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Unveiling the optical stack: the construction of TFT displays 

Unveiling the optical stack: the construction of TFT displays



John Stone, sales director, Relec Electronics


TFT (Thin Film Transistor) displays, a common type of LCD (Liquid Crystal Display), have revolutionised the world of digital display technology with their high resolution, low power consumption and compact size. At the heart of this technology lies the intricate structure known as the ‘optical stack’, a combination of multiple layers, each with a specific function contributing to the overall performance of the display.

Introduction to TFT displays

TFT displays leverage the properties of liquid crystals sandwiched between two glass panels. Each pixel in a TFT display consists of a tiny thin-film transistor that controls the orientation of the liquid crystal molecules, thus regulating the light’s passage through the pixel.

Anatomy of the optical stack

The optical stack is a critical part of the TFT display and is composed of several layers:

Backlight: The bottommost layer of the optical stack is the backlight. It produces the light that will ultimately pass through the various layers to create the image seen by the viewer. The backlight commonly uses LEDs, offering high brightness, longevity and energy efficiency.

Light guide plate: This layer guides the light from the LEDs evenly across the display. It uses a series of microscopic dots to direct the light towards the viewer.

Diffuser: The diffuser ensures that the light is spread uniformly across the entire display, eliminating any hotspots or irregularities, and contributing to a consistent image quality.

Prism sheets: These layers enhance the brightness and viewing angles of the display by refracting the light from the diffuser towards the viewer.

Polariser: The first polariser, known as the lower polariser, ensures that the light entering the liquid crystal layer vibrates in only one direction.

Glass substrate with TFTs and colour filters: This layer houses the tiny thin-film transistors that control the orientation of the liquid crystals. Each pixel also has a colour filter (red, green, or blue) that gives the pixel its colour.

Liquid crystal layer: The liquid crystals in this layer rotate under the control of the TFTs, thereby modulating the light’s passage through the pixel.

Upper polariser: The upper polariser, oriented perpendicular to the lower one, permits or blocks the passage of light based on the orientation of the liquid crystals.

Working together to create the final image

The operation of a TFT display involves the seamless interaction of all these layers. The backlight emits light which is then guided, diffused and directed by the light guide plate, diffuser and prism sheets, respectively. The polarised light then passes through the TFT and liquid crystal layers, where its intensity is modulated to create the grayscale value of the pixel. The colour filter assigns the pixel’s colour, and the final image is then observed through the upper polariser.

TFT displays exemplify how meticulous design and fine layering in the optical stack can result in vibrant, high-resolution images. The understanding and optimisation of these layers are crucial for enhancing display performance, thereby pushing the boundaries of what is possible in digital display technology.

The beauty of a TFT display is much more than skin deep. Behind the vibrant colours and sharp images lies a complex world of optical engineering and materials science, a testament to human ingenuity and the relentless pursuit of technological advancement.


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