Everything You Ever Wanted to Know About Led Backlights but Were Afraid to Ask
LEDs (Light Emitting Diodes) are found in almost all LCD technologies from Segment, Graphic, and Character, to TFT displays.
Although, their operation is simple, a DC (Direct Current) and a few volts, there are many things to keep in mind when choosing the best option for your LCD design.
Below are key terms used in LED backlighting.
HALF-LIFE
This is the amount of time (in hours) before the backlight will be half as bright as when it was first turned on. This is not the amount of time for it to burn out.
A common half-life for LEDs is 70K hours. Factors that will decrease half-life include overdriving the LED (too much current) and operating in extreme temperatures.
In the past, many LCDs used an EL (Electroluminescence) backlight. These are typically less bright and only have a half-life of 3K – 5K hours. They also run on AC (alternating current) power.
POSITIVE MODE AND NEGATIVE MODE LCD
Positive Mode:
A positive mode LCD means the characters/icons are darker in color and the background is lighter. Positive mode is readable with or without the backlight being on.
Negative Mode:
Negative mode means the character/icon is light in color and the background is dark. The LED backlight must be on for the display to be readable. Because negative mode LCDs require the backlight to be on 100% of the time while in use, the half-life of the LED will be lower.
RGB
An RGB (Red, Green, Blue) LED is a single LED that can be modulated to generate any color on the spectrum except black.
Most people think of TFT displays when they talk about RGB LEDs. However, TFTs actually have white backlights that shine through RGB pixels. This allows them to generate between 64,000 and 64,000,000 unique colors depending on the BPP (bits per pixel).
However, RGB LEDs are also found in older, monochrome technologies such as Segment, Character, and Dot matrix (Graphic) displays. The segments and pixels do not change color, but the background color can change based on the LEDs.
Some applications use backlight color to indicate the condition of a product or its operating environment. An example would be an LCD used to monitor a motor.
- A Green background would indicate that all systems are normal.
- Yellow warns of a possible error or slight temperature increase.
- Red demands immediate action such as a failure, overload, or overheating.
UNITS OF BRIGHTNESS
NITs are one of the units used to measure the brightness of an LED. One Nit is equivalent to one candela per square meter. It is common to see both nits and cd/m2 used interchangeably as the measure for the brightness of a display. The higher the nits, the brighter the display.
Typically high nit counts are seen in products that will be used in high ambient light conditions, most notably, outdoor products that will be used in direct sunlight. While the brightness required to allow for sunlight readability is subjective, we recommend a minimum of 700 nits and prefer a brightness of 1000 nits or more for sunlight readable applications.
INCREASING BRIGHTNESS
1. Increase Current
This is an easy fix accomplished by removing or reducing the current limiting resistor. This will draw more current and shorten the LED’s half-life. This does not mean any display can be used and all you need to do is increase the power until you find the optimal brightness. It is important to review the displays brightness specifications to ensure it can safely operate at the required levels. Do not power the backlight beyond it’s maximum threshold as it can damage the product.
2. Use a Transmissive Polarizer
Transmissive allows all the light behind the polarizer to pass through to the user, producing a brighter backlight, but the backlight must be on for the display to be readable. Not the best option for battery powered.
3. PWM
Pulse Width Modulation turns the LED on for a short time and then off for a short time. The on and off happen so quickly, that the user cannot see the fade.
This requires additional circuitry.