Dr. Bortrum
08/06/2009
Lighting Up a TV
Some weeks ago I intended to write about light-emitting diodes (LEDs), inspired by an article by Shuji Nakamura, inventor of the blue gallium nitride LED. Then I found that I had already written about that article a few months earlier. I now realize that the article contained some interesting material I had not covered in the earlier column. However, I'm once again diverted from writing about LEDs per se. After reading a report headlined "Watching television in a new light" by David Colker of the Los Angeles Times in last Sunday's August 2 Star-Ledger, I realized I was totally ignorant of what goes on inside my liquid crystal display (LCD) high-definition TV (HDTV). For over a year, my wife and I have been thoroughly enjoying our 40-inch Sony Bravia HDTV, a gift from our StocksandNews editor Brian Trumbore.
The Star-Ledger report dealt with the backlighting of liquid crystal display (LCD) TVs by LEDs. To tell the truth, I had not even thought of the fact my HDTV had a backlight. Not only that, but I realized that I had no idea of what happens to that light as it makes its way to form the beautiful pictures we see on our TV screen. Longtime readers will know that a prime reason I've been writing these columns over the past decade is to convince or con myself into thinking that I understand the subject of a given column.
First, let me say I am not unaware of LCDs. My Casio SL-750 Film Card is a solar cell-powered credit card-size calculator with a liquid crystal display that I've relied upon for simple calculations for some 10-20 years. Not only do the solar cells require sunlight or some other source of light to power the calculator but there also has to be light reflecting off the liquid crystal display to see the numbers. But what about our LCD TV? If it worked like my calculator, we would have to shine a bright light on the TV in order to see the picture. In a darkened room we wouldn't see anything.
Hence the backlight in our HDTV. Now I understand why the TV emits a fair amount of heat when it's on. The backlight is what is known as a cold-cathode fluorescent lamp (CCFL). Apparently the cold cathode isn't all that cold and, actually, there could be a sizeable number of CCFLs lighting up my HDTV. I've just looked at my Sony manual and it takes 235 watts to power the TV set, not much less than it takes to power an old bulky TV from the pre-HDTV days. Aside from my manual, what follows has been gleaned from a host of Web sites of the various TV manufacturers and from an extensive consideration of the technology and history of HDTV on the Wikipedia site.
So, let's follow the white light given off by our CCFLs out to the front of the TV and the beautiful image that results. First we have to know something about two things - polarization of light and liquid crystals. The white light coming out of the CCFLs is a mix of light waves oscillating in all different directions. The first thing we do is pass the light through a glass sheet with a polarizing film on one side. Only the light waves oscillating in the appropriate direction get through this polarizer. You may have a pair of polarizing sunglasses and, if you look at some light reflecting off a body of water, for example, you may rotate the glasses and notice that the light darkens or brightens, depending on how it was polarized. The sunglasses block light vibrating in the wrong orientation.
In our HDTV, perhaps only half or less of the white light gets through the first polarizer, Not only that, but the light that does get through the first polarizer has to go through another sheet of glass with a polarizing film oriented to block the light that got through the first polarizer! Hey, we've got to do something or there won't be any light showing up on our TV screen! We've got to stick something in between the two polarizers that will change the orientation of enough of the light so that it can pass through that second polarizer and form a picture on our TV screen. The answer is liquid crystals and modern electronic circuitry.
Liquid crystals are sort of strange compounds that can behave and flow like a liquid but also can act as though they are crystalline. I won't try to explain them any further except to say that many liquid crystals form little rod-like crystals. These rods in a liquid crystal film are typically in a twisted shape that bends or changes the orientation of the light that travels through them. Now that light can pass through that second polarizer and end up on your TV screen. If you apply a voltage to that twisted liquid crystal rod, you can untwist it and the light passing through will not change orientation and will be blocked by the second polarizer. That part of the picture will be black.
