---
title: "Stroboscopic effect and its prevention"
subject: "Industrial Power Distribution and Illumination"
chapter: 6
key: "semester_6/industrial_power_distribution_and_illumination:stroboscopic-effect-prevention"
source: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/stroboscopic-effect-prevention
site: BCT Engineering
---

# Stroboscopic effect and its prevention

Industrial Power Distribution and Illumination · Chapter 6: System Components for Industrial Illumination

- Asked in: 2073 Bhadra, 4 marks; 2072 Magh, 4 marks; 2069 Bhadra, 4 marks (asked 3 times)
- Video: [Method To Reduce Stroboscopic Effect By Subhash Sir || 💥5th Sem Elect || @IBTechnicalClasses](https://www.youtube.com/watch?v=x8FfVl7tk2c) — IB TECHNICAL CLASSES
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/industrial_power_distribution_and_illumination/stroboscopic-effect-prevention
- Whole chapter: https://www.bctengineer.com/md/semester_6/industrial_power_distribution_and_illumination/chapter-6

## Question

Define stroboscopic effect. How it can be prevented in factory lighting design.

## Short answer

Stroboscopic effect is the illusion that rotating machinery appears stationary or moving slowly under flickering light, a safety hazard. Prevention involves distributing luminaires across three phases, using lead-lag twin-tube circuits, high-frequency electronic ballasts, flicker-free LED drivers, or supplementary incandescent local lighting.

## Answer

**Stroboscopic effect** is the illusion in which a rotating or moving object appears to be **stationary, moving slowly, or moving backwards** when lit by a light source whose output flickers at a regular rate. Discharge lamps (fluorescent, sodium, mercury) and poor LED drivers on 50 Hz supply give light that rises and falls 100 times per second. If a machine part rotates at a speed that is a multiple of this flicker, it looks still, which is dangerous in factories because workers may touch a running lathe chuck, fan or saw.

#### Prevention in factory lighting

1. **Distribute lamps over three phases:** adjacent luminaires (or the tubes of a twin/three-lamp fitting) are connected to R, Y and B phases. The light dips of each phase occur at different instants, so the total light is nearly steady.
2. **Lead–lag (twin-tube) circuit:** in a twin fluorescent fitting, one tube is fed through an inductive ballast (current lags by about 60°) and the other through a ballast with a series capacitor (current leads by about 60°), so their currents are about 120° apart and their light pulses do not coincide.
3. **High-frequency electronic ballasts:** operate lamps at 20–40 kHz, so the flicker is far too fast to cause the effect.
4. **Flicker-free LED drivers:** use constant-current drivers with low output ripple (low flicker percentage) instead of cheap capacitor drivers.
5. **Local lighting with incandescent/halogen lamps** on machines with rotating parts; the filament's thermal inertia smooths out the variation.
6. **Mix of light sources** in the same area so that the flicker of one is filled by another.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. What is the stroboscopic effect?
   A. A lamp getting brighter over time
   B. An illusion where a rotating object appears stationary, slow or reversed under flickering light
   C. A lamp failing to start
   D. Colour change of lamp with age
   Answer: B. Light flickering at 100 Hz can make a rotating part at a matching speed seem still, which is dangerous. (Key point: Definition)

2. Why is the effect significant on 50 Hz supply with discharge lamps?
   A. Light output rises and falls 100 times per second
   B. The light is continuous DC
   C. Lamps emit only UV
   D. Voltage is 11 kV
   Answer: A. Light follows power, which pulses at twice the supply frequency, 100 Hz. (Key point: Cause)

3. How does connecting adjacent luminaires to R, Y and B phases help?
   A. It increases brightness
   B. Dips in each phase occur at different instants so total light is nearly steady
   C. It lowers voltage
   D. It removes the need for ballast
   Answer: B. Phase-shifted light pulses fill in each other's dips. (Key point: Three-phase connection)

## Related in Industrial Power Distribution and Illumination

- [Requirements and installation of factory lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Afactory-lighting-installation-types) — Industrial Power Distribution and Illumination, chapter 7
- [Factory hall 55.75 m × 40.25 m LED lighting](https://www.bctengineer.com/question.md?key=semester_6%2Findustrial_power_distribution_and_illumination%3Afactory-hall-led-55x40) — Industrial Power Distribution and Illumination, chapter 7, asked 4×
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