Thermal Imaging with Photon-Counting Detectors.
The first quantitative evaluation of a photon -counting detector as a thermal imager is described. Photon -counters overcome problems of limited spatial resolution and the need for complex cooling and scanning subsystems, which are typical of conventional thermal imagers. Photon -counting imaging also has proven useful in pattern recognition. Three methods for thermal imaging with photon-counting detectors are investigated. These methods include directly imaging the thermal object with the photon-counter, upconverting the thermal radiation with an infrared phosphor, and upconverting the thermal radiation with a four-wave mixing process. We present, to our knowledge, the first thermal images obtained using a photon-counter. Figures of merit are developed to compare the methods. An extension to the Noise Equivalent Differential Temperature frequently used to evaluate conventional thermal imagers is derived. Image quality is evaluated using the Modulation Transfer Function of the total system, which is derived using a cross-spectral density formalism. The number of detected photons required to distinguish between thermal objects is used as a metric of the pattern recognition capabilities of each imaging system. Images of thermal objects in the 600^ circ-800^circK temperature range are presented. The images were recorded without upconversion, using only a photon-counting detector with a blue-sensitive photocathode. The expected performance using red-sensitive photocathodes is computed numerically; the four orders of magnitude improvement in count rate indicates that thermal imaging without upconversion could be extended to object temperatures in the 400^ circK range. A phosphor upconversion system is analyzed and measured values for the upconversion efficiency and the phosphorescent noise are presented. The analysis shows that the phosphor upconversion system does not perform as well as the direct-imaging method of thermal detection. A sum-frequency upconversion system that uses sodium vapor as the upconversion medium is designed. Analysis shows that the small upconversion efficiency associated with unfocused, continuous-wave pump beams causes the sum -frequency system not to perform as well as direct-imaging of thermal objects. Sum-frequency upconversion may offer advantages in imaging laser-illuminated objects. The first upconverted images of CO_2 laser radiation using unfocused, continuous-wave pump lasers are presented. The measured upconversion efficiency is shown to agree well with theoretical predictions.
- Pub Date:
- Physics: Optics