the failure of the TFT transistor. |
Main cause | Microscopic defects (dust, process fluctuations) during the production process are the main cause; Subsequent physical damage (compression, static electricity) is the secondary cause. |
Is it repairable | It cannot be permanently fixed through software or 'repair video'. This is a permanent hardware damage, and any software claiming to be repairable is only a temporary visual interference that cannot repair the physical structure. |
What to do | Newly purchased equipment: During the inspection period, according to the manufacturer's warranty policy (many brands have stricter standards for highlights, such as "no highlight guarantee"), if the number or location of highlights exceeds the promised range, a replacement can be requested. Out of warranty equipment: A single bright spot usually does not affect the main use, but if you are very concerned or there are more bright spots, the only way is to replace the entire screen panel, which incurs higher maintenance costs. |
Simply put, the highlight of a LCD screen is like a small ink dot with a printing error on a book, which is a microscopic defect that is difficult to completely avoid during the manufacturing process. It represents that the "switch" of that pixel is damaged and stuck in the open position.
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