Bill Hammack explains how LED backlit LCD monitors work

posted Mar 17th 2011 2:01pm by Mike Szczys
filed under: video hacks

We had a basic understanding of how LCD monitors worked, and you may too. But the thing is, [Bill Hammack] doesn’t just explain the basics. Since he’s the Engineer Guy he explains the engineering principles behind how LED backlit LCD screens operate. But he does it in a way that everyone can understand.

After the break we’ve embedded his five-minute video. In it you’ll see him strip down a monitor to the back plate and then build it up piece-by-piece. We enjoyed his discussion of how the diffuser panels work together to even out and distribute the light. Theses are made of several layers and, although we knew they were there from working with salvaged LCD screens, we never knew quite what they were doing. He also covers how each liquid crystal cell works along with polarizing sheets to either block or allow light passage. And he’ll bring it on home by show how thin-film transistors in each subpixel of the screen work to multiplex the display, just like we did with that pumpkin back in October.

Comments [12]

tagged: lcd, led

12 Responses to Bill Hammack explains how LED backlit LCD monitors work

Tony says:

March 17, 2011 at 2:26 pm

Excellent explanation, tear down, build up and animations. Before I thought that liquid crystal just magically changed between light and dark. The truth that it actually goes from twisting the polarization light between 0 to 90 degrees is much cooler.

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Munden says:

March 17, 2011 at 2:36 pm

Wow! Great explanation. However, I’m still going to answer those who ask me how LCD screens work with, “magic!”

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Nick McClanahan says:

March 17, 2011 at 6:18 pm

Kind of off-topic, but what program do you think they use to do the animations?

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billengineerguy says:

March 18, 2011 at 2:30 pm

The rotation of the red rods was done by Patrick in maya; the rest I did with still images I drew in gimp then animated in final cut pro.

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Brian says:

March 17, 2011 at 6:53 pm

As someone who tore apart the guts of an old (damaged) LCD to see what’s inside, I found this video much more informative and very well-done.

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FireSokar says:

March 17, 2011 at 7:18 pm

Excellent, one of the better articles I have seen posted on this website. Please post more when they become available.

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SteveO says:

March 17, 2011 at 10:33 pm

there was some awe in that video.

that was the best, most concise, and easy to follow example of led back lighting, fiber optics, diffusion, polarization, RGB, TFT, LCD, refresh rate, and video signal humanly possible in 4 minutes and 53 seconds.

completely unbelievably awesome.

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Little_Neo says:

March 18, 2011 at 1:02 am

This video was really interesting.
What I really found awesome is that he is demonstrating on the same piece of crap monitor that I have. It made it even better for me to understand.

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xtremegamer says:

March 18, 2011 at 1:56 am

we demand a series of all popular technology (lolz!)

,greetz

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NateOcean says:

March 18, 2011 at 9:43 am

Excellent tear down, explanation, and animation.

At around 4:30 Hammack mentions that the speed at which rows receive information is “so fast that your brain blends it into a fluid image”. This may require more explanation.

Both CRT-based monitors and LCD-based monitors require at least 24 images per second (24 frames per second) for you to perceive continuity of motion (for a motion picture or animation). Much below 24 fps, you you perceive the result as a jerky slide show. This is called Critical Fusion Frequency.

However, CRT-based monitors suffer from an additional constraint that does not (for the most part) affect LCD-based monitors. CRT monitors must be continuously refreshed, and rapidly enough to avoid annoying flicker. This is called Flicker Free Refresh Rate.

Generally, it’s around 60 flashes per second. 60-hertz is used in America to match the power line frequency. However, in Europe 50-hertz is instead used to match the 50-hertz power mains frequency there, and generally most people still see an annoying flicker.

LCD monitors don’t suffer from that same effect. When you turn on the drive transistor at a certain sub-pixel site, you in effect “write” a voltage to that site, and that voltage remains there until a new voltage is written. That voltage determines how much the twisted helix liquid crystal material will be disrupted, and that determines how transparent or opaque that site will be.

However, in the absence of a continuous refresh process, the voltage will remain there indefinitely. (Nothing’s perfect, and the voltage will leak away after many seconds.)

That’s not true for CRTs. Stop the refresh process, and the image is gone in an instant.

People are affected differently by CRT flicker. Younger people perceive it more than older. Women are more annoyed by it than men. You tend to see it more in your peripheral vision than straight on. This can also be a problem for some people with epilepsy.

Put another way, for a static image on the screen, with an LCD monitor, each pixel is pretty much an unwavering light source. But for the CRT monitor, even for a static image, each pixel is pulsed to the intended brightness then rapidly fades only to be zapped again 1/60th second later.

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Dax says:

March 18, 2011 at 2:43 pm

Although for all intents and purposes, an LCD screen will still flicker because of two effects:

1) A CFL tube for a backlight operates on high voltage AC from a transformer, and will flicker at the rate at which its booster circuit operates.

2) A LED backlight will flicker at the frequency of the PWM dimmer that is used to control the brightness of the backlight.

Especially early monitors simply ran the backlight from the mains voltage through a simple transformer, so that would make it blink at 50/60 Hz.