By varying the voltage, the amount of twisting can be controlled along with the amount of light passing through the crystal and the polarizer. What amazes me is that in my HDTV there are some 6 million of these little rods neatly oriented in rows and columns on the grid that lies between the two polarizers. But wait, we haven't talked about color. Each of these 6 million rods is paired with a filter, either red, green or blue. Each of these 6 million rod-filter combinations is known as a sub-pixel. The green, red and blue light from each set of three sub-pixels combines on the TV screen to form a pixel. Hence there are 2 million pixels that make up the picture on my HDTV. (More precisely, according to my manual,1920 "dots" times 1080 lines, or 2,073,600 pixels, according to my trusty LCD calculator.).
Now, I had not thought about any electronic circuitry involved in the LCD display but if pressed, I would have speculated that each liquid crystal was addressed by grid of connections ("wires") with voltages fed to the intersections of in the grid, each intersection corresponding to one liquid crystal. Wrong! That's too simple. Such an arrangement involves voltage leakage problems and liquid crystals along the way would get distorted, twisted or untwisted, and the picture would not be sharp but fuzzy. So, each liquid crystal has a little transistor and capacitor associated with it . Now we have 6 million transistors and possibly more capacitors in my HDTV. The transistors and capacitors operate at voltages high enough that the corresponding liquid crystals won't get "triggered" by any low level voltage leaks.
With the grid of electrical connections and transistors and capacitors filling up the spaces among the liquid crystals the areas between the crystals are dark, absorbing or reflecting light and when all is said and done, only about a tenth or less of the light coming from the backlight CCFLs emerges on my TV screen.
So, to summarize, we have a backlight that emits white light. The white light gets polarized as it passes through a polarizer. Depending on the voltages on the transistors at each liquid crystal, the amount of light getting through each red, green or blue filter , and through the second polarizer, is determined by how much the voltage has twisted or untwisted the liquid crystal. Finally, on the screen all the 6 millions streams of light get combined into 2 million pixels that give us HDTV.
But what about that article in the Star-Ledger on LED backlighting? Instead of those fluorescent lamps, another approach to backlighting is to use LEDs, which use less power than fluorescent lamps and also don't take up as much space as the round fluorescent lamps. This means a cooler, thinner TV. Some TVs use LED backlighting and have been for some time. However, the cost of the LED backlit TVs has been quite high compared to other CCFL-backlit TVs. The Ledger article quotes a price of $4,000 for a 46-inch Sony set. The set is only 1.2 inches thick! With further progress in LEDs and mass production, the LED backlit TVs are expected to become less expensive and the LED backlighting also has some advantages insofar as picture quality is concerned.
Let's consider just one advantage of LED backlit TVs. If the backlighting is provided by LEDs spread out over the back of the TV, the set will be capable of "local dimming". This means that in dark areas of the picture the backlight can be dimmed to allow more detail in the darker areas and also blacker blacks. Another approach is to backlight with LEDs only feeding light from the edges of the backlight area rather than the LEDs being spread out over the backlight area. This approach does not allow local dimming.
However, as far as I'm concerned, the ultimate TV is the "true'' LED HDTV in which the LEDs themselves are the sub-pixels, with no backlighting and no polarizers, just the LEDs and electronic circuitry to drive them. Back in the 1960s at Bell Labs, we envisioned the day when there would be red, green and blue LEDs and hence the possibility of a color TV. However, at that point we only had red LEDs bright enough to light up pushbuttons on the telephone and did not dream that the LEDs would ever be as bright as they are today. With all the amazingly bright blue, green, red and white LEDs of today, I wouldn't be surprised to see "real" LED TVs take over the market in my lifetime and I'm 81 years old!
On sober reflection, let me modify that last statement. I've decided I would be shocked if "real" LED TVs took over the market in my lifetime. To make a structure with millions of trios of red, green and blue LEDs serving as sub-pixels seems pretty daunting. More likely would be white LEDs paired with red, green and blue filters. Or maybe the organic LED (OLED) technology will finally be developed to give bright LEDs lasting long enough to be commercially viable. And hey, at 81, I'll be lucky to be around tomorrow, let alone another 10 years!
Next column on August 20.
Allen F. Bortrum
