3.1

Signal Transfer Function (SITF) and Noise-Equivalent Temperature Difference (NETD)

The Signal Transfer Function (SITF) is the variation of the response of the detector to a variation of the temperature of the scene. In the measurement of the SITF, the response of the system to two different scene temperatures is measured. The two temperatures should be close to the temperature at which the NETD is required due to the non-linearity of the SITF with the temperature.

In the case of this work, the SITF has been characterized between scene temperatures of 25 C and 35 C for NETD measurements at 30 C. A typical graph of the response of a microbolometer detector line (mean response of the 512 pixels) to a step in the temperature of the scene blackbody between 25 C and 35 C is presented in Figure 2. The response of the detector is measured after the analog to digital converter (ADC) of the Readout Integrated Circuit (ROIC), being therefore given in counts. Its value depends not only on the response of the pixels but also on the different ROIC parameters along the signal transfer chain (such as gain, comparator voltage thresholds, integration time and clock frequency).

Figure 2:

Example of measurement of the mean detector response (over the 512 pixels) to a change of the temperature of the blackbody as a function of acquisition number. The temperature of the blackbody is varied after 100-150 acquisitions. Measurement parameters: Temperature of TEC 20 C, integration time 140 ms, clock frequency 2 MHz.

The SITF is then calculated as

for each pixel in the array, where M₂ and M₁ are the mean of the response of each pixel over several acquisitions for temperatures of scene T₂ and T₁ .

The Noise Equivalent Temperature Difference (NETD) measures the increment of temperature of the scene that produces a peak signal-to-noise ratio of 1 under flood illumination [6, 7]. For the NETD characterization using the radiometric package, a set of more than 1000 acquisitions were taken at the scene temperature of the measurement. The low frequency component was then eliminated by applying a high-pass filter to the collected data. The RMS of the filtered data represents the high frequency noise used for the NETD calculation. Another method used for the calculation of the high frequency noise is based on the computation of the Power Spectral Density (PSD) of the temporal data (the noise power spectrum) [8]. The PSD is given in counts²/Hz and the usual shape shows a 1/f feature followed by a plateau at higher frequencies (Figure 3 (b)). The level of this plateau is used to estimate the instantaneous RMS noise by assuming that it represents the white noise level across the full bandwidth.

Figure 3:

(a) Mean response of the 512 pixels of the line 2 of a microbolometer pixel array to a scene at a constant temperature of 25 C. Collection time for 1000 acquisitions is around 9 min; (b) Power density spectrum of the signal from (a) after removal of the DC component. The white noise level is indicated by the red line; (c) High frequency noise extracted from temporal response raw data from (a). Measurement conditions: Type IIB ROIC 140 ms integration time, 2 MHz clock frequency, TEC T 20 C, BB T 25 C.

In the current measurements, the time to collect 1000 acquisitions is 9 min. Therefore, the sampling period is 0.54 sec and the sampling frequency Fs is 1.85 Hz. A “one-sided” “pwelch” function has been utilized. Therefore, all the spectral power is contained in the range of frequencies from 0 Hz to Fs/2 (i.e. 0.93 Hz, the Nyquist frequency). By integrating the noise floor (indicated by a red line in Figure 3 (b)) over the frequency range from 0 Hz to 0.93 Hz, the instantaneous RMS noise can be obtained. Figure 3 (b) shows the PSD of the measured signal shown in Figure 3 (a). The high frequency noise filtered using a highpass filter is shown in Figure 3 (c).

The NETD is then calculated as the high frequency temporal noise per pixel divided by the SITF

The value of NETD highly depends on the optical components integrated in the package derived from specific mission requirements. Filter spectral band and bandwidth, window transmission, temperature of operation and integration time have an influence on the value of measured NETD. Furthermore, the NETD is also dependent on the temperature of the scene at which it is measured. Therefore, the scene temperature should be specified when reporting a value of NETD.

INO has developed different types of microbolometer pixels that can be fabricated on top of the 512×3 ROIC and that will be reported in a future publication. Several of those pixels have been packaged in the radiometric package in order to characterize their performance. As an example, an NETD as low as 23 mK was measured for a high resistivity pixel packaged in a radiometric package for which the combined transmission of the window and filter are shown in Figure 4 (measurement conditions: flood irradiation with no front optics, f/1, 140 ms integration time, temperature of the detector of 10 C and temperature of the scene of 30 C).

