This is a NDIR (Non-Dispersive Infrared) CO2 sensor. This means that it works by measuring the amount of infrared (IR) light absorbed by CO2 molecules at a specific wavelength. These sensors utilize the principle that CO2 molecules absorb infrared light at a wavelength of 4260 nm. By shining IR light through a sample of air and measuring how much light is absorbed at that specific wavelength, the sensor can accurately determine the concentration of CO2 in the air.

The Spectrum below shows the response of IR excitation of different gases :

Source : Wikipedia
Credit : Daniel Popa and Florin Udrea

The Sensor has a an infrared light source, often an LED or infrared lamp, which emits infrared light. The light passes through a light tube or chamber containing the air sample being measured. An optical filter is placed in front of the infrared detector. This filter is designed to allow only the specific wavelength of infrared light absorbed by CO2 (around 4260 nm) to pass through. The infrared detector measures the amount of infrared light that passes through that filter. The amount of light absorbed by CO2 molecules is directly proportional to the concentration of CO2 in the air sample. The detector's reading is then processed by the sensor's electronics to calculate and display the CO2 concentration. In order to compensate for humidity and temperature the sensor has a nifty algorithm. That's why the sensor also supplies temperature and humidity.

The different CO2 levels cause different levels of well-beeing. It is commonly understood, that the following concentrations corespond to those different symptoms :

CONCENTRATION [PPM]	EFFECTS
400 ... 1000	background (normal) outdoor air levels
1001 ... 2000	complaints of drowsiness and poor air
2001 ... 5000	headaches, sleepiness, and stagnant, stale, stuffy air poor concentration, loss of attention
> 5000	workplace exposure limit (as 8-hour TWA) in most jurisdictions

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