Guide to non-Contact Temperature Measurement
Non-contact temperature measurement is the preferred technique for small, moving, or inaccessible objects; dynamic processes that require fast response; and temperatures > 1000C (1832F). To select the best non-contact temperature measurement device for a particular application, it is essential to understand the basics of temperature measurement technology and parameters, as well as the features offered by the various measurement systems currently available.

Defining the Terms
Temperature. Temperature is one expression for the kinetic energy of the vibrating atoms and molecules of matter. This energy can be measured by various secondary phenomena, e.g., change of volume or pressure, electrical resistance, electromagnetic force, electron surface charge, or emission of electromagnetic radiation. The most frequently used temperature scales are Celsius and Fahrenheit, which divide the difference between the freezing and boiling points of water into 100 and 180, respectively.

The thermodynamic scale begins at absolute zero, or 0 Kelvin, the point at which all atoms cease vibrating and no kinetic energy is dissipated.

0 K = -273.15°C = -459.67°

IR Radiation. Infrared is that portion of the electromagnetic spectrum that lies beyond the visible (blue to red, 0.4-0.75 m) response of the human eye. IR wavelengths extend from 0.75 um to 1,000 m, where the shortest microwaves (radar) begin. Because IR radiation is predominantly generated by heat, it is called thermal radiation.

For the purpose of radiation thermometry, only portions of the IR spectrum are important. The spectrum is frequently divided into "atmospheric windows" that provide maximum loss-free transmission through water vapor in air:

0.7-1.3 µm; 1.4-1.8 µm;

2.0-2.5 µm; 3.2-4.3 µm;

4.8-5.3 µm; 8-14 µm

Thermometer. Most of the well-known thermometers, e.g., glass bulb mercury or alcohol, thermocouple, or resistance thermometer, must be placed in direct contact with the temperature source. Their useful measurement range is -100C to 1500C.

Radiation Thermometer. This non-contact thermometer determines the surface temperature of an object by intercepting and measuring the thermal radiation it emits (see Photo 2).

Emissivity. This quality defines the fraction of radiation emitted by an object as compared to that emitted by a perfect radiator (blackbody) at the same temperature. Emissivity is used to calculate the true temperature of an object from its measured brightness or spectral radiance. Emissivity is determined in part by the type of material and its surface condition, and may vary from close to zero (for a highly reflective mirror) to almost 1 (for a blackbody simulator). Because an object's emissivity may also vary with wavelength, a radiation thermometer with spectral response that matches regions of high emissivity for the target material should be selected. Emissivity values are listed in the literature for a variety of materials and spectral bands, or these values can be determined empirically.

Brightness/Single-Color Pyrometer. These devices measure and evaluate the intensity, or brightness, of the intercepted thermal radiation. Intensity, or, more generally, spectral radiance, is measured in a narrow wavelength band of the thermal spectrum. Band selection is dictated by the temperature range and the type of material to be measured.

The oldest brightness pyrometers compared optical brightness in the visible (red) spectrum at 0.65 µm by matching the glowing object to a hot "disappearing" filament. The term "single-color" derives from the single narrow wavelength band of red seen by the user. Instruments sensitized to measure in the IR Region are also called spectral radiation pyrometers or spectral radiation thermometers.

Ratio/Two-Color Pyrometer. This radiation thermometer measures temperatures on the basis of two (or more) discrete wavelengths. The ratio of the brightness in separate wavelengths corresponds to color in the visible spectrum. The use of two distinct, visible colors--typically red and green--has long been popular to infer color temperatures. More recently, the term has broadened from its initial usage to include wavelengths in the infrared. The advantage of ratio measuring is that temperature readings are greatly independent of emissivity fluctuations and/or sight path obscuration. The technique is generally used for temperatures above incandescence (700C, 1300F), but measurements down to 200C (400F) are also possible.

Measurement Parameters
Advanced radiation detection, optical and electronic signal processing sub-systems greatly extend the accuracy and performance capabilities of non-contact temperature instruments. For process control, standardized interfaces are available within the instruments that provide conditioned signal outputs optimized for specific applications.

Radiation Detection
Emissivity Adjustment. Temperature reading accuracy depends on the correct adjustment of the instrument to the target emissivity. Preset emissivity values can be used for on-line sensors to monitor targets of constant emissivity. Measurements on those materials with changing emissivities require an accurate and reproducible emissivity adjustment.

Surrounding Area Temperature. Thermal target radiation always contains stray radiation emitted by the environment surrounding the target area and reflected by the target=s surface. In practice, the ambient temperature is frequently presumed to be the same as the temperature of the sensor. If the target is exposed to a different thermal environment, e.g., inside a heated furnace, inside a cooled chamber, or outdoors facing the open sky, adjustments are necessary for accurate measurement. Separate sensors for the area surrounding the target may be used for automatic temperature calculation.

Sight Path Obscuration. Gases, water vapor, dust, and other aerosols in the sight path of a sensor may affect the temperature reading. Using one of the "atmospheric windows" in the IR region (i.e., areas of the IR spectrum that provide maximum loss-free transmission through water vapor) greatly reduces measurement errors. Since both optical channels are equally attenuated, ratio pyrometers are generally immune to sight path obscuration, and the signal color ratio remains unaffected.

Ambient Temperature Drift. By the nature of their design, radiation detectors are strongly affected by ambient temperature changes. To maintain high measurement accuracy, precise compensation of this temperature drift is required. Temperature drift is specified in error/C or error/F of ambient temperature change. To learn more about how Heitronics solves this problem with its chopped radiation technique, click here.
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