Infrared Thermometers Ensuring Accurate Temperature Screening

As temperature screening becomes standard practice in public spaces during the ongoing pandemic, many have questioned the reliability of infrared forehead thermometers. How do these devices work? What ensures their accuracy? This article examines the technology behind these essential tools.

Infrared thermometry offers two critical advantages: non-contact operation and rapid response. Measurements take mere milliseconds, eliminating the cross-contamination risks associated with contact thermometers. However, not all infrared devices are suitable for human temperature screening. Industrial infrared thermometers, designed for different purposes, should never be used for medical applications.

When choosing an infrared forehead thermometer, industry standards provide essential guidance. The ASTM E 1965-98 standard specifies that devices intended for skin temperature measurement must maintain accuracy within ±0.3°C (±0.54°F). For ear thermometers, the requirement tightens to ±0.2°C (±0.36°F). These specifications ensure reliable fever detection during health screenings.

Medical professionals must understand the distinction between skin temperature and core body temperature. FDA-approved medical thermometers typically feature both "adjusted" and "unadjusted" modes. The adjusted mode compensates for the difference between skin and core temperatures, while the unadjusted mode facilitates calibration.

In unadjusted mode, readings may appear approximately 3°C (5.4°F) lower than actual core temperature due to this natural variation. Additional factors like individual blood flow patterns, perspiration, and ambient temperature can further influence measurements. When screening reveals potential fever, confirmation with contact thermometers remains advisable.

All objects emit infrared energy proportional to their temperature. Infrared thermometers capture this energy through optical systems that focus it onto specialized sensors. Electronics then convert these signals into temperature readings. Contrary to some misconceptions, these devices measure thermal radiation—not laser emissions. While some industrial models include laser pointers for targeting, medical thermometers typically omit this feature to eliminate eye safety concerns.

Claims that infrared thermometers can harm the brain are scientifically unfounded. These devices operate passively, merely detecting rather than emitting energy. Even models with targeting lasers pose no risk when used properly, though direct eye exposure should always be avoided.

Sophisticated fever screening systems employ thermal imaging cameras to scan individuals entering facilities. The most advanced configurations incorporate blackbody radiation sources—objects with precisely known temperatures and emission properties—to maintain measurement accuracy. Unlike single-point thermometers, thermal cameras generate detailed heat maps of their subjects.

Calibration and verification procedures are essential for reliable temperature screening. Three key factors influence infrared thermometer performance:

• Emissivity: This property describes how efficiently a surface emits thermal radiation. Human skin typically has an emissivity of 0.98, though variations between 0.94 and 0.99 occur. Proper calibration must account for this characteristic.
• Wavelength: For near-room-temperature measurements, the 8-14µm wavelength range proves most effective, offering humidity resistance and sufficient signal strength.
• Geometry: As optical instruments, infrared thermometers have specific field-of-view characteristics described by their distance-to-spot ratio. Medical forehead thermometers typically operate within 10cm (4 inches) of the measurement site.

Regular calibration checks maintain thermometer accuracy. A standard verification process involves:

1. Placing the calibration device in a stable, draft-free environment
2. Setting the reference temperature to 37°C (98.6°F) with 0.98 emissivity
3. Allowing adequate stabilization time (15 minutes minimum)
4. Positioning the thermometer at the specified measurement distance
5. Recording multiple readings to establish average performance
6. Calculating any deviation from the reference standard

Proper calibration requires understanding these instruments' technical specifications and operating principles. While simplified methods using flat-plate calibrators provide basic verification, more sophisticated approaches yield greater accuracy for critical medical applications.