How do LCD TFT Displays and OLED Displays Work?

Monochrome Liquid Crystal Displays (LCDs) are composed of two glass panels with electrodes printed on the interior surfaces. Encased between these glass layers is a liquid crystal (LC) organic fluid. When the LCD is in its “off” state, meaning no voltage is applied, light passes through the display. However, in the “on” state, when voltage is applied, the LC fluid twists in response to the electric field, resulting in the blocking of light and the creation of dark areas on the LCD. By selectively applying voltage to the electrodes, various patterns can be generated. It’s important to note that LCDs are passive displays, meaning they do not emit their own light. Instead, they rely on ambient light in the surroundings or backlight illumination from sources like LED, Electroluminescent (EL), or Cold Cathode Fluorescent Lamp (CCFL) lighting.

LCD Backlighting

An LCD essentially functions as a reflective surface and requires a light source to make the displayed content visible. This can be achieved by utilizing available ambient light or by adding a backlight source positioned behind the LCD. There are several backlighting options available, but the primary three are as follows:

1. Electroluminescent (EL) Backlighting: EL backlights are extremely thin, lightweight, and provide consistent illumination. They come in various colors, with white and turquoise blue being the most popular choices for LCDs. Although their power consumption is low, they require voltages in the range of 80-100VAC, which are typically supplied by an inverter converting a 5, 12, or 24VDC input to AC output. ELs have a relatively low brightness of 15-20cd/m² and a limited lifespan of 10,000 to 15,000 hours to reach half brightness.

2. Light Emitting Diode (LED) Backlighting: LEDs have become the top choice for backlighting LCDs. They are available in a wide range of colors, including white and blue, and offer high brightness levels with a long operational life, typically ranging from 25,000 to 50,000 hours, depending on the color. These solid-state devices operate directly off 2-5VDC, though a current limiting resistor is advisable for LED protection. LEDs are either mounted in an array directly behind the LCD or along one edge within a diffuser assembly.

3. Cold Cathode Fluorescent Lamp (CCFL) Backlighting: CCFL backlighting offers low power consumption and produces a bright white light. Typically, CCFL lamps are positioned along the edges of the LCD with a diffuser assembly to evenly distribute the light across the rear of the display. CCFL lamps require 270-300VAC, supplied by an inverter that converts 5, 12, or 24VDC input to AC.

Viewing Modes

LCDs are available in three fundamental light transmission modes:

• Reflective Mode: This mode utilizes available or ambient light to illuminate the display by incorporating a reflector along with the rear polarizer. Reflective LCDs are best suited for outdoor or well-lit environments.

• Transflective Mode: Transflective LCDs combine reflective and transmissive features, with the rear polarizer having partial reflectivity. They are designed to be used with a backlight for visibility in all lighting conditions.

• Transmissive Mode: In this mode, LCDs have a transparent rear polarizer and do not reflect ambient light. To view the display, the backlight must be active at all times. This mode is most effective in low-light or dark conditions.