Bill Hammack Explains How LED Backlit LCD Monitors Work

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.


14 thoughts on “Bill Hammack Explains How LED Backlit LCD Monitors Work

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. Thank you, very detailed and informative! Is it possible that you can do a video on what causes back light bleeding? Almost every LED lit (specifically edge lit) TV/Monitor seems to suffer for BLB / flash lighting / clouding and if there’s anything a end user could do to try to correct it? I just got a UN55JU6500 which is edge lit and has some significant BLB. However, the UN40JU6500 doesn’t have this problem. Does scale play an issue in manufacturing causing BLB in larger screens or is it just a QA issue during manufacturing, maybe the light guides are slightly off during putting the layers together? The panels in both are not IPS/sPLS, I think they are VA/MVA.

  7. I have an LCD scereen for a computer that has a water stain on the inside of the screen. There’s the velvety screen that covers the actual LCD screen, I think? And between this velvet screen and the actual LCD display screen with its glass there are streaks of dry liquid that has a red color to it when the screen is on but showing black.

    Is it safe to sake the velvet part away from the LCD screen to clean this?

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