The Ultimate Guide to TFT Display Modules: Part 2

A backlight is a form of illumination used in LCD displays because these displays don’t produce light by themselves. In order to produce a visible image, they need some form of illumination – for example, ambient light or a special light source.

A backlight can illuminate the LCD from the side or back of the display panel. Backlights are most often used in smaller displays to increase their readability in low light conditions.

Backlights can be made up of:

• Light-emitting diodes (LEDs)
• An electroluminescent panel (ELP)
• Cold cathode fluorescent lamps (CCFLs)
• Hot cathode fluorescent lamps (HCFLs)
• External electrode fluorescent lamps (EEFLs)

Only ELP can give off uniform light over the entire display surface. Other backlights – like LEDs – need to use a diffuser. This is how they can provide even lighting coming from an uneven source.

Backlights come in many colors. Monochrome LCDs typically use yellow, green, blue, or white backlights. Color displays employ white backlights that cover most of the color spectrum.

What is a NIT? It’s a unit used to measure a backlight’s brightness. As a general rule, one nit equals the light produced by one candle.

The majority of LCD display backlights are LEDs. In most cases, they require the same power, which is applied to the LCD component. Manufacturers use two common methods to make the LED backlight brighter – they modify the current limiting resistor or integrate a transmissive polarizer.

Current limiting resistor

LED backlights need a current limiting resistor to reduce the driving current that reaches the backlight. The lower the value set for the resistor, the brighter the backlight will be. By reducing the resistor value, manufacturers also shorten the half-life of the LED backlight. The normal half-life can reach 50-70K hours; when overdriven, it can drop to 20K hours or less. Note that the half-life of the LED is the amount of time measured in hours for the LED to become half as bright as when it was first turned on.

This might not become an issue if the product in question has a short lifespan – or if the backlight will be rarely on. The customer needs to decide on the tradeoff between making the LED backlight brighter and dealing with a shorter half-life.

Transmissive polarizers

The other option to increase the LED brightness is by replacing the transflective polarizer with a transmissive polarizer. Every LCD includes two polarizers:

the front polarizer (facing the user), which is always transmissive,

and the rear polarizer (selected by the user).

The transmissive polarizer allows for more light to pass from the backlight without decreasing the half-life of the LED. However, using a transmissive polarizer might make the display less readable when the backlight is off. Transmissive isn’t recommended for battery-powered projects since, in this case, the backlights need to always be on.

Final notes:

The backlight can be engineered to be bright enough for outdoor signage or office viewing or include other optical light filters.

Electronic interface to the main system

Another thing to consider is the three aspects of the electronic interface connecting the main system:

Interface 1: Display electronic interface

Interface 2: Touch electronic interface

Interface 3: Backlight electronic interface to PWM

You can read more about them in this article.

Importance of assembly

Another key aspect is the physical connection between the touch panel and the LCD. When connecting the LCD to the touch panel, it’s important to remember the laws of physics. One cost-efficient solution is bonding by a double-sided adhesive gasket. The thickness of the gasket will depend on the size of the module.

The advantage of this type of bonding is its tightness – the air gap between the touch panel and the LCD display is free from any pollution and provides a clear picture. The tight assembly also protects the module from the humidity and even chemicals dissolved in the air that can present a danger to the device mechanics.

Testing

EMI emissions

To obtain certification and launch a product on the European market, the company needs to meet specific requirements of electromagnetic compatibility (EMC). Every electronic component emits some electromagnetic radiation, and that includes TFT LCD displays.

Electromagnetic Compatibility (EMC) is the ability of an electronic device to work properly in a specific electromagnetic environment without emitting any noises (EM Susceptibility) that might interfere with other devices that are located nearby.

Electromagnetic interference might be caused by the radiation emitted by the device, as well as the environment, but the compatibility is a characteristic for the entire device, not just its components like the display module. That’s why appropriate testing is essential before releasing the project to mass production.

Effects of ESD

Electrostatic discharge (ESD) refers to the sensation of an electric shock when walking across a carpet or opening a car door. The ESD definition offered by https://www.esda.org describes it as “the rapid, spontaneous transfer of electrostatic charge induced by a high electrostatic field.” Manufacturers employ specific strategies to limit the effect of ESD.

Shock and vibration testing

Thanks to these standard testing procedures, the manufacturer can ensure that a TFT display is durable and reliable. They allow the detection of problems and solving them to avoid costly failure in mass production.

• Vibration test – This test checks the mechanical resonance in all three directions (X, Y, and Z). Mechanical engineers use its results to optimize the design and improve the product.
• Shock test – the purpose of the shock test is to verify how rugged the design is in all three directions (X, Y, and Z) to optimize it.

Other issues to consider

Another key problem to consider is the startup process of the display which includes the initialization and requires a backlight inrush current.

If you’re looking for more expert advice about TFT display modules, please contact our consultants. At Melrose, we have years of experience in providing our clients with custom display solutions that make the most of the latest technologies.