We analyze the performance of a fiber optic temperature sensor in terms of the standard radiometric figures of merit to optimize its design from the measured fluorescence spectrum. We apply this analysis to an erbium-doped silica fiber employing the intensity ratio technique. We develop the expressions for the SNR, the responsivity in the band, and the noise-equivalent power to predict and optimize the performance of each spectral channel, considering a number of spectral subbands. Incorporating narrow bands for each transition, we improve the SNR by a factor of 5 for each channel. We evaluate and predict the sensitivity of the sensor, incorporating the intensity ratio, to be around 0.025 K-1. The temperature resolution of this highly linear sensor is estimated at a value of about 0.1 K for the temperature interval from below room temperature to above 500°C.