The term "flicker" is used to refer to the subjective perception of a change in light intensity. This is usually caused by periodically fluctuating voltage changes, i.e. small but frequently occurring voltage dips, in contrast to usually one-off voltage dips (see also "White paper on voltage dips from Janitza electronics GmbH"). The voltage dips that cause flicker can be measured using an algorithm described in DIN EN 61000-4-15. Flicker is caused by rapidly changing high load changes (transformer overloads) in welding machines, arc furnaces, photovoltaic systems, wind turbines, nuclear magnetic resonance tomographs, etc.

The fact that the personal perception of flicker depends on many subjective conditions, such as visual acuity, irritability of the retina in the human eye, the existing light conditions in general, and much more, causes difficulties. The perception threshold is very different for each person and can therefore only be determined statistically. For example, the flicker level of 1 was determined as the level at which the perception threshold applies to 50% of the persons tested. The perception threshold changes with the frequency of the voltage change under consideration. It is lowest at 8.8 Hz. This also shows that flicker is a measured value that must be measured over a certain frequency spectrum.

Very high flicker levels lead to increased maintenance costs and malfunctions in electronic equipment or even their destruction (power supplies).

In addition, the flicker causes employees, particularly those working at VDU workstations in office buildings, to tire more quickly, become irritable and lose concentration. The constant adaptation of the optic nerve to changing light conditions quickly becomes tiring and ultimately affects a person's overall perception. For this reason, limit values have been set in the power quality standard EN 50160, compliance with which should help to avoid the negative effects of flicker.

As flicker manifests itself in very rapid and small voltage variations, it can only be detected using very high-quality measurement technology. This is described in DIN EN 61000-4-15. The instantaneous value of the flicker is usually of less interest here, as longer-term effects should be considered, which is why the values short term flicker (PST) and long term flicker (PLT) have been defined. These are average values over 10 or 120 minutes.

The UMG 511 and UMG 605 measurement devices from Janitza electronics GmbH nevertheless measure all three values and comply with the DIN EN 61000-2-4 standard.

However, due to the empirical nature of flicker (see above), it is not a simple, linear measured value like voltage, for example. A flicker meter must therefore correctly record the empirically obtained test levels of square wave voltages and sinusoidal voltage variations for certain frequencies according to a given accuracy in order to be categorized into the respective flicker meter classes. The UMG 511 corresponds to class F1 and the UMG 605 to class F3 for flicker meters.

However, in order to obtain meaningful values, the values obtained in PST and PLT must be "classified". Since these are average values obtained at different frequencies, a high level of a single frequency would otherwise hardly carry any weight, i.e. the values must be weighted. In this case, higher values have a greater impact on the final value than a very low level.

The power quality standard EN 50160 standardizes a permissible flicker level of 1 in the power grid and thus provides a reference value for by which it can be analyzed. If the standard is permanently violated, measures should be taken to maintain the voltage (higher transformer ratings, dynamic compensation systems, UPS, etc.). For manufacturers of non-linear consumers, the flicker emission is standardized to PST=1 and PLT=0.65 (EN 61000-3-2). The simplest method of analyzing the value is to use the = 1 p.u. curve. P.u. stands for "unit of perception" and is the maximum tolerance level for the sensitivity of the human eye to light fluctuations. The value = 1 p.u. must not be exceeded even when all interferers are combined.