In the display industry LEDs are used as the light source for LCD panels and as the display pixels in direct view LED displays, eg video walls. The most common method for adjusting the brightness of these LEDs is a method called Pulse Width Modulation, typically just known as PWM (you will see this referred to in all Digital View controller board model specifications).
What is PWM (Pulse Width Modulation)?
As mentioned, the most common method for adjusting the brightness of an LED used as a backlight (for LCD panels) or as the main display component (for direct view LED displays) is by a method known as PWM (Pulse Width Modulation).
PWM is a method of changing the perceived and actual brightness of an LED by pulsing the power to the LED on and off. This is done very rapidly so there should be no perceptible flicker, but changing the average power will change the brightness.
Direct View LED
For a direct view LED as used in a video wall, the LED is both the light source and the image pixel. So the pixel will be adjusted to match the video data, for example at 60fps (frames per second) though other frame rates are common, for example, 24fps, 30fps, 120fps. PWM will be used to adjust the brightness of the pixel to match the video data. Here is a simple diagram that illustrates this based on an LED that is driven at 3840Hz (ie turned on and off 3840 times per second):
The above is just an example and the LED write frequency can be at other rates depending on the LED driver.
LCD With LED Backlight
In a typical LCD panel such as in a TV, monitor or laptop, there is a LED backlight that the user adjusts to their preference. This doesn’t typically receive image data so the PWM is purely to adjust the overall brightness. However with the introduction of HDR and mini-LEDs as backlights the brightness of the LEDs will be adjusted to match the image, maybe not at the same resolution but close to it.
A backlight LED is likely to be driven at around 100Hz to 400Hz however the driver circuit and the LED will enable much higher frequencies. The benefit of higher frequencies is greater granularity to the brightness adjustment and is relevant to certain applications such as low light dimming for night viewing.
Notes
- PWM is used in many other products such as for motor control in robotics, it is also relevant for LEDs used in other applications such as vehicle lights.
- Another method of brightness adjustment for LEDs is amplitude modulation.
- LEDs can be driven at extremely high frequencies, for example at 100,000Hz.
- A USA based backlight power specialist is Applied Concepts (www.acipower.com)