# Polarimetric Thermal Database¶

Collected by USA Army, the Polarimetric Thermal Database contains basically VIS and Thermal face images.

Follow bellow the description of the imager used to capture this device.

The polarimetric LWIR imager used to collect this database was developed by Polaris Sensor Technologies. The imager is based on the division-of-time spinning achromatic retarder (SAR) design that uses a spinning phase-retarder mounted in series with a linear wire-grid polarizer. This system, also referred to as a polarimeter, has a spectral response range of 7.5-11.1, using a Stirling-cooled mercury telluride focal plane array with pixel array dimensions of 640×480. A Fourier modulation technique is applied to the pixel readout, followed by a series expansion and inversion to compute the Stokes images. Data were recorded at 60 frames per second (fps) for this database, using a wide FOV of 10.6°×7.9°. Prior to collecting data for each subject, a two-point non-uniformity correction (NUC) was performed using a Mikron blackbody at 20°C and 40°C, which covers the range of typical facial temperatures (30°C-35°C). Data was recorded on a laptop using custom vendor software.

An array of four Basler Scout series cameras was used to collect the corresponding visible spectrum imagery. Two of the cameras are monochrome (model # scA640-70gm), with pixel array dimensions of 659×494. The other two cameras are color (model # scA640-70gc), with pixel array dimensions of 658×494.

The dataset contains 60 subjects in total. For VIS images (considered only the 87 pixels interpupil distance) there are 4 samples per subject with neutral expression (called baseline condition B) and 12 samples per subject varying the facial expression (called expression E). Such variability was introduced by asking the subject to count orally. In total there are 960 images for this modality. For the thermal images there are 4 types of thermal imagery based on the Stokes parameters ($$S_0$$, $$S_1$$, $$S_2$$ and $$S_3$$) commonly used to represent the polarization state. The thermal imagery is the following - $$S_0$$: The conventional thermal image - $$S_1$$ - $$S_2$$ - DoLP: The degree-of-linear-polarization (DoLP) describes the portion of an electromagnetic wave that is linearly polarized, as defined $$\frac{sqrt(S_{1}^{2} + S_{2}^{2})}{S_0}$$.

Since $$S_3$$ is very small and usually taken to be zero, the authors of the database decided not to provide this part of the data. The same facial expression variability introduced in VIS is introduced for Thermal images. The distance between the subject and the camera is the last source of variability introduced in the thermal images. There are 3 ranges: R1 (2.5m), R2 (5m) and R3 (7.5m). In total there are 11,520 images for this modality and for each subject they are split as the following:

Imagery/Range

R1 (B/E)

R2 (B/E)

R3 (B/E)

$$S_0$$

16 (8/8)

16 (8/8)

16 (8/8)

$$S_1$$

16 (8/8)

16 (8/8)

16 (8/8)

$$S_2$$

16 (8/8)

16 (8/8)

16 (8/8)

DoLP

16 (8/8)

16 (8/8)

16 (8/8)