Figure 4:

Measured total transmission of the window and the filter of the radiometric package used to package a 512×3 FPA detector with high resistivity pixels for which an NETD of 23 mK was measured (measurement conditions: flood irradiation with no front optics, f/1, 140 ms integration time, temperature of the detector of 10 C and temperature of the scene of 30 C).

3.2

Robustness to Ambient Temperature Variations

Variations in the temperature of the environment of the package induce a variation in the power of the radiation reaching the detector by both heating of the package (direct emission over the detector) and by variation of the temperature of the objects outside the field of view of the device (variation of the amount of radiation reaching the detector as stray light). In order to assess the radiometric stability of the package, both the robustness to a variation of the ambient temperature (i.e. heating of the package) and to stray radiation (i.e. radiation coming from outside the field of view of the package) need to be studied.

In order to minimize the influence of the variation of the temperature of the package cover on the sensor response, a temperature stabilized radiation shield was placed around the detector (Figure 1). The exterior of the shield as well as the interior of the package cover were plated in gold to minimize heat transfer by radiation between the package cover and the shield. Furthermore, the shield was designed to optimize its temperature control by the TEC.

To characterize the influence of a variation of the temperature of the ambient on the response of the detector a heating tape was placed around the detector cover. The temperature of the package was measured by means of a thermocouple placed on the package cover. The blackbody was set to a fixed temperature of 25 C. Acquisitions were taken while the package was heated. The response of the detector was plotted as a function of the temperature of the package cover. The experimental data was then fitted with a linear fit. The slope of the fitted line represents the variation of the response of the detector per degree of variation of the temperature of the package.

An example of measurement obtained using this method is shown in Figure 5.

Figure 5:

Radiometric stability of a package to a variation of the environment temperature (Type IIB ROIC with low resistivity pixel, 140 ms integration time, 2 MHz clock frequency, TEC T 20 C, BB T 25 C after vibration

The variation of the equivalent temperature of the scene due to a variation of the temperature of the package can be calculated as

where ΔResponse/ΔT_ambient is the slope of the graph of the response of the detector as a function of the temperature of the package in [counts/C] and the SITF is measured in counts/C.

From the results shown in Figure 5 it can be calculated that each degree of variation of the temperature of the package will induce a variation of the equivalent temperature of the scene seen by the detector of 0.1 C.

3.3

Robustness to Stray Light

The robustness to the stray light can be understood as variation of the equivalent temperature detected by the sensor per degree of variation of the temperature of the stray light.

The stray light has been represented in the experiment by an extended area blackbody at different temperatures that illuminates the detector outside its field-of-view. During the experiment, the field of view was masked by a mask at 25 C and the variation of the response of the detector with and without the stray light blackbody was measured for different stray light blackbody temperatures.

Figure 6 shows the variation of measured scene temperature due to the presence of the stray light as a function of the temperature of the stray light blackbody.

Figure 6:

Variation of the measured scene temperature when exposed to the stray light of an extended area blackbody that emits radiation outside the field of view of the package.

As it can be seen in Figure 6, the design of the shield has been demonstrated to be very effective in preventing stray radiation from reaching the detector.

3.4

Environmental Tests

The radiometric package was subjected to thermal cycling (MIL-STD-810 method 501) from -55 C to +85 in a dry atmosphere (non-operating) for 30 cycles. Figure 7 shows the thermal cycle used. The length of the cycles was extended in order to ensure a minimum of 20 min soak at each extreme temperature.

Figure 7:

Thermal cycle.

The package was also subjected to severe random vibration in three orthogonal axes according to Table 1. The PSD is a standard GEV-SE curve specified by NASA for small components [9].

Table 1:

Random vibration PSD breakpoints

A photo of INO’s vibration setup is shown in Figure 8.

Figure 8:

INO vibration setup.

Finally the device was also subjected to shock according to MIL-STD-810F Method 516.5 at 75 G level, duration of 11 ms 2 pulse per axis.

The performance of the device in terms of NETD and radiometric stability was completely unaltered after the environmental qualification campaign.