The noise equivalent temperature difference is defined as a uniform square black body target with temperature. It is in the uniform black body background with temperature. The thermal imager observes this target. When the signal-to-noise ratio of the system output is 1, the black body target and The temperature difference of the blackbody background is called the noise equivalent temperature difference. The noise equivalent temperature difference describes the temperature sensitivity characteristics of the infrared imaging system, and its relationship with the noise voltage is
Where is the F-number of the optical system; can be the noise voltage of any kind of noise or the total noise voltage; ； is the rate of change of the target radiated power with temperature.
For a scanning thermal imaging system, its noise is distributed in each subsystem. Due to the effect of scanning, it can be represented by temporal noise. The noise power spectrum is applied, and the system noise is equivalent to a noise source, which is inserted into the detector. The reference electronic filter for measuring NETD is generally used to simulate the filtering effect of the subsequent system of the detector of the first generation thermal imaging system. Finally, the bandwidth of the NETD and system noise can be used to find the system noise.
For gaze-type thermal imaging systems, second-generation NETD measurement points are often placed at the video signal output. Before the system is displayed, NETD is not sufficient to describe system noise. First, both NETD measurements and calculations require a reference filter to simulate subsequent system signal processing circuits. In fact, the signal processing of second-generation thermal imaging systems often appears before the measurement points of NETD. Second, Noise from signal processing inhomogeneities and focal plane inhomogeneities contributes significantly to system noise and even dominates, and NETD clearly cannot describe these noises.
In fact, the signal outputting the second-generation focal plane already contains various noises such as time-space random noise, time-independent spatial correlation noise, and spatial correlation time-independent noise. A three-dimensional noise analysis method has been introduced for this purpose, and three-dimensional refers to the horizontal, vertical, and temporal directions of the space. When the time-space random noise term is converted to the corresponding temperature, it is similar to the form of NETD. In fact, for gaze arrays, is often written as NETD. Since is mainly affected by fluctuation noise, its noise power spectrum is white noise power spectrum. Meanwhile, although other literature believe that other noises (called fixed noise) should have their non-spatial noise power spectrum, only the models considered in the current consideration are considered. The noise arms, and its spatial noise power spectrum is considered white noise power spectrum, the processing method is similar to, of course, this has a certain approximation, especially affecting the accuracy of detecting small targets. Although NETD is not enough to describe the second-generation thermal imaging system, because people are used to it, NETD is still used in the model for the second-generation thermal imaging system and is introduced by it.
Figure 14.1 shows a 320×240 infrared focal plane array imaging system at an ambient temperature of 20 °C.
The histogram of the noise equivalent temperature difference can be seen that the noise equivalent temperature difference between different rows is different, and the average value is about 0.10 °C.
We can examine the noise equivalent temperature difference of the system from three levels: pixel, row (or column) and the entire focal plane. Determining the Pixel Noise Equivalent temperature difference is commonly used to evaluate the performance of an infrared focal plane array; the noise equivalent temperature difference test for determining the line or the entire focal plane is typically used to evaluate an infrared thermal imaging system. The noise equivalent temperature difference test for the determined line or the entire focal plane must be done in the corrected state. In addition, the noise equivalent temperature difference is also a function of the detector's ambient operating temperature, so testing the noise equivalent temperature difference must indicate the ambient temperature.