---
title: "Opacity"
description: "Opacity is the percentage of light obscured by particulate matter in stack flue gas, measured continuously by a transmissometer (opacity monitor) installed in the stack. Opacity is the headline visual KPI for ESP performance and is permit-limited in most jurisdictions — typically 20–40% on a 6-minute rolling average, with absolute peaks limited to 60% for shorter periods."
canonical_url: "https://sylio.co/glossary/opacity"
last_updated: "2026-06-28T02:29:35.094Z"
---

**Opacity** is the percentage of light obscured by particulate matter in stack flue gas, measured continuously by a transmissometer (opacity monitor) installed in the stack. Opacity is the headline visual KPI for [ESP](/glossary/electrostatic-precipitator) performance and is permit-limited in most jurisdictions — typically 20–40% on a 6-minute rolling average, with absolute peaks limited to 60% for shorter periods.

## Opacity excursions

Opacity excursions are typically driven by:

- [ESP rapping](/glossary/esp-rapper) [re-entrainment](/glossary/re-entrainment) puffs
- [Baghouse](/glossary/baghouse) bag failure (sudden particulate breakthrough)
- [ESP back-corona](/glossary/back-corona) collapse
- Combustion upsets producing unusually high inlet particulate
- Soot-blower-triggered re-entrainment

## How sonic horns reduce opacity

[Sonic horns](/glossary/sonic-horn) deliver continuous gentle dust release rather than periodic aggressive rapping puffs. Plants retrofitting horns on opacity-limited ESPs commonly see 20–40% opacity-peak reduction without other changes — the headline business case for many sonic-horn ESP installations.

## Related terms

- [CEMS](/glossary/cems)
- [Electrostatic precipitator](/glossary/electrostatic-precipitator)
- [Re-entrainment](/glossary/re-entrainment)
- [Sonic horn](/glossary/sonic-horn)