LCD Displays: How to Make an LED Backlight Brighter
There are a few techniques that can be employed to make the LED backlight brighter, but if your goal is to compete with (or against) the direct sun light, there are better methods than to increase the brightness of the backlight.
“We need the brightest LED backlight possible for our new LCD design and we need to make sure that the LCD is sunlight readable.” This is a common request and the battle begins when engineers employ such techniques as PWM (pulse width modulation), reduction or elimination of current limiting resistors and overdriving the rated power limits with the final goal to make the backlight brighter.
Emissive & non-Emissive LCD modules
- Emissive displays do not require a backlight since they produce their own light; an example of this is an OLED (Organic Light-Emitting Diode). OLED’s are readable in both dark and daylight environments.
- Non-Emissive displays require a backlight to illuminate the LCD when used in low or no ambient lighting conditions. Character, Graphic and Segment displays fall into this category and are the subject of this article.
The backlight, as its name suggests, sits behind the LCD’s bottom polarizer and can be turned ON or OFF depending on the ambient lighting conditions.
What is a NIT? The brightness of a backlight is measured in ‘nits’. As a general rule, one nit = the light produced by one candle. (It’s a bit more technical than that, but the idea works out the same.)
The majority of LCD display backlights are LED (Light Emitting Diode) and in most cases require the same power that is applied to the LCD logic (the power that drives the LCD only).
Two common methods to make the LED backlight brighter include:
- Modifying the current limiting resistor.
- Integrating a Transmissive polarizer.
Current-Limiting-Resistor
LED backlights require a current limiting resistor to reduce the driving current reaching the backlight. The lower the resistor value, the brighter the backlight.
Reducing the resistor value shortens the half-life of the LED backlight. The normal half-life of a LED backlight can range from 50K hours to 70K hours, but when overdriven the half-life can drop to 20K hours or less. This may not be an issue if the product has a short lifespan or if the backlight is rarely on, such as a cell phone.
The customer needs to decide on the tradeoff between making the LED backlight brighter and a dealing with a shorter half-life.
Note: Half-life of the LED is the amount of time (in hours) for the LED to become half as bright as when it was first turned on. Half-Life is not when the backlight will burn out, but when it dims to half the brightness of when it was first turned on. MTBF (Mean Time Between Failures) is the amount of time before the backlight fails.
Transmissive polarizers: Making the LED backlight brighter
A second option to increasing the LED brightness is to replace the Transflective polarizer with a Transmissive polarizer.
Note: Each LCD contains two polarizers, the front polarizer (facing the user) is always Transmissive; the rear polarizer is selected by the user.
The Transmissive polarizer allows more light from the backlight to pass through without decreasing the half-life of the LED.
One disadvantage of the Transmissive polarizer is that the display is difficult to read when the backlight is off. Transmissive is not recommended for battery powered product since the backlight must always be on.
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