LCD vs OLED: How Phone and TV Screens Make Color
A screen builds its picture from tiny red, green and blue dots, setting the brightness of each one. An LCD shines white light from behind and uses liquid-crystal shutters to pass or block it, dot by dot. In an OLED, each dot makes its own light. This difference is why the two differ in how deep their blacks look and how easily burn-in happens.
What you'll learn
- That each dot on a screen (a pixel) is made of three even smaller colored dots
- How an LCD passes or blocks light, and the switch at each dot (the TFT)
- Why OLEDs can show true black, plus the facts on burn-in and flicker
- How to read spec-sheet words such as mini-LED, 120 Hz and ppi
Prerequisites: None. Reading "What Is a Transistor?" and "How Do LEDs Work?" first will make the parts on TFTs and OLEDs easier to follow.
Where you see it: display words on spec sheets
Spec sheets for phones and TVs list the type of display, such as "LCD", "OLED" or "Mini LED". Numbers like "120 Hz" and "460 ppi" also tell you something about how the screen works.
Look at a white screen through a magnifying glass, and you will see rows of tiny red, green and blue dots. Our story starts with these dots.
Pixels and subpixels: making color from three dots
The picture on a screen is a grid of tiny dots in rows and columns. The smallest dot that makes up the picture is called a pixel.
Each pixel is usually made of three even smaller dots: one red, one green and one blue. These smaller dots are called subpixels. Turn all three up and you see white; turn them all off and you see black. Change the mix of the three brightness levels, and you get all kinds of colors.
The dots are so small that your eye sees the three colors blended into one. If each color has 256 brightness levels, there are about 16.78 million combinations (256 × 256 × 256).
For example, the LCD on the iPhone 11 has 1792 × 828 pixels (Apple's specs), or about 4.45 million subpixels. The screen sets the brightness of each one separately.
How it works 1: LCD, a shutter that decides whether light gets through
A liquid crystal is a material that flows like a liquid, yet its molecules line up in the same direction. The molecules are shaped like long, thin rods, and they turn when a voltage is applied. A screen that uses this property is a liquid crystal display (LCD).
Liquid crystal does not make light by itself. So a panel that gives off white light sits behind the screen. This is called the backlight, and today it is usually made of white LEDs.
Light is a wave that vibrates at right angles to the direction it travels. Ordinary light is a mix of vibrations in many directions. A film that lets through vibrations in only one direction is called a polarizer. Stack two polarizers with their directions at right angles, and almost no light gets through.
The liquid crystal sits between these two polarizers. Depending on how its molecules are arranged, it rotates the direction in which the passing light vibrates. If the direction rotates, the light passes the second polarizer; if not, it is blocked. Turning the molecules with a voltage lets the screen adjust brightness finely. In other words, the liquid crystal works as a shutter that a voltage opens and closes.
On top sits a film that lets only one color through at each subpixel: red, green or blue. This is called a color filter.
In this design, much of the light is lost along the way. When light vibrates in random directions, the first polarizer stops about half of it. The color filter also passes only one of the three colors. So at most about one-sixth of the light gets out (our simple calculation).
How it works 2: TFTs, a switch for every subpixel
To give each of millions of subpixels its own voltage, every subpixel gets its own switch. This switch is a transistor, a part that uses a voltage to let a current flow or stop it (see "What Is a Transistor?").
The transistors in a screen are built on a glass plate by stacking very thin films. That is why they are called thin-film transistors (TFTs). Ordinary chips are different: they are built on a thin disc of crystalline silicon, called a wafer.
The TFTs sit where thin wires running across and down the screen cross. A row line turns on all the TFTs in one row at once. Each column line carries a voltage that matches the brightness of its subpixel. The screen writes one row at a time. When it reaches the bottom, it starts again from the top.
Even after a TFT turns off, the written voltage stays in a tiny part that stores charge, called a capacitor. So the brightness holds until the next rewrite.
TFT materials: amorphous silicon, LTPS and IGZO
The film material of a TFT sets how easily electrons move inside it. This ease of movement is called mobility. The higher the mobility, the smaller a TFT can be and the faster it can switch.
The material used most widely for a long time is a silicon film whose atoms are not neatly ordered. This is called amorphous silicon. It can be made cheaply on large sheets of glass, but its weak point is that electrons do not move easily in it.
