Light Emitting Diodes (LEDs) are often used for lighting, indicator lights, displays and screens. LED displays should be tested both in darkness and in direct sunlight if they are intended for outdoor use.
LEDs are often cycled on and off very fast for purposes of multiplexing or for adjusting the light intensity. Generally, this cycling goes so fast that no flickering is visible when you look at it. However, the flickering may be visible as stripes or dots if the light is moving or if the person looking at the light is moving. LEDs that are used at roads as traffic lights or as guiding lights on dark roads should either not be cycled or have such a high cycle frequency that they don't appear to flicker by persons who drive by. Even if you can't "see" the flickering itself, your brain can capture it, even at thousands "flashes" per second, and it results in the path of eye micromovement. Such path may be adversely affected by flickering, so some people experience visual discomfort and accelerated tiredness when they have to work with flickering surfaces. It might make a minor differences to other people as well, so you may find paying extra money for "flicker-free" backlight and preferring LCD over LED displays reasonable. The actual estimates for the perceivable flickering rate may be about 100 or more times higher and not mentioned or simply silenced in the "research" sponsored by the equipment makers. The facts, which are normally not mentioned is that you would see a different "picture" on flickering and flicker-free surfaces when moving your eye focus from one point to another - for the duration of that path, even if your brain produce you a "final", approximated version of the image. Depending on the conditions, such differences may be observed for the frequencies up to tens of kilohertz (estimated) or even up to 100KHz. It also appears that equipment makers had been well-aware of such negative effects, as they already started producing some "flicker-free" monitor models, where the backlighting is powered by direct current instead of PWM.