Working Principles and Pros/Cons Comparison of LCD and OLED

01. Working Principles of LCD and OLED

1.1. LCD Working Principle

The cross-sectional diagram of an LCD is as follows, consisting mainly of 7 layers​ from bottom to top: Backlight Layer, Vertical Polarizer, Positive Electrode Circuit, Liquid Crystal Layer, Negative Electrode Circuit, Horizontal Polarizer, and Color Filter.

• The bottom-most Backlight Layer​ emits white light, which passes through the colored Color Filter​ to become light of the corresponding color. Note: the backlight is not an independent LED lamp under each pixel; rather, the entire screen shares one large backlight layer.

• When voltage is applied to the Positive Electrode Circuit, it penetrates the Liquid Crystal Layer, connects to the Negative Electrode Circuit​ to form a loop. This voltage drives the liquid crystal molecules to twist. At this point, the liquid crystal layer, like a shutter, blocks part of the light. By controlling the voltage magnitude, we can control the twist angle of the liquid crystal molecules, thereby controlling the brightness of the red, green, and blue sub-pixels. Changing the brightness ratio allows mixing any desired color.

1.2. OLED Working Principle

OLED stands for Organic Light-Emitting Diode. Like LED screens, it also has three sub-pixels. The difference is that it lacks a liquid crystal layer and a backlight layer; it is itself a specially designed self-emissive diode. By controlling the voltage, and thus the current flowing through the diode to change its brightness, the color ratio of each sub-pixel can be controlled, ultimately mixing the desired color.

• OLED has no backlight layer. Each pixel can be controlled independently for on/off. Therefore, unlike LCD screens which must light up the entire backlight layer when turned on, in an OLED screen, each pixel can be understood as an independently controlled lamp. This enables features like Always-On Display (AOD), where part of the pixels can be lit at low brightness and refresh rate to show time, notifications, and other important information when the phone is locked.

02. Pros and Cons Comparison of LCD and OLED

2.1. OLED Has Lower Power Consumption

• An LCD screen, once turned on, lights up the entire backlight and consumes power continuously.

• An OLED screen only consumes more power than LCD when displaying a pure white image. However, each OLED pixel's brightness and on/off state can be controlled independently, allowing for differential brightness reduction or even turning pixels off. Therefore, unless you stay on a white screen for a long time, OLED generally offers longer battery life.

2.2. OLED Has Higher Contrast Ratio

• Contrast ratio refers to the brightness ratio between the brightest white and the darkest black. A higher ratio means more vibrant and saturated colors.

• For an LCD to display pure black, ideally, the liquid crystal molecules would close completely to block all emitted backlight. However, they cannot achieve complete closure. When displaying black, a small amount of white light leaks through, so what is seen is not pure black but a significantly dimmed gray. This characteristic prevents LCDs from displaying true, pure black.

• OLED has no backlight, and each pixel is independently controlled. To display black, power to those pixels can be completely cut off, resulting in no light emission and achieving true, pure black. Thus, OLED screens have a higher contrast ratio.

2.3. OLED Has Faster Response Time, Advantageous for Dynamic Display

• When displaying dynamic content, each pixel needs to switch colors quickly. The time required for this switch is the screen's response time. If the response time is too long, pixels cannot change color in time, causing afterimages as the previous frame hasn't fully disappeared before the next appears.

• LCDs switch colors by controlling the twist of liquid crystal molecules. The speed of this twist directly determines the LCD's gray-to-gray (GtG) response time. This twisting speed is temperature-dependent, slowing down at lower temperatures, leading to noticeable ghosting in LCD screens in cold environments.

• OLED screens lack a liquid crystal layer and are thus not limited by it. While OLEDs do have a response time, it is very short for color-to-color transitions, showing almost no ghosting. The switch between pure black and pure white takes slightly longer but is still generally shorter than most LCDs. Therefore, OLED has an inherent advantage in displaying dynamic content.

2.4. OLED Can Be Thinner and is Foldable

• OLED screens, lacking the backlight and liquid crystal layers, are much easier to make thin and can be bent significantly, like paper. This bendability allows for easy implementation of curved screens, significantly enhancing the perceived quality of devices.

• LCD screens, besides having the backlight and liquid crystal layers, also contain polarizers, making them much thicker. The backlight and liquid crystal layers use rigid substrates, allowing only very slight bending, typically seen only in large panels like desktop monitors.

2.5. OLED Has Shorter Lifespan

• In LCDs, the backlight layer emits light. The liquid crystal layer only controls light blocking, and the filter only changes light color; neither emits light. Voltage is applied to the non-emissive liquid crystal layer.

• In OLEDs, voltage is applied directly to the self-emissive diodes, causing frequent electron migration within the OLED's emissive layer. Combined with the fact that the emissive layer itself is made of organic materials (which age more easily than inorganic materials), and the self-emission property, this directly leads to a significantly shorter screen lifespan for OLEDs compared to LCDs.

2.6. LCD Suffers from Light Bleed

• Due to the presence of the backlight layer in LCD screens, and the fact that the screen panel must be assembled into a device, light from the backlight can easily leak out at the seams between the screen and the device's frame. When displaying a pure black image, large halos of light can appear at the borders, known as "light bleed."

2.7. OLED is Prone to "Burn-in"

• In LCDs, the backlight is a single unit, and all pixels age uniformly.

In OLEDs, each pixel emits light independently, meaning different areas of the screen age at different rates depending on usage. For example, if area A consistently displays blue, the blue sub-pixels there will degrade faster. Later, when displaying a uniform color, that area's blue will be slightly dimmer, causing a persistent afterimage, as if the image is "burned" into the screen. This phenomenon is called "burn-in" (not physical burning, but uneven pixel aging leading to color differen

2.8. Both LCD and OLED Can Cause Eye Strain, but Differently

• Screen brightness needs to be controllable to match ambient light. Two primary methods are used: PWM (Pulse Width Modulation)​ and DC (Direct Current) Dimming.

• DC Dimming​ is straightforward: it directly controls voltage to change brightness. Higher voltage equals brighter light. As the light source remains continuously on, it does not cause flicker-related eye strain.

• PWM Dimming​ adjusts brightness by varying the duty cycle (on/off time ratio) of the light. A larger duty cycle means brighter light. It controls brightness by switching the light on and off, which creates flicker. A drawback is potential eye strain; higher flicker frequencies are less noticeable.

• Due to its inherent characteristics, if an OLED screen uses DC dimming at very low brightness levels, the overly low voltage causes an uneven, "muddy" screen effect, significantly impacting image quality. Therefore, OLED screens cannot effectively use DC dimming and primarily rely on PWM.

• Furthermore, because OLED's organic materials are prone to aging, they cannot use high-frequency PWM (which is less perceptible) and are often limited to low-frequency PWM dimming​ (typically up to around 250Hz). Some visually sensitive individuals can perceive this flicker, making it more likely to cause visual fatigue.

• The part of screen-emitted light most harmful to the eyes is high-energy blue light in the 420-440 nm​ wavelength range, which can cause irreversible damage to the retina. Traditional LCD screen backlights use several bright LED lamps with light guide plates. A significant portion of the high-energy blue light emitted by these LEDs falls within this range.

• In summary: LCDs have strong blue light​ which is harmful, and OLEDs use PWM dimming​ which is also harmful. Some low-end screens even use PWM dimming on LCDs to cut costs. Which is more harmful is not definitively clear.