Originally Posted by JinMTVT
can someone explain to me how they manage to get even light distribution
from edge lit + diffusor on large screens ???
They can't, so they shoot for the best approximation.
You can't actually run a true diffuser on a typical edge-lit TV. First of all, edge light doesn't lend itself to typical diffusion. Second of all, the depth doesn't permit would we'd normally think of as diffusion.
So edge-lit sets rely on two tricks to achieve light diffusion:
1) Light guides: long, thin pieces of plastic that the light it sent into from the side. These are crafted in such a way that as the light travels down the guide, some
of it escapes at each spot along the way. The guides are shaped such that the light gets distributed across half
the width of the TV (on most modern LED LCD TVs you have two LED bars on the sides, if you had top-bottom bars, which are used very occasionally like last year's Samsung 65D8000 apparently, you'd cover half the height). So basically, the guides are slightly wedge shaped. There are some examples here:http://www.ciri.org.nz/downloads/Lightpipe%20design.pdf
2) Diffusion films: Typically, in front of the light guide (or bonded to it, depending on the design) there will be a layer of diffusion film which is designed to smooth out the light hitting it so it doesn't "hot spot" or appear to be a point source. This is a pretty good diagram of how a diffusion film is placed in concert with the light guide and light source.http://www.kimoto.co.jp/english/products/light.html
Now, here's the thing. If the light guide is positioned perfectly with respect to the light source such that the angle and entry gap are as intended, even plastic light guides can theoretically provide a fairly
uniform illumination. The problem is that the relative placement of all those items are rarely done with the precision of someone's CAD drawing.
It's fairly likely that if uniformity could be brought comfortably inside of +/- 10%, most people would never see any artifacts like flashlighting, clouding, etc. But it's clear that few TVs are anywhere near this good. And, in fact, variances of 20% or more are common.
The fact is that simply using an array of LEDs behind the display, it's easier to achieve uniformity. The consistency of output between
LEDs is going to be easier to achieve than having to not only achieve that, but also spread the light around (edge-lit sets need to do both... there are still a lot of LEDs to be "uniform" with).
Now, if you give up some thinness and go behind the screen with full array, you can also diffuse the light more aggressively, making the job even easier. It's no surprise that sets like last year's Sharp 73x line did a very good with uniformity, except in the corners (which was mostly due to design decisions leaving those areas without enough lighting).
The new "direct LED" sets uses relatively few LEDs in a "full array" type design (relatively few vs. a true full array set). These are somewhat higher power, set somewhat farther from the LCD panel, and are therefore (a) somewhat better diffused and (b) somewhat more likely to cast overlapping light. Ironically, these cheap designs will probably outperform much more expensive edge-lit sets on uniformity
, despite being thicker, cheaper, and somewhat less desirable.
Sharp claims that this year's edge-lit sets will compare favorably to full array sets on uniformity metrics. I have not yet experienced any of these sets, nor have I seen any reports about them. It will be interesting.