TV Blooming Explained: Why Bright Objects Grow Halos

What is blooming on a TV?

Blooming is the soft glow, or halo, that spreads around a bright object on a dark background on an LCD television with local dimming. It happens because the backlight is switched up and down in zones that are far larger than the pixels in front of them, so light meant for a white subtitle or a single star spills into the black around it. Some blooming is normal behaviour for this kind of screen. A glow that ignores the picture is a different matter.

Why do backlight zones create a halo?

An LCD panel does not make its own light. It acts as a very fine shutter in front of a backlight, and local dimming (the feature that brightens or dims sections of that backlight to match the picture) is how the set gets deeper black without giving up bright highlights. RTINGS describes it plainly in its lighting-zone test notes: local dimming "dims the LEDs behind darker parts of the screen and can even brighten the rest of the screen when more power is available."

The trouble is scale. A 4K panel has 3,840 × 2,160 pixels, a little over 8.2 million, while each backlight zone sits behind a large block of them. When a small bright object lands inside a zone, the television has to light the whole zone to show it, and the shutter in front cannot block every bit of that light from the surrounding black pixels. The TestUFO blooming test page puts it in one line: "blooming (halos) can become visible at the boundaries of local dimming zones." The same page notes that the effect becomes more visible "when the background is black and the display has fewer local dimming zones."

Which TVs bloom, and which do not?

Blooming belongs to backlit LCD televisions, including most sets sold as LED or mini-LED. Smaller mini-LED light sources can support more zones, which tends to shrink the halo, but the label alone does not tell you how a particular set handles it.

Self-emissive screens, where each pixel produces its own light, sidestep the problem. TestUFO states that OLEDs and direct-view MiniLED or MicroLED displays with no LCD layer "are capable of full brightness pixels adjacent to full blacks, and will show no blooming." RTINGS makes the same point for its zone tests, noting that they do not matter for OLED televisions, "as they have perfect contrast without the need for zone dimming." Our OLED versus LCD guide covers the wider trade-offs, including brightness and room light.

Layout matters too. A full-array backlight sits directly behind the image, so its glow tends to cluster around the bright object. An edge-lit backlight feeds light in from the sides, so in our judgement its dimming is coarser and excess light is more likely to show as a brighter strip than a neat ring.

Why do subtitles and small bright objects bloom the most?

The worst case for any zone-based backlight is a small, very bright object surrounded by true black. White subtitles over a dark film scene, end credits, a star field, a single streetlight, a cursor moving across a black menu: each asks the television to light a zone hard for the sake of a few pixels. RTINGS lists exactly these conditions for its zone-precision test, saying it matters "in dark scenes with bright lights or well-lit subjects or for subtitles on dark backgrounds."

Large bright areas rarely bloom in a way you notice, because the whole region is meant to be lit. A sunlit window filling a third of the screen does not reveal its zone edges; a single candle in a black room does. That is why a set can look flawless on a bright shop-floor reel and then glow around every line of dialogue at home.

Subtitles are the easiest of these to soften without touching the television. Many streaming apps let you change how captions look. Netflix's subtitle appearance help page says "You can change the font, size, shadow, and background color of subtitles and closed captions," and notes that connected TVs offer fewer options than a web browser. A semi-opaque dark box behind the text, or a slightly smaller size, gives the backlight a less extreme job.

Does viewing angle make blooming worse?

Often, yes. Sitting off to one side, you look through the LCD layer at a slant, and on many panels the shutter blocks backlight less effectively at an angle than straight on, so a halo that was faint from the centre seat can look wider and greyer from the end of the sofa. RTINGS once changed the way it filmed its local-dimming video for exactly this reason, noting in its test-bench changelog that it began "filming it at an angle, so the blooming is more visible."

Panel type plays a part here. RTINGS' black uniformity notes say vertical alignment (VA) panels "tend to have much higher contrast ratios," while in-plane switching (IPS) panels have worse contrast ratios and more backlight issues. Higher native contrast means the panel holds back more of the light spilling from a bright zone. Our viewing angle guide explains how to judge a set from every seat you actually use.