Shining a laser on this film turns it into a mass of tiny crystals. The result is LTPS (low-temperature polysilicon). Electrons move far more easily in it (it is made by laser annealing), and it is common in phone screens with fine pixels.
The third material is IGZO, an oxide made of indium, gallium, zinc and oxygen. In 2004, Hideo Hosono and colleagues at Tokyo Institute of Technology (now Institute of Science Tokyo) reported TFTs made of it in the journal Nature. In 2012, Sharp started producing LCD panels that use IGZO TFTs.
The mobility of IGZO is more than 10 times that of amorphous silicon (though lower than LTPS). Its other strength is that very little current leaks when it is off. The written voltage holds for a long time. So for a still image, the screen can rewrite less often and save power.
How it works 3: OLED, where each subpixel makes its own light
In an OLED screen, each subpixel gives off its own light. OLED stands for organic light-emitting diode. "Organic" refers to materials built on a framework of carbon. The light comes from electroluminescence, which means light given off when an electric current flows.
The way it lights up is much like an LED. Several very thin layers of organic material are stacked between two electrodes. When a current flows, electrons enter from one side and holes (empty spots left by missing electrons) enter from the other. They meet inside the layers and give off light. Red, green and blue each use a different material.
Today's OLEDs trace back to a device with two stacked organic layers. Tang and VanSlyke of Kodak in the US reported it in 1987. The layers are made by methods such as vacuum evaporation, which turns the material into vapor in a vacuum so it coats the surface.
An OLED gives off light only while a current flows. So each subpixel has at least two TFTs. One is the switch that writes the voltage. The other keeps a current flowing according to that voltage. An LCD usually needs only one.
Black levels and contrast
On an LCD, the backlight stays on even when the screen shows black. The liquid-crystal shutter cannot block the light completely, so a little leaks through. That is why black can look grayish in a dark room.
On an OLED, a black subpixel is switched off and gives off no light, so black is truly dark. The ratio between the brightness of the brightest white and the darkest black is called the contrast ratio.
In Apple's specs (typical values), the iPhone 11 with an LCD is 1400:1. The iPhone 15 with an OLED is 2,000,000:1, far higher, because its black is almost zero. Companies measure this in different ways, so you cannot simply compare these numbers with other brands' figures.
OLED's weak point: burn-in
OLED materials slowly wear out and grow dimmer, depending on how long and how brightly they have been lit. If the same bright image stays on screen for a long time, that area wears more than its surroundings. Then a faint trace of the old image remains, even when you show something else. This is called burn-in.
According to Apple, burn-in can happen when the same high-contrast image stays on screen at high brightness for a long time. To counter it, the phone records how each pixel is used and corrects its brightness. An LCD has no light-emitting material in each pixel, so this kind of burn-in does not happen.
| Rule of thumb | LCD | OLED |
|---|---|---|
| Black in a dark room | Can look a little gray | Deep black |
| Same image for hours | Little risk of burn-in | Watch out for burn-in |
These are general tendencies, and products differ a lot.
Mini-LED and microLED
Mini-LED splits an LCD's backlight into many small LEDs. Behind dark parts of the picture the LEDs dim, and behind bright parts they brighten. This lets even an LCD show deeper black. According to Apple, the 2021 iPad Pro (12.9-inch) has over 10,000 LEDs, grouped into more than 2,500 zones.
However, the zones are much larger than pixels, and the screen is still an LCD. A bright object on a black background can have a faint glow around it.
In a microLED screen, each subpixel is itself a tiny LED. Like OLED, each dot makes its own light. But the material is not organic: it is an inorganic semiconductor such as gallium nitride. The article "How Do LEDs Work?" does the math on why placing tens of millions of LEDs with none missing is hard.
Refresh rate: how many times per second the screen redraws
A screen redraws its picture dozens of times every second. This count is called the refresh rate, measured in hertz (Hz). At 60 Hz, the screen redraws 60 times a second, or about every 16.7 ms. At 120 Hz, it redraws about every 8.3 ms, so scrolling looks smoother.
Each redraw takes power, because a voltage must be written to every TFT. So engineers found ways to redraw less often when the picture is not moving. A screen design for this is called LTPO. It uses fast LTPS TFTs and low-leakage oxide TFTs side by side in the same screen.
