Flickering backlight - just a myth?
Author: Dr.-Ing. Erich Roos
When looking at a modern LCD monitor, some people feel disturbed by annoying flicker, even if the monitor, e.g. in word processing, shows a static image. Why is that and what can you do about it?
What is perceived as flicker is a significant change of the brightness of the light that falls into our eyes at short time intervals. At about 3 light-dark shifts per second (3 Hz), such changes of brightness are very noticeable. At higher frequencies an annoying disturbance is produced, the maximum of which is at about 6 to 10 Hz. From about 20 Hz the harassment significantly decreases, and above about 50 Hz flicker expands into a constant light impression. The frequency of this transition point is called the flicker-fusion frequency (FFF).
For many years, modern methods for moving images have been working with frame rates that are significantly above 20 Hz. In most cases, LCD screens use a frame rate of 60 Hz. With this, one normally should be on the safe side, but why do some people still see a flicker?
Characteristics of displays
When looking at the screen with concentration, a high visibility stimulus arises, which lets almost all other environmental perceptions disappear. Not only the central (foveal) vision is addressed but also the side (peripheral) vision, where the flicker-fusion is significantly higher in general. Whereas many people do not perceive flicker when directly looking at old CRT monitors and televisions with 50 Hz, they notice very clearly visible flicker out of the corner of their eye. Twenty years ago, CRTs with frame rates of at least 72 Hz were considered flicker-free (TCO 92).
Especially among young people, the individual flicker-fusion frequency might be still much higher. As a result of test series, values of more than 100 Hz were obtained. Due to the high brightness and contrast ratio of current LCD screens, the flicker threshold is shifted upwards. Overall, the actual flicker-fusion frequency is a very individual value, strongly influenced by technical, personal and situational parameters. In practice, it is situated somewhere between 40 and 120 Hz, but usually between 50 and 60 Hz.
Thus, the usual frame rate of 60 Hz could definitely be too low for sensitive people, but this only applies to moving image sequences. At a screen change, LCD monitors will switch seamlessly to the next image, and fixed images, such as a text window, should appear flicker-free in each case. So how is it a static image still flickers?
Little known, another source of flicker, the effect of which is completely independent from the frame rate, is the background illumination of the LCD monitor, also referred to as backlight. Cold Cathode Fluorescent Lamps (CCFL) or Light Emitting Diodes (LEDs) produce a white light that shines through the transparent panel to the front. The screen content, which is formed by the liquid crystals in the panel (LCD), is made visible through this white light. Eventually, the crystals determine colour and brightness of each pixel.
Sometimes the full brightness reach by such a screen is not needed. In a darkened room, for example, a monitor that is always working with full backlight power can be very stressful for the eyes. It would be obvious to control the brightness of the pixels with the transparency of the liquid crystals, but this approach would leave the darkest point (black level) always the same, while the lighter would be increasingly limited. As a result, the image contrast, described as the ratio of the brightest and darkest pixel, would be getting worse with increasing regulation and the picture would become dull.
This disadvantage can be avoided if one always utilizes the maximum transparency range of the crystals and if the brightness of the backlight is changed instead. The easiest way would surely be to lower the operating voltage, but this would result in only a small control range because the light sources that are used operate correctly only within relatively narrow limits. Therefore, the commonly used alternative is to periodically turn off the backlight for a split second. As long as this interval is short enough, it is not noticed by the eye, but the impression of brightness decreases with increasing length of the dark phases. This procedure is known as pulse width modulation (PWM).
Brightness progression with CCFL at PWM (140 Hz).
So here it is, the ominous second source of flicker: Once you turn down the brightness control on an LCD monitor, light-dark switching occurs in the backlight, which modulates the brightness and will be recognised as disturbing as the frame rate by sensitive people. With one very significant difference: The backlight flickers not only at image refreshes but all the time, which consequently affects not just videos and games, but particularly the typical office applications with their large bright backgrounds: word processing, spreadsheet, email client, file directories, etc.
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