Why does blooming look worse in a dark room?

Your eyes adapt to the room. In a dark room the black around a subtitle is the darkest thing you can see, so even a faint grey glow stands out sharply. In a lit room the same glow is competing with ambient light on the screen and in your peripheral vision, and much of it disappears. RTINGS says good zone control matters most "when you're watching dark scenes, especially in a dark room where low contrast is most noticeable."

That suggests a cheap, reversible fix: a small lamp placed so it does not reflect in the screen can make night-time halos less conspicuous, at the cost of a slightly lighter sense of black.

How do local-dimming settings change the halo?

Most LCD televisions with zone dimming offer a strength setting, often labelled low, medium and high, sometimes buried inside picture modes. A stronger setting pushes dark zones down harder and bright zones up harder, which deepens black but sharpens the contrast between a lit zone and its neighbours. RTINGS' advice in its lighting-zone test notes is direct: "Higher settings are often more aggressive, so you might see a sharper delineation between bright and dark areas on the screen. This results in higher contrast, but it can also be more distracting in some content. If you see this and it bothers you, try switching to a lower setting."

Turning dimming off removes zone halos entirely but replaces them with a uniformly lifted, greyer black. For a set whose dimming is particularly harsh, RTINGS says it would "usually recommend leaving the local dimming off entirely if the TV allows it." RTINGS also scores game-mode dimming as a separate test, so expect halos to change when you switch into a game mode.

Change one thing at a time and judge it on the same scene. The local dimming guide explains why zone counts alone do not predict the result.

Is blooming normal, or is something wrong with my TV?

RTINGS is blunt about expectations: "No local dimming feature is perfect," and haloing "is the most common" of the issues. Blooming that behaves like this is expected:

• The glow appears only around bright objects on dark backgrounds, and moves with them.
• It shrinks or grows when you change the local-dimming strength.
• It looks worse off-centre and in a dark room, and fades when the lights are on.

A few things are not blooming and are worth a closer look. Clouding and flashlighting are backlight unevenness that sits in fixed places regardless of what is on screen. RTINGS describes flashlighting as looking like "a flashlight pointed across the corners of the television" and clouding as "brighter patches on parts of the screen," and says both come from the TV "improperly diffusing the backlight." It adds that "black uniformity can vary between individual units," so an exchange for the same model can be worth trying. A bright rectangle that stays lit on a black screen, a zone that stays dark when the picture is bright, or flickering blocks that do not track the content all point to a backlight or firmware issue rather than ordinary halo.

For any of those, check for a firmware update, reset the picture settings to defaults, and then contact the retailer or manufacturer while the set is within its return or warranty period. Do not open the back of the television or press on the panel to chase a fault; uniformity and backlight problems are a job for the manufacturer's service team.

One more possibility deserves a mention. TestUFO cautions that lens effects, "e.g. fingerprints on lens, cataracts, etc," can create blooming-style glow that is not caused by the display. If you see rings around streetlights, headlights and lamps as well as the television, the NHS lists "seeing glowing rings (halos) around lights" among cataract symptoms and advises seeing an optician.

How can you check a TV for blooming in five minutes?

1. Set the room as you normally watch, including the usual lighting.
2. Play a dark film scene with white subtitles, then end credits on black.
3. Watch from the centre seat and then from the furthest seat anyone uses.
4. Step the local-dimming setting through each level on the same scene.
5. Pause on a fully black screen and look for fixed bright patches or corners.

If the halos follow the subtitles and ease with a lower setting, you are looking at the normal cost of zone dimming, and the choice is how much black depth you want to trade for a cleaner edge. Our first picture settings guide is a good next stop for the rest of the menu.

Sources

• RTINGS: Our TV Picture Quality Tests: Lighting Zone Precision And Transitions
• RTINGS: Black Uniformity Of TVs: Clouding And Flashlighting
• RTINGS: TV Test Bench 1.3 changelog
• TestUFO: Full Array Local Dimming (FALD) / MiniLED Blooming Test
• Netflix Help Center: How to change the appearance of subtitles and closed captions
• NHS: Cataracts