A portable, stable, standards-quality radiation thermometer was invented that can measure temperatures between -50 °C (-58 °F) to 150 °C (302 °F). The corresponding infrared wavelengths are from 8 to 14 micrometers.

(1) Infrared (IR) light from a fixed-temperature calibrated source (at right, not shown) enters the thermometer enclosure through this lens, which focuses the radiation onto a “field stop.” (2) A circular metal chopper slices the IR beam into a sequence of pulses. (3) The first lens inside the central cylinder converts the light from the field stop to a parallel beam. (4) The light passes through this insulated cylinder about 30 cm (12 in.) long, which is temperature-controlled by a feedback system. Stray radiation is blocked by another stop. (5) A second lens focuses the light onto a pyroelectric detector. (6) The detector output is routed to an amplifier that boosts the signal to readily readable levels. (NIST)

In addition to clinical medicine, temperatures in that region are of urgent importance in applications where contact is not appropriate or feasible; for example, surgeons need to measure the temperature of organs prior to transplant. Modern farmers need accurate temperatures when handling, storing, and processing food. Satellites require non-contact thermometers for measuring temperatures on land and the surface of the sea.

Conventional radiation thermometers often contain little more than a lens for focusing the infrared radiation, and a pyroelectric sensor — a device that converts heat energy into an electrical signal. The measurements can be affected by temperature differences along the thermometer and by temperature outside the instrument.

The new Ambient-Radiation Thermometer (ART) is fitted with a suite of interior thermometers that constantly gauges temperatures at different points in the instrument. Those readings are sent to a feedback loop system that keeps the 30-cm (12-inch) cylinder containing the detector assembly at a constant temperature of 23 °C (72 °F).

After it has been calibrated against standards-grade contact thermometers, the instrument can remain stable to within a few thousandths of a degree for months under continuous operation. That makes the system very promising for applications that involve remote sensing over long periods.

There are several methods of making very-high-accuracy temperature measurements but few are well-suited to field work. Platinum resistance thermometers are fragile and need frequent recalibration. The standard temperature source for transferring that calibration to the ART involves a heat-source cavity inside about 42 liters (11 gallons) of liquid.

For more information, contact Howard Yoon at This email address is being protected from spambots. You need JavaScript enabled to view it.; 301-975-2482.


Tech Briefs Magazine

This article first appeared in the September, 2019 issue of Tech Briefs Magazine.

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