An NDIR (Non-Dispersive Infrared) sensor is a relatively simple yet highly effective spectroscopic device used to measure the concentration of specific gases—most commonly Carbon Dioxide (CO2).
They are favored because they are “non-consuming” (they don’t use up a chemical reagent to work) and can last for years without much maintenance.
How it Works: The Physics of Light
The fundamental principle behind an NDIR sensor is that many gas molecules absorb infrared light at very specific wavelengths.
The Source: An infrared lamp or LED sends a beam of light through a tube filled with the air sample.
The Absorption: As the light passes through, the target gas molecules (like CO2) “catch” or absorb specific wavelengths of that light.
The Filter: Before reaching the detector, the light passes through an optical filter. This filter blocks all wavelengths except the one the target gas absorbs (e.g., CO2).
The Detector: The sensor measures how much light is left. If the light is dim, it means there is a high concentration of the gas. If the light is bright, the gas concentration is low.
Key Components
IR Lamp: Usually a specialized incandescent bulb or a micro-glow source.
Sample Chamber: A tube (often gold-plated) designed to reflect the light and maximize the path length for better accuracy.
Optical Filter: The “gatekeeper” that ensures the sensor only looks for one specific gas.
Infrared Detector: Usually a thermopile or a pyroelectric sensor that converts light energy into an electrical signal.
Why use NDIR?
Longevity: Since the gas doesn’t touch the electronic components directly, the sensors don’t “wear out” like electrochemical sensors.
Specificity: They are excellent at ignoring “interference” from other gases.
Stability: They don’t drift as much over time, making them the gold standard for indoor air quality (IAQ) monitoring.
Fun Fact: Because NDIR sensors rely on physical light absorption, they can be affected by changes in air pressure. Most high-end sensors include a pressure compensation algorithm to keep readings accurate at different altitudes.