Description of the Flickermeter
The measurement of flicker induced by the EUT is made using a Flickermeter. The instrument output is given in units of flicker severity (Pst), a value of one being the acceptable human tolerance limit. The workings of this instrument are a fascinating mimic of the way that a human perceives flicker. The instrument employs a series of electrical filters to model the response of the human eye and brain to light flicker. The design of Flickermeters is prescribed in EN60868.
Figure 3: Simplified Block Diagram of a Flickermeter
The first process of the Flickermeter is to separate the waveform producing the voltage change from the continuous mains level signal. This demodulation process involves squaring and filtering to leave the modulating voltage change waveform.
This waveform then goes through the simulation filters. A filter with an 8.8Hz peak simulates the lamp to eye response, whilst a squaring operation and a sliding mean filter simulate the non-linear averaging process carried out by the eye and brain. The signal which results from these blocks is a representation of flicker as perceived by a human observer.
The extent to which flicker is annoying to the observer depends on its level and its rate of occurrence. For more complex voltage fluctuations, annoyance depends on how often certain levels of fluctuation are exceeded. This model leads to the need for some statistical analysis of the signal. This is carried-out by the formulation of a distribution which relates to the proportion of time each particular level of flicker is exceeded. After 10 minutes of data accumulation, certain key levels are taken from the distribution to calculate the so-called short-term flicker severity (Pst). This value should be less than one in order for a given appliance to conform to the standard.
Whilst the design of the Flickermeter is complex, there is a good choice of commercial meters which can be purchased that take care of the details. Despite the underlying complexities, from the point of view of the person carrying out the compliance tests on the EUT, the procedure boils down to a relatively simple electrical measurement using calibrated equipment.