An LCD does not emit an image. It starts with a white backlight that is always on, and then removes light from it — layer by layer, pixel by pixel — until what is left is a picture. Everything an LCD does well and everything that goes wrong with it follows from that one fact: the black is only as good as the blocking, and a pixel fails when the shutter controlling it stops responding. This page walks through the layers in order and shows exactly where dead and stuck pixels come from.
On this page
- What are the layers of an LCD screen? — the sandwich that turns a white lamp into a picture
- How do liquid crystals control light? — the electrical shutter behind every sub-pixel
- How do sub-pixels create colour? — three filters, and why your eye sees one colour instead of three dots
- What is the difference between IPS, VA and TN panels? — three ways of arranging the same crystals, with different trade-offs
- How is OLED different from LCD? — no backlight at all, and what that changes about failure
- Why do LCD screens get dead and stuck pixels? — where in the stack each fault happens, and which one is fixable
- How do you test an LCD screen? — what each solid colour reveals about the layers
What are the layers of an LCD screen?
An LCD panel is a stack of layers, and light passes through all of them on its way to you:
- The backlight. A sheet of white LEDs across the back of the panel, on whenever the screen is on. It never changes to form the image.
- A diffuser. Spreads that light evenly so the picture is not brighter near the LEDs.
- The first polariser. Filters light down to a single orientation, like a letterbox that only passes light waving one way.
- The liquid crystal layer, divided into cells — one per sub-pixel — each with its own transistor.
- Colour filters. Red, green and blue, one over each cell.
- The second polariser, rotated 90° from the first.
The two crossed polarisers are the key. On their own they block essentially everything: light that gets through the first is at the wrong orientation for the second. The liquid crystal sits between them and decides, cell by cell, whether to rotate the light so it can pass.
How do liquid crystals control light?
Liquid crystals are the reason the technology has its name: a state of matter that flows like a liquid while its molecules stay aligned like a crystal. That alignment can be changed with a small voltage, and it changes what happens to light passing through.
With no voltage, the molecules sit in a twisted arrangement that rotates the light by 90° as it travels through — which lines it up perfectly with the second polariser. The light passes and the sub-pixel is bright.
With voltage applied, the molecules straighten out. The light is no longer rotated, hits the second polariser at the wrong orientation, and is blocked. The sub-pixel goes dark. Vary the voltage in between and you vary the brightness continuously, which is how a panel produces shades rather than just on and off.
So each sub-pixel is an electrically controlled shutter, and the panel is millions of them operating independently. Nothing here generates light — every cell can only subtract from the backlight behind it. That is why LCD black is really very dim grey, and why a completely dark room shows up the difference. Our screen contrast test shows how well your panel separates those dark shades.
How do sub-pixels create colour?
Every pixel is three sub-pixels side by side, each with a colour filter over it: one red, one green, one blue. The liquid crystal cell behind each one sets how much light gets through, and your eye blends the three into a single perceived colour because they are far too small to resolve separately at normal viewing distance.
All three at full brightness gives white. All three closed gives black. Red and green with blue closed gives yellow; green and blue gives cyan. With 256 levels per sub-pixel that is roughly 16.7 million combinations, which is where the “16.7 million colours” on a spec sheet comes from.
Modern phone panels pack over 300 pixels per inch, which is about the point at which the human eye stops resolving individual dots at arm’s length. That density is also why a single failed sub-pixel on a phone is far harder to spot than the same fault on a large, coarse television panel.
What is the difference between IPS, VA and TN panels?
All three are LCDs using the layers above. What differs is how the crystals are arranged and which way they move, and that produces genuinely different screens:
- IPS aligns the crystals parallel to the panel and rotates them in that plane. Best colour accuracy and by far the best viewing angles, which is why it dominates phones, tablets and anything used for design work. Historically slower, though modern IPS is fast enough for most gaming.
- VA aligns them perpendicular to the panel. Blocks light much more effectively when closed, so contrast is far higher and blacks look genuinely dark — the usual choice for televisions. The trade-off is colour shift when you move off-centre.
- TN is the original twisted arrangement. Cheap and very fast, but with noticeably worse colour and viewing angles. Now mostly confined to budget and high-refresh gaming displays.
Panel type also changes what pixel faults you should expect. Cheaper panels generally show more of them, simply because the manufacturing tolerances are looser.
How is OLED different from LCD?
OLED removes the backlight, the polarisers and the liquid crystal entirely. Each sub-pixel is an organic compound that produces its own light when current passes through it. To display black, an OLED pixel simply switches off — there is no lamp behind it to block, so black is genuinely black and contrast is effectively unlimited.
That difference changes how the two technologies fail, which matters a great deal if you are diagnosing a fault:
- LCD fails toward stuck pixels. A crystal cell that stops responding lets backlight through when it should be blocking, giving a permanently lit dot — often recoverable.
- OLED fails toward dead pixels. There is no backlight to leak, so a failing sub-pixel goes dark rather than bright — not recoverable.
- OLED also ages unevenly. The organic compounds dim with use and blue dims fastest, which is what causes burn-in. LCD does not do this.
Pixel faults on OLED covers why the LCD fixes do not transfer.
Why do LCD screens get dead and stuck pixels?
Now the structure explains the faults exactly. Each sub-pixel depends on its own thin-film transistor, and a 1080p panel holds around six million of them.
A dead pixel is a transistor that has stopped delivering power altogether. The cell cannot be driven, all three sub-pixels stay dark, and the pixel reads as a black speck. There is nothing left to send a signal to, which is precisely why no software can recover it.
A stuck pixel is a transistor that still has power but has stopped switching. The cell is frozen in one position, letting a fixed amount of light through its colour filter regardless of what the panel is told to show. Because the electronics are still live, rapidly cycling colours through it can sometimes shake it loose — which is the whole basis of the flashing method.
Backlight bleed is a third thing entirely, and gets misdiagnosed as a pixel fault constantly. It is light escaping around the edges of the panel where the layers do not seal perfectly, visible as brighter patches at the corners of a black screen. No pixel is faulty and no software will change it.
How do you test an LCD screen?
Knowing the structure makes testing straightforward, because each full-screen colour interrogates a different layer:
- Black exposes anything letting light through when it should not: stuck pixels, backlight bleed and clouding.
- White exposes anything not letting light through: dead pixels, and dust on the glass.
- Red, green and blue isolate individual sub-pixels. One failed sub-pixel hides on its own colour and shows clearly on the other two.
- Grey exposes uniformity problems and banding across the backlight.
How to run a dead pixel test covers the preparation that makes the result trustworthy, and our LCD screen health guide adds contrast, uniformity and backlight checks to the pixel test.
