Physical Sciences Inc. (PSI) designed, constructed, and delivered a prototype passive imaging pyrometer capable of accurately measuring and controlling the temperature distribution across a specified surface. The device was originally designed, under a NASA SBIR contract (solicitation year 1985) monitored by Jet Propulsion Laboratory, for space application with a material processing acoustic levitation furnace, and thus the software allowed for registration of a surface that was allowed free motion within a defined range.

The multicolor imaging pyrometer was designed specifically to measure temperatures from 900 to 2500 K with spatial resolution of 2 mm. These parameters can be extended and customized for other applications.

Like all pyrometers, this instrument determines the temperature of a material by measuring the emitted radiation. However, unlike most other pyrometers, temperature measurement errors associated with a lack of knowledge about the heated sample's emissivity are minimized by utilizing an optical system that operates at short wavelengths compared to the peak of the blackbody spectrum for the temperature range of interest. In this regime the radiant power increases faster than exponentially with temperature.

Because of this extreme sensitivity to temperature, the emissivity of the source plays a relatively small role in determining the emitted power. The short wavelengths therefore provide more accurate measurements of the temperature than can be made using longer wavelengths, assuming equally poor knowledge of the sample's emissivity. The penalty paid for this accuracy is that the dynamic range of the pyrometer's CCD image detector places rather narrow limits on the range of temperatures that can be measured at a single short wavelength. To cover a broad range of temperature, six wavelengths – each sensitive to a specific temperature range – are provided.

An optical head projects distinct images at all six wavelengths onto the detector concurrently. Computerized data acquisition and analysis enables the user to capture data and store it on videotape, interactively examine and interpret the data either immediately after capture or upon replay of the videotape, or to automatically analyze the pyrometer's output. In the latter mode, the pyrometer provides a display of the object's temperature distribution in a false color image on a video monitor, a calculation of the temperature distribution's mode value, and a capability to use that value in a feedback loop for controlling the object's temperature.

The pyrometer technology was later extended under a National Science Foundation grant to allow rapid nonintrusive measurement of particulate and gas temperatures in extremely hostile environments. The resulting optical temperature monitor, again employing multiple wavelength measurements, was originally sold commercially as GasTemp®. Following this successful demonstration and commercialization, PSI established a subsidiary to manufacture and enhance the product. It was soon apparent that this company had additional opportunities in the environmental monitoring instrumentation market, and a second optical instrument, the SpectraScan®line, was developed. SpectraScan directly measures trace concentrations of ammonia, hydrogen fluoride, hydrogen sulfide, and other regulated gases in either extractive or open path modes.

As the applications of SpectraScan became clear, the company's name was changed to Spectrum Diagnostix, and the SpectraTemp™ optical temperature monitor was introduced. The Massachusetts Capital Resource Co. and The Venture Capital Fund of New England invested $1.7 million, allowing Spectrum Diagnostix to grow to almost 20 employees. In 1996, Spectrum Diagnostix was sold to PSI's marketing partner, Bovar Western Research, which now manufactures and distributes SpectraTemp™It provides continuous accurate and reliable temperature monitoring of hot particulate-laden gas streams.

SpectraTemp determines particulate temperature by measuring light emissions from the particulates at three different wavelengths. These are selected to avoid interferences from cool heat-transfer surfaces (e.g., boiler walls). SpectraTemp provides a temperature range of 350ºC (675ºF) to 1600ºC (2900ºF). Its design gives it a unique capability to determine a line-of-sight average temperature all the way across a particulate-laden stream. Automatic calibration, fault detection, and alarms are incorporated into SpectraTemp, and both analog and digital outputs are supplied for interfacing with plant control systems.

SpectraTemp has most commonly been used for monitoring furnace-exit gas temperature in utility and industrial boilers fired by coal, heavy oil, wood, or municipal waste. In these installations, SpectraTemp reduces operating costs by providing better control of furnace-wall blowers and soot blowers, better burner control (tilt and excess air), and superior steam-temperature control.

SpectraTemp can also be used for improved process optimization in other industrial processes where accurate control of the temperature of particulate-laden gas streams is important. Examples of such processes include waste incinerators, cement kilns, and smelters. For more information on SpectraTemp!"contact Norman Stein, Bovar Western Research, (713) 789-1084.


    • Three-wavelength optical temperature monitor
    • Lowest measurable temperature:
      • 350ºC (675ºF)
    • Highest measurable temperature:
      • 1600ºC (2900ºF)
    • Absolute accuracy:
      • +/- 25ºC (+/- 50ºF)
    • Relative accuracy:
      • +/- 15°C (+/- 30ºF)
    • Measurement field of view:
      • 6º cone
    • Outputs:
      • Analog:
        • 0-10 VDC; 4-20 mA
      • Digital:
        • RS-232
    • Electrical classifications:
      • General purpose
    • Physical dimensions and weight:
      • 1200 × 250 × 180 mm (46 × 9.5 × 7 in.)
      • 18.2 kg (40 lb.
    • Port:
      • 50 mm (2 in.) port in boiler sidewall

This work was done by Physical Sciences Inc. under a NASA SBIR Contract monitored by Jet Propulsion Laboratory. Information on customized imaging pyrometers can be provided by George Caledonia, Physical Sciences Inc., 20 New England Business Center, Andover, MA 01810; (978) 689-0003.

NASA Tech Briefs Magazine

This article first appeared in the January, 1998 issue of NASA Tech Briefs Magazine.

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