Building biologists know exactly what's up, when speaking about the disadvantages of cheap LED lights.
To the eye flickering of light sources is barely noticeable, but possibly harmful for your health. A Flicker Meter serves to detect or measure the flicker factor.
If brightness varations slowly take place, like voltage fluctuiations in the power grid, the human eyes perceive them as considerable. Modern light sources like LEDs get clocked to current controlling. Depending on the clock frequency and the clock proportion a LED is also changing its brightness. The eye perceives it only limited. Studies suggest that this unconscious influence of light might harm your health.
Building biologists pay their attention to healthy constructions and installation of lighting. They have to know this characteristic and often determine the flicker factor with special developed measuring devices like a flicker meter.
A flicker meter is a complex instrument, because it determines different perception limits, temporal interval, frequency and
the intensity of light fluctuations. The flicker factor is calculated by a mathematical reference. A maximum and minimum light intensity over a certain time is used for this calculation for example by a photodetector of an oscilloscope. The difference between the maximum and the minimum value, related to the maximum value is the calculated one. An alternative measurement is the calculation method with the difference of the maximum value to the minimum value, in relationship to the sum of max and min.
The frequency of the alternating signal from max to min value is the flicker frequency. The (still known) light bulb supplies a flicker frequency of 100Hz, caused from the AC of the power grid of 50Hz for instance. The sine wave of the mains frequency reaches the maximum amount of voltage twice per period, that's why the light frequency of light and dark is doubled. This is known as the flicker frequency.
This shows us that even the well-known bulb was not consistent and also fluctuated in brightness.
Fluorescent bulbs on the other hand, show a significant higher frequency, depending on their electronic ballasts.
If you want to determine the waveness of light from LEDs, their properties particulary depend on the connected LED driver. Possible are factors from 0% to 100%, entirely depending on whether the diode current is regulated via pusle width modulation or a linear procedure.
Flicker Factor [%] = (Smax - Smin) / Smax
S is proportional to the brightness of the signal strenght value.
For more information to the function it is referred to the standard 61000-3-3 or this english technical article.