There are a few different types of LEDs available in the market - Full-Array LED TV, Edge-Lit LED TV (without local dimming); Edge-Lit LED TVs (with local dimming) and so on.
Local dimming is a technology which allows certain zones of LEDs to be dimmed or brightened corresponding to the image on the screen. If the image on the screen demands that certain areas be dark, then the TV can automaticall dimm the LEDs in that area of the screen. The end result is a very crisp image.
Plasma on the other hand doesn't need local dimming, because each pixel itself in plasma is a light source, whereas LED TVs need a light source.
I feel both Plasma and LEDs have its own advantages and disadvantages. Anyhow, the answer to you question, in short is LED TVs with local dimming has crisp image. If you are going for LED TV, Full-Array, will be a good choice. Full-Array LED TVs have local dimming capability.
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