A non-contact infrared thermometer (NCIT), often called a “thermometer gun,” measures surface temperature without physical contact. These devices are common in homes, businesses, and clinics due to their speed and hygienic operation. The accuracy of an NCIT reading depends on understanding the science behind the measurement and the numerous factors that can interfere with it. While they offer a quick screening tool, recognizing their limitations and best use practices is important.
The Science Behind Non-Contact Readings
NCITs operate on the principle that all objects above absolute zero emit thermal radiation within the infrared spectrum. The device uses a lens to focus this invisible energy onto a detector called a thermopile. The thermopile converts the absorbed radiation into an electrical signal, which the circuitry translates into a numerical temperature reading.
The accuracy of this conversion relies on “emissivity,” the relative ability of a surface to emit thermal energy. A perfect emitter (a black body) has an emissivity of 1.0, but real objects, like human skin, have a lower value. Most NCITs designed for human use are pre-set with an emissivity of around 0.98, which is necessary to correctly calculate the temperature from the detected radiation.
Common Sources of Measurement Error
Distance is a frequent source of error between the thermometer and the person. Each device has an optimal distance-to-spot ratio that defines the area being measured. Holding the thermometer too far away may measure background temperatures instead of just the forehead.
Environmental conditions significantly impact the surface temperature being measured. Drafts, air conditioning, or direct sunlight can rapidly cool or heat the skin, causing inaccurate readings. Moisture, such as sweat or cosmetics on the skin, can also interfere with infrared emission, resulting in a lower-than-actual temperature reading. Furthermore, physical obstructions like hair, hats, or glasses can block the sensor’s view, measuring the obstruction’s temperature rather than the body’s.
Best Practices for Maximizing Accuracy
To obtain the most reliable reading, both the device and the subject should acclimate to the measurement environment. It is recommended that both remain in the measurement area, ideally a draft-free space with a stable ambient temperature, for 10 to 30 minutes before testing. Before use, the sensor lens should be cleaned according to the manufacturer’s directions, as dust or smudges obstruct the path of the infrared energy.
The thermometer must be aimed correctly, typically at the center of the forehead, while holding the device perpendicular to the skin. Users should consult the device’s instructions for the correct distance to ensure the sensor focuses solely on the target area. Taking multiple readings and using the highest or average value helps account for momentary fluctuations.
Surface Temperature vs. Core Body Temperature
An NCIT measures the temperature of the skin’s surface, which is different from the body’s internal, or core, temperature. Core body temperature is tightly regulated by the hypothalamus and is the standard for detecting fever. Skin temperature is naturally lower and more variable, constantly changing as the body regulates heat in response to the environment.
Manufacturers of clinical NCITs attempt to bridge this difference by incorporating a mathematical algorithm. This algorithm adjusts the measured surface temperature to estimate an equivalent oral or core temperature reading. While this feature makes the reading more clinically relevant, it introduces an extra layer of estimation, which is a source of potential discrepancy when comparing NCIT results to traditional thermometers.