With the increase of flight speed, infrared (IR) seeker may be seriously affected by high temperature shock-layer in front of dome. Shock-layer radiance is produced by molecular processes and has implications for sensor waveband selection. This paper examines the impacts of high temperature shock-layer IR radiance from a molecular radiometry point of view. We derived the mathematical expressions of rotational quantum number of maximum intensity and the corresponding wavenumber of P and R branch. The theoretical calculation and the simulation based on HITRAN database are conducted. Results show that with the increasing of temperature, the rotational quantum number of maximum intensity will be larger. Ultimately, the wavenumber of P and R branch moves towards long wavelength and short wavelength direction respectively. The conclusions subserve to select sensor waveband.
In order to study the optical transmission and radiation characteristics in a hypersonic shock layers over infrared optical window, an approximation calculation approach is proposed. With this method, the formal solution for shock layer radiative transport equation is derived considering both target signature and windows thermal radiance. Consequently, the transmissivity of heating shock layer can be expressed quantitatively, which shows that the shock layer is optically thick in the 4~4.5μm band mainly. Besides, the radiance flux density of shock layer is calculated when temperature is 800K, pressure is 7atm and thickness is 1cm. Shock layer radiance is found to be larger in the mid-wave infrared band compared with long-wave infrared band and short-wave infrared band.
Polarization imaging focuses on degrees of linear polarization (DoLP) and angles of polarization (AoP). This paper introduces a polarization imaging set which employs a LWIR polarization imaging system along with a set of cooperative self-made target panels to acquire polarized radiation. The sampling flowchart is given and the process of polarization acquisition demonstrated. The experiment shows that larger the observation angle, stronger DoLP signal and weaker radiance lowers the contrast of polarization image.
Equivalent extinction ratio and polarization orientation are two significant parameters representing the performance of a polarized pixel in an integrated micropolarizer array camera. With manufacturing and integrating errors of the micropolarizer array, equivalent extinction ratios are nonuniform and polarization orientations of polarized pixels deviate from their nominal values. Measuring the equivalent extinction ratio and the polarization orientation of each polarized pixel by rotating a polarizer at a tiny step is extremely time-consuming and even inaccurate. Therefore, this paper proposes a calibration method for the equivalent extinction ratio and the polarization orientation of each polarized pixel. Its principle is derived by theorizing the relationship between an orientation of a linearly polarized incident light and its digital output of a polarized pixel. In experiment, this derived principle is applied to an integrated micropolarizer array camera. Experimental result proves that calibrated equivalent extinction ratios generally vary from 4.5 to 10, with a mean of 7.939 and a standard variation of 1.053.
Integrating microgrid polarimeters on focal plane array (FPA) of an infrared detector causes non-uniformity of polarization response. In order to reduce the effect of polarization non-uniformity, this paper constructs an experimental setup for capturing raw flat-field images and proposes a procedure for acquiring non-uniform calibration (NUC) matrix and calibrating raw polarization images. The proposed procedure takes the incident radiation as a polarization vector and offers a calibration matrix for each pixel. Both our matrix calibration and two-point calibration are applied to our mid-wavelength infrared (MWIR) polarization imaging system with integrated microgrid polarimeters. Compared with two point calibration, our matrix calibration reduces non-uniformity by 30~40% under condition of flat-field data test with polarization. The ourdoor scene observation experiment indicates that our calibration can effectively reduce polarization non-uniformity and improve the image quality of our MWIR polarization imaging system.
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