The half life of LED backlights
The half life of LED backlights need to be taken into consideration for those LCD displays that incorporate a LED backlight.
Two types display modules
Display modules can be broken down into two categories: Emissive and non-Emissive. Emissive displays are modules that produce their own light and can be seen in low or no-light ambient conditions, these include OLEDs (Organic Light-Emitting Diode) and VFDs (Vacuum Fluorescent Display)
Non-Emissive displays require a backlight to be seen in no light conditions. These include segment, character, graphic and UWVD displays.
Since the majority of backlights integrated in non-emissive displays are LED, this article will not cover other backlight technologies such as: EL (Electroluminescence) and CCFL (cold-cathode fluorescent lamps), but will be covered in future articles.
The half life of LED backlights
LEDs slowly dim over time; half life is the time, in hours, of how long the backlight can operate before its brightness is at 50% of what it originally was when first turned on.
So if the half-life is rated at 50K hours, then the brightness of your backlight will be half as bright at 50K hours. If the backlight burns for an additional 50k hours, it will be half as bright as it was at 50K hours, or in other words, the LED will only be 25% as bright as when it was first turned on.
See chart below.
What is the MTBF of a LED backlight?
MTBF (Mean time between failures) is the estimated number of hours before the backlight burns out or goes dark. Some estimates of this are as high as 1,000,000 hours, but take into account several factors that occur in the real world that will shorten this estimate.
What reduces the lifetime of a LED backlight?
In a perfect world backlights would maintain their brightness for several years, but harsh environmental conditions can shorten the life of the LED backlight. These include: Excessive current, Excessive heat and Vibration
LED backlights are designed for an optimum voltage and current, for this reason we add a current limiting resistor to the LCD or directly to the backlight to limit the current that is supplied to the backlight.
At times, customers will reduce the resistance value of the current limiting resistor to make the backlight brighter. This lowers the half-life hours and is not recommended. One solution to increase the brightness of the LED is to keep the recommended current limiting resistor and replace the transflective polarizer with a transmissive polarizer.
On June 26, 1990, Phoenix Arizona expressed a record high temperature of 122 degrees Fahrenheit, as a result the runway was too hot and airplanes were not allowed to take off and land.
This type of heat can shorten the half life of LED’s. If possible try to keep a LED backlights away from power supplies, high wattage resistors and other heat producing components.
Although LED backlights are solid state, their half-life is reduced when in environments of shock and heavy vibration.
There are ways to increase the half life of LED backlights, these include:
- Reducing current
- Integrating a PWM (pulse width modulation)
- Dimming the backlight when not in use.