### This method offers advantages over related prior Ramanspectroscopy- based methods.

A method of optical thermometry, now undergoing development, involves low-resolution measurement of the spectrum of spontaneous Raman scattering (SRS) from N_{2} and O_{2} molecules. The method is especially suitable for measuring temperatures in high-pressure combustion environments that contain N_{2}, O_{2}, or N_{2}/O_{2} mixtures (including air).

- It involves analysis in the frequency (equivalently, wavelength) domain, in contradistinction to analysis in the intensity domain in prior methods; and
- It involves low-resolution measurement of what amounts to predominantly the rotational Raman spectra of N
_{2}and O_{2}, in contradistinction to higher-resolution measurement of the vibrational Raman spectrum of N_{2}only in prior methods.

_{2}bands near the laser wavelength. The deliberate choice of lower resolution makes it acceptable to use wider spectrograph slits and thereby to collect more light to obtain greater signal-to-noise ratios. A further advantage of lower resolution is the independence of the spectra on pressure broadening effects.

According to theoretical simulations, the rotational Raman spectrum of N_{2} widens with increasing temperature (see Figure 1). This is because at higher temperature, greater proportions of rotational states having higher energies become excited. Consequently, it should be possible to establish a relationship between the width W_{d} of the envelope of the rotational Raman spectrum and the temperature and to express this relationship as a conversion formula for determining the temperature from W_{d} of a measured spectrum; this is the basic principle of the present method. The method as described thus far would be simple, were it not for the facts that (1) the rotational Raman spectra of N_{2} and O_{2} overlap and (2) almost any practical combustion system contains N_{2} and O_{2}. The net effect of the superposition of the N_{2} and O_{2} rotational Raman spectra is to produce a taller, narrower version of the spectrum of pure N_{2}, the amount of narrowing depending on the relative proportions of N_{2} and O_{2}.

To account for this narrowing, it becomes necessary to generate and use a more comprehensive conversion formula, as illustrated in Figure 2. First, the envelopes of rotational SRS spectra of N_{2} and O_{2} are calculated theoretically over a range of temperature at a certain pressure to obtain the conversion formulas for N_{2} and pure O_{2}. Then a blended conversion formula is obtained as a weighted average, wherein the weighting factors are determined by the relative proportions of N_{2} and O_{2} as measured or calculated by independent means. (The independent means could be measurements of vibrational Raman spectra of N_{2} and O_{2} or a chemical-equilibrium calculation.) Finally, the measured W_{d} is inserted into the blended conversion formula, yielding the temperature.

*This work was done by Quang-Viet Nguyen of Glenn Research Center and Jun Kojima of Ohio Aerospace Institute.*

*Inquiries concerning rights for the commercial use of this invention should be addressed to NASA Glenn Research Center, Innovative Partnerships Office, Attn: Steve Fedor, Mail Stop 4–8, 21000 Brookpark Road, Cleveland, Ohio 44135. Refer to LEW-18100-1.*