Flicker, PstLM, and SVM

What is Flicker?

Flicker refers to variations in luminance over time—rapid changes in light intensity or brightness, making the light appear to fluctuate or pulse.

Flicker Frequency

Human response to flicker depends on frequency, measured in Hertz (Hz), indicating how often a light source switches on and off or varies in intensity per second. One Hz equals one cycle or flicker per second. As the frequency increases, changes in brightness between minimum and maximum levels become less noticeable to the eye.

🔹3–70 Hz: Flicker is clearly visible.

🔹Up to ~100 Hz: Flicker is noticeable to most, depending on light conditions.

🔹100–200 Hz: Flicker is less noticeable but may be subconsciously detected.

🔹Above 200 Hz: Flicker is not perceptible to the human eye.

How to Measure Flicker?

Two common ways to measure flicker are percent flicker (modulation %) and flicker index.

Percent Flicker

Percent flicker measures the magnitude of perceived flicker. It is calculated by taking the difference between the maximum and minimum light output during a single cycle, dividing it by their sum, and expressing the result as a percentage.

Formula:
Flicker % = 100 × (A – B) / (A + B)

A= Maximum Value

B= Minimum Value
0% = no flicker; 100% = highest flicker.

IEEE 1789 is the most widely used standard for regulating flicker in LEDs.

Recommended Practice 1 (RP1):

🔹Below 90 Hz: Modulation (%) < 0.025 × frequency

🔹90 to 1,250 Hz: Modulation (%) < 0.08 × frequency

🔹Above 1,250 Hz: No restriction on modulation (%)

Recommended Practice 2 (RP2 – for NOEL):

🔹Below 90 Hz: Modulation (%) < 0.01 × frequency

🔹90 to 3,000 Hz: Modulation (%) < 0.0333 × frequency

🔹Above 3,000 Hz: No restriction on modulation (%)

Most LED fixtures flicker at twice the mains frequency (50 Hz in Europe, 60 Hz in the USA and Canada), leading to these typical flicker values:

50 Hz mains (100 Hz flicker):

🔹RP1 (Good): Max modulation = 8.0%

🔹RP2 (Best): Max modulation = 3.3%

60 Hz mains (120 Hz flicker):

🔹RP1 (Good): Max modulation = 9.6%

🔹RP2 (Best): Max modulation = 4.0%

Flicker Index

Flicker index is calculated by determining the average light output during a cycle, then measuring the area above and below this average. The index is the ratio of the area above the average to the total area.

Formula:
Flicker Index = Area above average / (Area above + Area below)
Ranges from 0 to 1; lower values = less visible flicker.

What is PstLM and SVM?

PstLM (Short-Term Light Modulation)

Applies to flicker in the 0–80 Hz range. A value of 1.0 means a 50% chance that an average observer will perceive flicker. A value of 0 is ideal; values ≤1.0 are acceptable.

SVM (Stroboscopic Visibility Measure)

Covers the 80–2,000 Hz range. An SVM of 1.0 is the visibility threshold for an average observer.

🔹An LED luminaire is considered flicker-free if: SVM ≤ 0.4 and PstLM ≤ 1.0