Hiện tượng Flicker

Chapter 1: Flicker happens. But does it have to?

Introduction

Nearly all AC-powered traditional light sources exhibit some degree of periodic modulation or flicker.
The effects of undesired flicker from artificial light sources have been a persistent problem, going back
as far as the 19th century when gas lamps became widespread. An article published in Cyclopedia of
Mechanical Engineering in 1908 describes the use of coal gas used for shop lighting: “this light is
objectionable on account of the fact that it vitiates the air…the light flickers, even in still air, which
makes it very trying to one’s eyes when engaged on fine work.”

Although desirable in some situations and not perceived equally by all people, flicker is a phenomenon
that should be avoided—or at least minimized—in most lighting applications. This article presents
results of testing performed on numerous solid-state lighting (SSL) luminaires, showing the amount of
flicker found in various types of these luminaires; and suggests some measures that enable lighting
designers to minimize flicker in SSL-based lighting products.

Physiology

The second volume of The Illuminating Engineer, published in 1908, features an article that discusses
the results of experiments to determine the, “vanishing-flicker frequency” – the threshold where the
effect is no longer observed. This is now known as the flicker fusion threshold or rate, and is
influenced by six factors.

  1. Frequency of the light modulation
  2. Amplitude of the light modulation
  3. Average illumination intensity
  4. Wavelength
  5. Position on the retina at which stimulation occurs
  6. Degree of light or dark adaptation

Studies have shown that visible flicker is usually noticed at frequencies below 100 Hz and invisible
above 500 Hz. Both visible and non-visible flicker are of concern. The negative physiological effects of
flicker are well studied and documented. They include headaches, migraines, eyestrain, distraction,
and in severe cases, epileptic seizures. Issues with the strobe effect—which causes objects to appear
to be moving at different rates than they truly are on factory floors, roadways, and sports lighting—
have been the topic of numerous studies.

Metrics and Industry Standards

Although flicker has been studied in great detail, an official industry standard does not exist to fully
quantify the effects of flickering light sources; however, one well-known method is defined by the
Illuminating Engineering Society (IES) in the RP-16-10 standard, where percent flicker is a relative
measure of the cyclic variation in the amplitude of a light source, and flicker index is a measure of the
cyclic variation taking into account the shape of the waveform. Figure 1 shows a sample waveform
and how percent flicker and flicker index are calculated.1 The drawback to this method is that it
addresses only two of the six factors previously mentioned. In addition, it assumes that a light source
will always flicker at a fixed frequency and amplitude, and does not address random, erratic events
that cause flicker, such as a sudden decrease in electrical current or voltage.

Flicker happens. But does it have to? Figure 1

Figure 1:Calculation of percent flicker and flicker index

 

Còn tiếp

P/s: Trích từ tài liệu được cập nhật từ Cree - Flicker