The basic operating principle is the same in both sensors. Both measure optical absorption at an absorption wavelength characteristic to the gas being measured. This signal is then compared to a nearby non-absorbing wavelength for reference. The CARBOCAP® non-dispersive infrared (NDIR) sensor uses a broadband light source and a tunable filter to measure absorption over an entire carbon dioxide absorption band in the 4.2 µm mid-infrared region. The SPECTRACAP® tunable diode laser (TDL) uses a narrow band tunable light source to measure a single oxygen absorption peak in the 760 nm visible/near infrared region.