eFunda: Introduction to Pyrometers

Overview

A Pyrometer, or radiation thermometer, is a non-contact instrument that detects an object's surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.

Typical Broadband Pyrometer

The wavelength of thermal radiation ranges from 0.1 to 100 µm (4 ~ 4,000 µin), i.e., from the deep ultraviolet (UV) across the visible spectrum to the middle of the infrared region (IR).

The Electromagnetic Radiation Spectrum
Courtesy of the Advanced Light Source, Berkeley Lab
(Ernest Orlando Lawrence Berkeley National Laboratory)

Pyrometers are essentially photodetectors which are capable of absorbing energy, or measuring the EM wave intensity, at a particular wavelength or within a certain range of wavelengths.

Common pyrometers include:

•	Optical Pyrometer (a.k.a. Brightness Pyrometer or Disappearing Filament Pyrometer)
-		Designed for thermal radiation in the visible spectrum.
-		Utilizes a visual comparison between a calibrated light source and the targeted surface. When the filament and the target have the same temperature, their thermal radiation intensity will match causing the filament to disappear as it blends into the targeted surface in the background.
-		When the filament disappears, the current passing through the filament can be converted into a temperature reading.

•	Infrared Pyrometer
-		Designed for thermal radiation in the infrared region (0.75 ~ 1000 µm; 30 µin ~ 0.04 in) usually 2 ~ 14 µm (80 ~ 550 µin)
-		Constructed from pyroelectric materials, e.g., triglisine sulfate (TGS), lithium tantalate (LiTaO₃), or polyvinylidene fluoride (PVDF).
-		Similar to the charge generated by stressed piezoelectric materials, a pyroelectric charge dissipates in time. Hence, a rotating shutter is required to interrupt the incoming radiation to obtain a stable output.

Pros and Cons

•	Pros:
	-	Non-contact measurement
	-	Fast response time
	-	Good stability

•	Cons:
	-	Expensive
	-	Accuracy maybe affected by suspended dust, smoke, and thermal background radiation

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