What people are describing
On a dark screen, an LCD can show brighter areas that should be black. Along one or more edges, or in a corner, a glow appears. This is usually called backlight bleed. In other cases, soft, irregular lighter patches float in the middle of the picture, like faint clouds. This is often called clouding or mura, a term borrowed from a Japanese word for unevenness.
Both come from the same basic fact. An LCD does not make its own light. It filters light from a separate source, and the filter is never completely closed. Whatever small amount of light gets through where it should not is what you see.
Why light gets through
The explanation of how liquid crystals control light covers the details of the filter. The key point here is that a black pixel is not one that is off; it is one where the liquid crystal layer is arranged to block as much light as possible. Blocking is good but not perfect, so a bright backlight behind a dark picture always leaves a faint glow.
Beyond that baseline, several things can make some areas brighter than others:
- Edge-lit designs. Many LCDs place lights along the edges and spread the light across the screen with a guide plate. Light can escape near the edges before it is fully spread, and small variations in the plate matter. The general job of LCD backlights is described elsewhere.
- Frame pressure. The layers in a screen are held in a frame. If the frame presses slightly too hard or unevenly, it can stress the layers and let more light through in that spot. This is a common explanation for bright corners or edges and for glow that changes slightly with how the screen is mounted or handled.
- Small variations in layers. Films and diffusers that spread light are made to tight tolerances, but not zero tolerances. Tiny differences from one unit to the next lead to patches.
- Heat. A screen that warms during use can expand slightly, which sometimes changes the pressure pattern. A glow that gets worse after hours on may be partly thermal.
- Panel type. Different panel technologies have different baseline black levels and different tendencies. VA panels typically have deeper blacks than IPS panels, which makes any leakage on them stand out against a darker background, while IPS panels typically show a fainter overall glow that can be more visible from the side. These are tendencies, not guarantees, and individual units vary. The articles on IPS panels and VA panels cover this in more detail.
Bleed, clouding and glow are not the same thing
It helps to separate the effects, because they have different causes and are judged differently.
Edge or corner bleed. A bright band or spot at a border, usually most visible on a black picture in a dark room. It often stays in the same place.
Clouding. Soft, uneven patches that can appear anywhere, often mid-screen. These are usually most visible on a dark gray picture rather than pure black, and less so when the picture is moving or detailed.
IPS glow. A hazy glow, often yellowish or whitish, that appears in the corners when you look at a dark screen from an angle and fades or shifts when you move. It is related to the way the panel’s layers behave off-axis rather than to leakage in the frame, and it is generally considered a characteristic of the panel type. Viewing angles discusses why colors and contrast shift as you move.
Blooming. On screens with local dimming, a bright object on a dark background can have a halo because the lighting zones are larger than the object. This is a related but distinct effect, discussed in the article on mini LED backlights and local dimming.
How it is judged
Because some leakage exists on nearly every LCD, the question is how much is acceptable. A few general points apply.
It depends on the room. Bleed is mostly invisible in a lit room or with bright content. It shows up in a dark room with dark scenes, such as night scenes in films, dark game levels and loading screens. If you rarely watch dark content in the dark, you may never notice it.
Camera photos exaggerate it. Photos of a dark screen often show glowing edges that look much worse than they do in person, because the camera’s exposure is raised. A comparison between photos of different screens is not a fair test.
A black test screen is a rough check. Displaying a fully black picture in a dark room, at normal viewing brightness and from a normal viewing distance, shows how a unit behaves. Test patterns can be useful here, and measuring evenness more systematically is the work of screen uniformity testing. Brightness settings change the result: higher brightness usually makes bleed more visible.
Contrast figures don’t tell the whole story. A screen can have a good overall contrast ratio and still have uneven black levels.
Makers’ policies vary. Many manufacturers treat some bleed as normal, and the amount that qualifies as a defect is often not defined precisely. If you suspect a unit is well outside normal, checking the warranty and return terms is the practical next step. A replacement unit may behave differently, since this is partly a matter of luck.
What can be done
You usually cannot fix backlight leakage yourself. A few things may help the appearance, though:
- Lower the brightness to a comfortable level for the room.
- Use a little bias lighting or room light behind the screen, which makes black levels seem less contrasty with the glow.
- Check the mounting. Some tight wall mounts or stands can press on the frame, so an unevenly tightened mount can influence the result. Monitor mounting is mainly about height and position, but the way the screen is held can matter.
- Avoid pressing on the screen or its frame.
Opening a screen to loosen the frame or adjust layers is generally not advised, as it risks damage and can void a warranty.
Not a sign of failure
Bleed and clouding are quality variations in how a panel was made, not signs that a screen is wearing out. They tend to stay roughly the same, though thermal and mounting changes can shift them a bit. For most uses they are a minor matter; for dark-room film watching or careful image work they may be worth testing for before buying, which is easier if a return period exists.