No thermometer “diagnoses” — it estimates body temperature. Accuracy depends on using the right type correctly for the person’s age. For a baby under 3 months, any fever is urgent.
Temporal (forehead) infrared
A temporal thermometer reads the infrared heat radiating from the forehead, often with a quick scan or a no-touch hover. It’s fast, hygienic, and great for screening and for not waking a sleeping child. Modern models are calibrated to report an estimated body temperature, so you generally shouldn’t add a degree yourself. See the infrared thermometer guide.Tympanic (ear) infrared
An ear thermometer measures infrared heat from the eardrum, which shares blood supply with the brain’s temperature center. It’s quick and works well for older babies, children, and adults, but technique matters — a poor angle or earwax can skew it. It’s generally not recommended for newborns.Digital stick (contact)
A digital probe measures temperature directly in the mouth (oral), underarm (axillary), or rectum. Rectal readings are the most accurate for infants; oral works for cooperative older kids and adults; underarm is convenient but reads lower than core temperature. These are the go-to for confirming a fever.What about old mercury or glass thermometers?
Mercury glass thermometers are now discouraged because a breakage releases toxic mercury. Modern digital and infrared devices are safer, faster, and easier to read, and they’ve effectively replaced glass thermometers for home use.Which type should I use?
Match the type to the situation: infrared forehead or ear for fast, no-fuss screening; a digital stick for confirmation and for the youngest infants. Using one method consistently — and noting which you used — gives the most comparable numbers over time. SonoHealth’s ThermoPRO (forehead and ear infrared) and ThermoMax cover everyday home needs.Related: Forehead vs. Ear vs. Oral · How to Take an Accurate Temperature · Choosing a Thermometer · Infrared Thermometer Guide