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What are the differences between IPS TFT screens and TN TFT screens

By Austin Li

Before I start writing about the differences between IPS TFT screens and TN TFT screens, let me tell you how TFT screens work in the first place.

Computer display screens have to do two things in essence.

1. Light. There must be a source of light. In a dark room, the monitor will still be bright. But on those regular calculators, you won't see anything in a dark room. Thus monitors must have some kind of light emitting source in them. Fluorescent tubes or LEDs or some kind of source must exist.

2. Color. With just the light source, we won't be able to see any different colors unless there was a way that we could separate the light emitted by the light source and selectively shine certain colors through and certain colors not through.

If you had a white LED at the back of the display without a color filter, all that you can see is white color. We need some sort of color filter that can both open and close to select which lights to let through.

Now we get to a few main conclusions. A monitor must have:

• Light source
• Color filter
• A gate for each filter that can be turn on and off.

What this gate is in monitors is Liquid Crystal. The crystal changes shape when an electrical charge is applied through it, which changes its optical qualities and thus changes the amount of light that passes through or blocked by the liquid crystal gate.

The object that controls the electrical charge is the transistor, embedded into each gate. These transistors are printed on a thin transparent film (save the transistor itself) giving the name Thin Film Transistor (TFT) of TFT displays.

The difference between IPS and TN is the arrangement of the transistors and liquid crystal. The reason why IPS screens are better suited to color reproduction and greater viewing angle is due to the arrangement of polarizers and crystal orientation. The arrangement of crystals in a TN display causes something called "angular retardation" in the light because of the structure of crystals.

I'll use an example here to demonstrate that difference. Get a soda bottle and put it on your screen, with the top drinking side facing you. When facing directly at the screen, you will only see the top side of the soda can. When you move your head around (especially vertically) you see a different composition of the soda can. This is what a TN screen looks like. The soda can represents how the crystals are arranged. Now let’s see an IPS screen.

Get the same soda can, but this time let the side of the bottle face you. The top and bottom should point to the left and right side of the screen. Now when you move your head around (especially vertically), the shape that you see doesn't really change much at all, it may be a different portion of the can, but its the same curvature. This allows better viewing angles, and more consistent reproduction of color due to less distortion properties.

IPS screens are more expensive because of four main reasons:

1. Their better light properties causes them to be in demand with professionals such as surgeons, photographers, or other occupations where color reproduction is paramount. Even without regards to manufacturing price, professional tools often are marked up just because they are in demand with professionals.

2. To maximize the benefits of better light properties in the crystals, the precision of shades per color channel is upped from 6 bits (TN displays) to 8 bits (IPS displays), this again increases its manufacturing cost. TN displays have poor color reproduction due to the limited bits, and the method of "dithering" manufacturers use to overcome the 18-bit limit has its own flaws.

3. More transistors are used to control the crystals, causing higher prices. The transistors also block light from the light source. Thus a stronger light source is required to achieve light intensity equal to TN displays.

4. A stronger light source often means more electricity, which causes heat issues and higher electric bills. This is rectified by increasing the efficiency of the light source, meaning LED back lights or some more complex design of polarizers.

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