To rectify alternating current, it is initially transformed into a pulsed current at a frequency that is double the mains frequency—120 Hz in the United States and 100 Hz across European nations. A consistent direct current is attained by smoothing out these pulsations. Nonetheless, achieving an entirely flat waveform is impractical, resulting in light flicker—periodic variations in the luminous flux emitted by a fixture.
Acceptable Frequency
The human eye perceives images as a continuous stream when presented at a rate of no less than 48 frames per second. In the sphere of film projection, the standard frame rate is 24 fps; however, the shutter mechanism blocks light twice within each frame, effectively doubling the frequency. Experiencing a film at 48 fps for an hour and a half feels instinctively natural.
Yet, a cinematic experience encompasses merely a fraction of our visual field. If ambient illumination flickers—even at frequencies of 100 to 120 Hz—it can subconsciously influence the nervous system, inducing fatigue. Contemporary research has illuminated that human vision is entirely impervious to flicker beyond 1,250 Hz. At frequencies exceeding 300 Hz, the ramifications of flicker on the human eye become trivial.
High-Frequency Flicker Fixtures
Certain lighting fixtures energize LEDs with pulsed current, particularly those designed for dimming. In such instances, pulse frequencies surpass 1,250 Hz.
A recent breakthrough, Pure Li-Fi, facilitates data transmission via LED illumination. Information is conveyed to the user by modulating the luminous flux over time, with flicker frequencies escalating to several megahertz. Conversely, data is transmitted through an infrared channel.
Flicker Percentage
Beyond frequency, the flicker percentage emerges as another pivotal parameter, calculated by the formula:
E-max denotes the maximum illuminance during the flicker cycle
E-min signifies the minimum illuminance during the flicker cycle
E-avg represents the average illuminance
These values are ascertained at multiple points using specialized instruments.
The flicker percentage is governed by national standards. For instance, according to recommendations from the Department of Energy, it should not exceed 9.6% at 120 Hz. In healthcare facilities and certain specialized environments, the threshold is set at 4%. Outdoor lighting typically evades flicker regulations; however, due to the pervasive use of surveillance cameras, it is generally maintained below 10%.
Leading manufacturers engineer LED drivers to ensure flicker percentages remain beneath 10%. Elevated values are more prevalent in economical drivers, which often suffer from diminished reliability and energy efficiency.
Stroboscopic Effect
This phenomenon is frequently observed in cinema when an antiquated carriage with large, spoked wheels traverses the screen. It may appear as though the wheels are either stationary or rotating in reverse. This optical illusion arises because images are captured at discrete intervals of time.
Elevated flicker levels can also engender the stroboscopic effect in real life. In industrial environments, this can pose significant safety hazards to workers. Consequently, the flicker percentage of lighting fixtures utilized in manufacturing must be meticulously maintained at the lowest possible levels.
