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TV Science: LCD and LED TVs

Posted by Science Explored on January 22, 2013

1080p may soon be a thing of the past as 4K takes hold.  If you just started sweating, don’t worry – you’re not back in calculus class.  In fact, these numbers and figures hit much closer to home than you think.  These figures refer to TV resolution, which may undergo a transformation with the arrival of 4K resolution in the next few years.  This transition may be as big as when HDTV arrived more recently.  Before we make that transition and drool over the ultra-realistic images on our brand new flat screens, maybe we should understand TVs as they stand today. 


“LED/LCD displays (the same types of screens you find in many handheld devices like a smartphone) are commonplace today.  First of all, let’s define two terms:

LED:  Light emitting diode, or tiny bulbs that don’t use a filament to light up, making them last far longer than incandescent bulbs (traditionallight bulbs).

LCD:  Liquid crystal display, or a series of moving crystals that change shape to create different colors when white light shines through them.

Whether a TV at Best Buy or Wal-Mart is called an LED or LCD TV, both work in very similar ways.  Both rely on using white light to generate the colors seen on a television screen – think about a prism splitting the sun’s white light into a rainbow of colors.  The difference lies in how they make this white light.

Pixels

LCD TVs rely on a group of substances called liquid crystals that were discovered in 1888 by Austrian chemist Friedrich Reinitzer.  Atoms in these crystals can move like liquids but maintain a neat, ordered structure like a solid.   The liquid crystals used in a TV or computer screen are transparent and will react to electricity by unwinding.  In an LCD TV, liquid crystals are sandwiched between two layers of glass that are surrounded by electrodes (conductors that can control the movement of electricity) and polarizing filters (filters that can block light or let it pass through); this sandwich is called a subpixel.  Subpixels are grouped into bundles of three called a pixel (3 subpixles = 1 pixel).  Each subpixel has a red, green, or blue film so that it displays that color when white light shines through it.  A computer screen, phone, or TV can easily have thousands of pixels.

A processor inside the LCD TV increases or decreases the amount of electricity that reaches each twisted liquid crystal inside the subpixel.  As the processor sends more electricity to a certain subpixel, the liquid crystal unwinds.  When it unwinds, less light passes through the polarizing filters.  The result is that that subpixel’s color gets dimmer (for example, more electricity to the green subpixel means that the entire pixel would look more red + blue, or purple, since there is hardly any green).  With more electricity, that pixel can become completely black as all the subpixel’s liquid crystals unwind.  By carefully controlling the flow of electricity to each subpixel, each pixel can display a different color and thus show a whole image on the screen.  If no electricity is sent to the electrodes of a pixel, all three subpixels will shine their colors and show white.

It is important to realize that the pixels donot make light.  They just let light pass through them in different ways to change the color of the light moving through.  The light actually comes from the back of the TV.  Here is where we get the difference between an “LED TV” and an “LCD TV.”  LCD TVs use fluorescent tubes, much like the ones in classroom ceilings, to make white light that passes through the pixels to show the colors seen on the TV screen.  LED TVs, on the other hand, use LEDs to make white light that passes through the pixels to make all the colors you see when watching your favorite show or playing a game on the computer.  Both use liquid crystals.  Both have pixels.  The difference is just how they make their white light.  That means that LED TVs are really just LED-lit LCD TVs. 

So what about that 4K TV?  Pixels are always getting smaller – this means that you can cram more of them onto a screen.  The basic technology of LED/LCD screens is not going to change anytime soon.  On a 4K screen, there will be literally thousands more pixels in a smaller area than ever before.  That means images will be sharper and more realistic than before.  Still, don’t get your hopes up about having a TV marathon with your new 4K display. Until they become easier to make, 4K televisions will be much more expensive than their currently available LED/LCD cousins.

TEKS:  5.6A, 6.9C, P.7AB, P.7D

 

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Topics: Science Explored, LED, pixel, LCD

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