where is the angle of incident with respect to the normal incident radiation and in the present set up it was taken as 90◦
Fig. 5 shows the plot of resistance as a function of blackbody temperature (TBB) and detector temperature (TD) when device was in contact with the IR source. The exponential decrease in the resistance with respect to increase in the body temperature was observed. The effective power of device/sensor for different distances (d) of source to detector was calculated using Eq. (4). Fig. 6 shows the graph of resistance against effective power of detector. From the graph it was noted that the effective power absorbed by the thermo sensor decreases with respect to increase in the distance between the source and the detector. The maximum and minimum power was obtained when the distance (d) is 1 cm and
5 cm respectively.
Table 1 represents the electrical characteristics such as resistance, thermistor constant (ˇ), TCR and responsivity in terms of (R/R)/mW of thick film NTC thermistor as an IR detector. The IR responsivity was calculated by taking the electrical output per mili watt of the incident energy (Fig. 6). The resistance, thermistor constant and TCR of the thermistor were 540 K, 4045 K and −4.51%/K respectively with the responsitivity of 28.79/mW. It may be noted here that Vittal [14] has reported the IR responsivity of 87/W for lead oxide based thick film NTC thermistors which is much lower than that of the present case which are completely ‘lead free’ thick film NTC thermistor. Karanth et al. [10] also reported the thin film thermistor prepared using oxides of Mn–Ni–Co having lower thermistor value of constant (2186 K), higher sheet resistance 5 × 105/sq and the responsivity, and detectivity of the infrared detector 3000V W−1 and 4.4 × 108 cm Hz1/2W−1 at operating frequency of 10 Hz. In this case also, the resistance is quite higher than that of our reported value and also lower thermistor constant and resposivity than the present case of lead free thick film thermo sensor.