The uncooled microbolometer array
LIR uses an uncooled microbolometer array (UMBA) as the detector with 328 × 248 pixels. The internal layout of the camera is shown in Fig. 1 (Fukuhara et al. 2011). The UMBA does not use a cryogenic system, which makes the camera lightweight and small. However, components of the camera still need to be thermally controlled to prevent imaging noise. Therefore, temperature of the UMBA is controlled at 313 ± 0.01 K by a Peltier device. The germanium (Ge) lens on the camera and the baseplate on which the camera is mounted are kept thermally stable by heaters. The power supply unit, which is not thermally controlled, is separated from the camera so that the temperature in the vicinity of the detector does not increase. A baffle resides on the outside of the spacecraft to prevent solar radiation from directly entering the detector; however, its temperature varies drastically with the solar incident angle. Therefore, the camera is thermally isolated from the baffle.
Sensitivity of each pixel of the UMBA used in the LIR has a large inhomogeneity, which is called as on-chip fixed pattern noise (OFPN) (Fukuhara et al. 2011). The OFPN is partially subtracted from an image with the calibration data in the analog circuit of LIR. The residual component can also be eliminated from the target image by subtracting a dark image acquired by taking a shutter. Images are acquired and accumulated continuously at a rate of 60 Hz in order to reduce random noise; we defined the number of accumulations as “m.” The procedure is continuously repeated up to 32 times within 120 s, and resultant images are further accumulated; we defined the number of repetitions as “n.” Digital electronics (DE) in the spacecraft process the first and second accumulations. Then, the averaged image is stored in a data recorder and subsequently sent to the ground station.
Conversion of data counts to brightness temperature
The original LIR image sent from the spacecraft expresses brightness detected with 12 bits per each pixel. The data number for each pixel is converted to brightness temperature based on the reference data, which were derived from the blackbody images acquired during calibration tests at Institute of Space and Astronautical Science (ISAS) of JAXA, in a vacuum environment before launch (Fukuhara et al. 2011). The blackbody used in the experiment was thermally controlled 5 K steps from 210 to 250 K, which corresponds to the expected temperature range at the cloud tops of Venus (e.g., Taylor 1980). The original brightness contrast depends on the shutter temperature, which is not thermally controlled. Therefore, the heater attached to the baseplate of LIR controlled the temperature of the camera to balance the shutter temperature at 297, 300, and 303 K when the blackbody was set at each temperature. An offset of the brightness contrast caused by variation of the shutter temperature can be eliminated from each pixel by
$$I_{\text{s}} = I_{\text{o}} + C_{\text{s}} \left( {T_{\text{s}} - T_{0} } \right) ,$$
(1)
where I
s and I
o are the brightness values after and before the correction, respectively; C
s is the coefficient experimentally derived in Taguchi et al. (2012); and T
s and T
0 are the shutter temperature measured at the time of observation and a standard temperature properly defined for the correction based on the calibration experiments, respectively.
Since integration of the spectral radiance I from 8 μm (λ
1) to 12 μm (λ
2) is a linear function of I
s in Eq. (1), it can be represented by
$$B = \int_{{\lambda_{1} }}^{{\lambda_{2} }} {I{\text{d}}\lambda = G \times I_{\text{s}} + C_{\text{o}} } ,$$
(2)
where G and C
o are the gain and offset, respectively, derived from blackbody images obtained in the laboratory. When emissivity of the thermal infrared radiation from Venus is assumed to be unity, brightness temperature T for each pixel in an image is derived from the Planck’s law. On the other hand, T can also be approximated by a power series of B as follows in case the target temperature is from 210 to 250 K:
$$T = \mathop \sum \limits_{i = 0}^{n} a_{i} B^{i} ,$$
(3)
where the coefficients a
i
are estimated from the blackbody images and n = 7. In this paper, formula (3) is used for converting the thermal contrast to the brightness temperature.