Fast Infrared Spectrometer

John H. Glenn Research Center, Cleveland, Ohio

A spectrometer has been developed for acquiring transient emission and absorption spectra in the wavelength range from 1.2 to 5.0 µm. It could be used to characterize flames, turbulence, and other transient phenomena that interact with infrared radiation. Heretofore, success in the study of such phenomena has been limited by the inability of infrared spectrometers to acquire data at sufficiently high speeds. In contrast, the present infrared spectrometer measures the spectrum at a repetition frequency of 390 Hz. The high speed of this instrument could also make it attractive for such commercial applications as monitoring food, pharmaceutical, and petroleum products in process streams and on production lines.

The spectrometer optics include a chopper, two prisms that serve as dispersers, and parabolic optical-path-folding mirrors. The spectrally dispersed light is projected onto a 160-pixel linear array of lead selenide photodetectors. The spectrometer also includes electronic circuitry for controlling the chopper, synchronizing readout from the pixels with the chopping cycle, and sending data to an external computer or data logger, all at the repetition frequency of 390 Hz.

Spectra From Five Flames were measured by the fast infrared spectrometer.

The spectrometer is housed in an assembly with overall dimensions of 222 by 165 by 89 mm. A data logger for use with the spectrometer in a special application (a drop-tower rig) is mounted in a separate housing with overall dimensions of 432 by 140 by 190 mm. Because of requirements specific to that application, the data logger stores the data on an easily removable Personal Computer Memory Card International Association (PCMCIA) card. The spectrometer with the data logger is the fastest and smallest instrument of its kind.

The spectrometer was calibrated by use of a black-body radiation source at a temperature of 980 K along with narrow-pass-band filters at wavelengths of 2.56, 2.71, 4.26, and 4.33 µm. In a further calibration experiment, flames fed by alcohol, butane, propane, candle oil, and siloxane were used as radiation sources (see figure). The temperatures of the flames were calculated on the basis of the flame spectra and found to range from 2,175 K for the candle-oil flame to 2,475 K for the alcohol flame. These values are close to the expected adiabatic flame temperatures, and the fact that the lowest temperature was that of the candle flame is consistent with the expectation that the radiative loss from that flame would be greater than from the other flames. Therefore, the calculated temperatures seem reasonable.

This work was done by Yudaya Sivathanu and Rony Joseph of En'Urga Inc. for Glenn Research Center.

Inquiries concerning rights for the commercial use of this invention should be addressed to