PstLM
PstLM — short-term flicker severity, light-side variant
Definition
PstLM is a single-scalar perceptual metric for how annoying flicker from a light source is to a static human observer. PstLM = 1.0 corresponds to the visibility threshold — the level at which an average observer experiences flicker in 50 % of cases. It is produced by the light flickermeter specified in IEC TR 61547-1:2015 Annex A, and is the metric EU Ecodesign uses for SLR (Single Lighting Regulation) compliance.
Verbatim from the source
“Pst[EUT] = 1, the EUT is experienced to have equal flicker behaviour as a 60 W incandescent lamp by an average observer. I.e. in 50 % of the cases flicker is experienced and in 50 % of the cases no flicker is experienced.”
Technical detail
Validity range: 0.3 Hz ≤ f ≤ ~90 Hz. Below 0.3 Hz the statistical evaluation becomes unreliable; above 90 Hz the human visual system stops behaving as a temporal low-pass sensor and other metrics (SVM, PAVM) apply.
Computation pipeline (per IEC TR 61547-1:2015, Figure A.2): (a) illuminance adapter normalises the input to relative fluctuation; (b) bandpass weighting filter applies the LM weighting from Annex A Eq A.5; (c) instantaneous flicker sensation block squares and 300-ms-LPFs the filtered signal; (d) statistical evaluation produces Pst from a percentile bank over the observation window.
Minimum observation window: 120 seconds per IEC. Longer captures (180 s is typical) give more stable results because the percentile statistics converge.
Importantly, PstLM is what professional hardware flicker meters (UPRtek MF250N, Gigahertz-Optik, Asensetek) report. It is NOT just percent flicker or flicker index dressed up — it is a perceptually-weighted metric where the human visual system's frequency response is baked into the weighting filter.
Where it appears
- FlickerSpec Used here
Computed on a 180-second tripod-based capture (Pro tier, iOS, v1.2.0+). The 240 fps camera signal is upsampled 8× to 1920 Hz before the IEC weighting and statistical pipeline runs.
Related terms
- IEC TR 61547-1:2015
IEC TR 61547-1:2015 is the international Technical Report that defines a standardized light flickermeter — a model of human perceptual respo…
- IEEE 1789-2015
IEEE 1789-2015 is an IEEE recommended practice that defines low-risk and no-effect frequency / modulation-depth thresholds for current modul…
- Percent flicker
Percent flicker is the relative variation in light output of a source over one full modulation cycle, expressed as a percentage. 0% means no…
- SVM
SVM is a perceptual metric for the stroboscopic effect — the apparent stuttering or jagged motion of a moving object under modulated light. …
- PAVM
PAVM is a perceptual metric for the phantom-array effect — the chain of bright dots that appears when a viewer moves their eyes across a hig…