Current technology constraints prevent the production of color-accurate OLED displays without employing Pulse Width Modulation (PWM). This is primarily because color integrity varies with the current amplitude. The most effective method for achieving consistent color when adjusting brightness is to manipulate the pulse width while maintaining the same amplitude.
Although increasing the frequency could benefit individuals sensitive to flicker, it would subsequently lead to higher energy consumption. This challenge arises because color shifts occur with intensity changes. To manage intensity while retaining consistent color, the device operates at full power for brief intervals, switching off for similar durations, thus relying on the human eye’s persistence of vision to average the changes.
PWM plays a pivotal role here, similar to how dimmers for incandescent bulbs function—by truncating sine waves in AC systems. Likewise, PWM modifies portions of the DC signal, which complicates the use of these dimmers with LED and CFL bulbs. Employing a variable resistor to reduce current would waste energy as heat, thus proving impractical in devices like smartphones or tablets.
PWM has long been favored for its energy efficiency in both LEDs and OLEDs in flat panel displays. While DC and AC refer to whether the waveform is constant or exhibits frequency content, PWM is categorized as AC, representing a square wave with a varying duty cycle. It is simpler and more efficient to produce than sine waves, functioning effectively as an on/off switch.
Recent discussions surrounding DC dimming focus on establishing a controllable but quasi-constant current source. Since LEDs are diodes (the “D” in LED), they maintain a nearly constant voltage that correlates power with the current flowing through. The eye strain some experience due to PWM’s operations raises questions about why modern LCDs with LED backlights seem less problematic.
Historically, LCDs exhibited similar issues, but advancements in technology, such as phosphor-coated white backlights that are energized by blue or UV LEDs, likely mitigate these effects. The phosphor’s delayed energy release, akin to cathode-ray tubes (CRTs), helps average out the flicker, so users may not notice the PWM flickering as prominently. Furthermore, since the phosphor generates color rather than the LED itself, any underlying color shifts are potentially minimized, reducing the harshness of PWM effects.
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