You can see the difference between the Samsung Galaxy S20 Ultra 5G on the left, which has a medium duty cycle (around 60%), and the Huawei P40 Pro and the Oppo Find X2 Pro, which have long duty cycles (roughly 90% the black lines show that the OLEDs are turned off, albeit briefly): The video below was shot with a Phantom VEO-E 340L camera at 1500 fps (as were the other videos further below), slowed down to 4 fps to show display pulse-width modulation (PWM) - the white areas separated by black lines that extend across the screen when brightness diminishes at regular intervals. At lower PWM frequencies, flicker can become much more noticeable, which helps explain why reading text or watching videos in bed at night is more likely to cause headaches and eyestrain than when viewing screens in brighter conditions. Perceived brightness levels for 25%, 50%, 75%, and 100% PWM duty cycles.Ī significant disadvantage to using PWM technology can be that when a display adjusts to its minimum brightness in very dim or completely dark ambient light conditions, the duty cycle is very short and the interval when the diode is off is proportionately much longer (for example, minimum brightness may translate to a 10% duty cycle, meaning that the diode is off for 90% of the period). The chart below illustrates how different PWMs affect the perceived brightness of a display: Second, as the human eye may experience flicker sensitivity up to about 250 Hz (at least for most people), it should come as no surprise that OLED displays are more likely to cause eyestrain than LCDs.Īn on/off modulation pair is called a period, and the amount of time that the diode is switched on in a period is called a duty cycle. For one thing, OLED displays and LCDs show PWM at different frequency ranges - the PWM of OLED displays range from ~50 to ~500 Hz, whereas the PWM of LCDs starts at around 1000 Hz or higher. So both LCDs and OLED displays power their light sources differently, but both technologies are subject to flicker effect however, it is usually more noticeable on OLED displays than on LCDs. How dim depends on how long the diodes are off versus how long they are on: the longer they’re off, the dimmer the screen will appear. Because we normally should not be able to see this switching between off and on (in other words, the flicker!), our brains are fooled into perceiving the screen as simply dimmer overall (a phenomenon known as the “brain averaging effect”).
So how do phone manufacturers dim displays? They make use of a technique called pulse-width modulation (PWM), which means that they turn the diodes off and on at varying rates. Because of a diode’s intrinsic physical properties, it cannot be dimmed by changing the intensity of the current (mA) without impacting the color of the light. Since both LCDs and OLED smartphone displays are composed of light-emitting diodes, let’s describe how these diodes are driven.
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By contrast, in an OLED display, every pixel is itself an OLED that produces its own light. LCDs don’t emit their own light rather, they are back-illuminated by a strip of LEDs whose light intensity is quite powerful so as to compensate for the brightness drop due to the low transmission rate of the LCD panel (caused mainly by the RGB color filter). But wait! Why do they flicker? Well, let’s remember that smartphone display hardware is based on either LCD (liquid crystal display) or OLED (organic light-emitting diode) technology. Given the ubiquity of smartphones, it is unfortunate that the flicker on their displays (especially OLED displays) is still an issue for many people. This involuntary physiological response can certainly explain why we may have a headache and particularly why our eyes can feel tired after looking at a display for an extended period of time - they have been working hard! (This is especially true when looking at a display in dark ambient conditions, such as reading in bed with the lights turned off, for reasons we’ll touch on a bit more below.) What causes flicker on smartphones? While we may not be consciously aware of the flicker phenomenon, it’s important to understand that our eyes still physically respond to it - that is, our irises expand and contract in response to these changes in brightness. Photo credit: DXOMARK for illustration only What is flicker?įlicker is a quick oscillation of light output between on and off it is measured in hertz (Hz) to quantify the frequency at which the oscillation occurs.
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