In 2019, Apple announced that the always-on display of the Apple Watch Series 5 used LTPO. In 2021, it announced the iPhone 13 Pro, whose screen varies its rate from 10 to 120 Hz. In 2022, the iPhone 14 Pro could drop as low as 1 Hz. This let Apple add an always-on display that keeps showing the screen instead of turning it off.
Display driver ICs: chips that turn numbers into voltages
How bright each subpixel should be is decided, as numbers, by the phone's main processor chip (the SoC). A display driver IC receives those numbers. It turns them into a voltage for each column line and sends them out. Other chips in a phone are covered in "What Chips Are Inside a Smartphone?"
The iPhone 15 Pro screen has 2556 rows and redraws up to 120 times a second (Apple's specs). That leaves about 3.3 µs (3.3 millionths of a second) for each row. This rough figure ignores the short pauses between redraws, but it shows how fast the TFTs must switch.
Brightness control and flicker
You can change an OLED's brightness by changing the current through it. But at very small currents, tiny differences between TFTs start to show. Brightness and color then tend to look uneven.
So many OLEDs, especially when dimmed, switch on and off faster than the eye can notice. They set the brightness by the share of time the light is on. This method is called pulse-width modulation (PWM), and it is also used to dim LED lighting.
Most people do not notice this flicker. But some people report eye strain or discomfort. Research on how flicker speed relates to eye strain continues, and there is no clear conclusion yet.
Resolution and ppi: putting sharpness into numbers
How fine the pixels are is given as the number of pixels along one inch (2.54 cm). This is called ppi (pixels per inch). With the same number of pixels, a smaller screen has a higher ppi.
Let's work it out for the iPhone 15. The screen has 2556 × 1179 pixels, and its diagonal, measured as a rectangle, is 6.12 inches (Apple's specs). The Pythagorean theorem gives the number of pixels along the diagonal. Add 2556 squared and 1179 squared, then take the square root: about 2,815. Divide by 6.12 and you get about 460, matching the 460 ppi in the specs.
So how fine is fine enough? A person with a visual acuity of 1.0 (often written 20/20) can tell apart dots 1 arcminute apart, which is 1/60 of a degree. Seen from 30 cm away, 1 arcminute spans about 0.087 mm. About 290 dots of that width fit in an inch (our calculation).
So when viewed from 30 cm, differences above about 300 ppi become hard to see for someone with 1.0 acuity. This is a simplified calculation, though. People with sharper eyes, or who hold the screen closer, can see finer detail.
Screens also differ in the range of colors they can show, called the color gamut. The iPhone 15 supports P3, a gamut wider than sRGB, the standard for the web (Apple's specs). P3 is based on DCI-P3, a range made for cinema.
Common misconceptions
Myth: An "LED TV" is a TV whose pixels are LEDs, and OLED TVs are one kind of LED TV
In fact, most products sold as "LED TVs" are LCD TVs with an LED backlight. The light comes from the LEDs at the back, while the liquid crystal and color filters create the color and brightness. OLED is a different type, where each pixel makes its own light. The similar names, LED and OLED, add to the confusion.
Myth: A higher resolution always looks sharper
In fact, viewing distance and your eyesight set a limit. As calculated above, about 300 ppi is a rough guide for a phone viewed from 30 cm. And if the original video has few pixels, adding pixels to the screen does not add detail. Resolution is easy to compare on spec sheets, so people tend to think bigger is always better.
Myth: One pixel is one point of light
In fact, one pixel is usually made of three subpixels: red, green and blue. Many OLED phones use a layout with twice as many green subpixels as red or blue, shared with neighboring pixels. The dots are so small that they normally blend into one color, so people tend to think of each pixel as a single light.
By the numbers
| Number | What it means |
|---|---|
| About 4.45 million | Subpixels on the iPhone 11's LCD (1792 × 828 pixels × 3; our calculation) |
| About 290 ppi | A rough guide to the finest detail a person with 1.0 acuity can see from 30 cm (our calculation) |
| About 3.3 µs | Time per row when redrawing 2556 rows at 120 Hz (our calculation) |
Learn more
- Related articles: "How Do LEDs Work?", "How Do Touchscreens Work?" (the other part stacked on a screen)
- See it in a figure: nMOS channel (a TFT also uses its gate voltage to open a path for current)
- See the differences in a table: Sputtering vs evaporation (evaporation is used to make OLED layers)
- Try the numbers: the image, audio and video data size calculator