ABSTRACT
Hg-Cd-Te (MCT) cameras can be used to analyze the thermal emission or the infrared reflective response of physical systems. However, measurements performed with this instrument need to be corrected for the thermal emission from the environment surrounding the camera. In this work we analyzed this effect under conditions typically met in field applications, when environmental temperature variations are common. The dark current signal on a Xeva MCT 320 CL TE4 camera was studied as a function of ambient temperature and the integration time used for image acquisition. The MCT sensor at the focal plane was kept at a constant nominal temperature of 210 K by a thermoelectric cooler unit throughout the experiment. Integration times for data acquisition varied between 2.0 to 12.0 ms. The camera body temperature was monitored within ±0.2°C, ranging from about 17.0°C to 27.0°C. The camera unit was allowed to reach thermal stabilization in a controlled-temperature lab before each measurement session. Both the integration time, and temperature range intervals were chosen to represent typical field deployment conditions. The average dark current signal showed a clear linear dependence with integration time, for a constant environmental temperature setting. The slope of this linear relation increased with the ambient temperature, whereas the intercept was insensitive to temperature changes. The standard deviation of the dark current signal was a function of integration time, but independent of the ambient temperature setting. These results allowed modeling the dark current signal as a function of the integration time and the camera body temperature. To minimize the dark current for a given integration time setting, measurements should be performed under the coldest possible conditions, in opposition to manufacturer recommendations. As a direct consequence of these results, the useful dynamic range for science applications with this MCT camera is reduced with increasing integration times and ambient temperatures. For instance, when acquiring images with 5 ms integration time, at 22°C ambient temperature, the resulting dark current signal reduces the maximum useful dynamic range in about 20%. The results shown here can be promptly adapted to other applications with MCT cameras, especially in situations with a non-controlled thermal environment, or when analyzing the reflective properties of cold targets.
ABSTRACT
The influence of the thickness of the nanostructured, mesoporous TiO2 film on several parameters determining the performance of a dye-sensitized solar cell is investigated both experimentally and theoretically. We pay special attention to the effect of the exchange current density in the dark, and we compare the values obtained by steady state measurements with values extracted from small perturbation techniques. We also evaluate the influence of exchange current density, the solar cell ideality factor, and the effective absorption coefficient of the cell on the optimal film thickness. The results show that the exchange current density in the dark is proportional to the TiO2 film thickness, however, the effective absorption coefficient is the parameter that ultimately defines the ideal thickness. We illustrate the importance of the exchange current density in the dark on the determination of the current-voltage characteristics and we show how an important improvement of the cell performance can be achieved by decreasing values of the total series resistance and the exchange current density in the dark.