Pulse oximeters are non-invasive devices clipped to fingers, toes, or earlobes that measure blood oxygen saturation (SpO2) and pulse rate using red/infrared LED light absorption through tissue. Oxygenated blood absorbs more infrared light; deoxygenated absorbs more red-enabling 95-100% accuracy in healthy patients, though less reliable below 85% or with dark skin, nail polish, or motion. Uses span hospitals (ICU monitoring, COVID-19 screening), homecare (COPD/asthma), and wearables for sleep apnea tracking. Fingertip models display SpO2 (normal: 95-100%), heart rate (60-100 bpm), and perfusion index; alarms alert for drops below 90%.