RESUMEN
Interest in the eSWIR band is growing due to focal plane array technology advancements with mercury cadmium telluride and type-II superlattice materials. As design and fabrication processes improve, eSWIR detector size, weight, and power can now be optimized. For some applications, it is desirable to have a smaller detector size. Reduced solar illumination in the 2 to 2.5 µm spectral range creates a fundamental limit to passive imaging performance in the eSWIR band where the resolution benefit of small detectors cannot out-compete the reduced SNR in photon-starved environments. This research explores the underlying theory using signal-to-noise ratio radiometry and modeled target discrimination performance to assess the optimal detector size for eSWIR dependent upon illumination conditions. Finally, we model continuous-wave laser illumination in the eSWIR band to compare the effect of detector size on active and passive imaging for long-range object discrimination.
RESUMEN
The spectral information contained in the reflective imaging bands can be exploited for specific tasks. Whether targeting or mapping, the visible (VIS), near-infrared (NIR), shortwave infrared (SWIR), extended shortwave infrared (eSWIR) bands perform very differently for every application. For any imaging project, high contrast is very important for good imagery. High contrast leads to more recognizable features within a scene and easier identifiable objects. For mapping, good background scene contrast gives prominent features more detail and their locations can be easily identified. For targeting, low background scene contrast reduces clutter, making it easier to detect objects of interest. The VIS, NIR, SWIR, and eSWIR bands are popular reflective bands to design daytime imaging systems for either task. Deciding on which band will have the best contrast for a specific task is one of the first things to study when designing an imaging system. By measuring urban and rural scenes in terms of equivalent reflectivity (ER), a direct comparison of these four bands can show the utility they provide. The systems used to measure scene contrast are designed to have the same spatial resolution and field of view (FOV). With these instantaneous FOV (IFOV) matched systems, the variance and 1D power spectral densities (PSDs) provide a quantitative comparison for the contrast among the four bands. The ER differences and resulting contrast measured among these four bands show that the eSWIR has the highest contrast in both urban and rural scenes.
RESUMEN
Daytime low-light conditions such as overcast, dawn, and dusk pose a challenge for object discrimination in the reflective bands, where the majority of illumination comes from reflected solar light. In reduced-illumination conditions, the sensor signal-to-noise ratio can suffer, inhibiting range performance for detecting, recognizing, and identifying objects of interest. This performance reduction is more apparent in the longer wavelengths where there is less solar light. Range performance models show a strong dependence on cloud type and thickness, as well as time of day across the reflective wavebands. Through an experimental and theoretical analysis of a passive sensitivity- and resolution-matched testbed, we compare Vis (0.4-0.7 µm), NIR (0.7-1 µm), SWIR (1-1.7 µm), and eSWIR (2-2.5 µm) to assess the limiting cases in which reduced illumination inhibits range performance. The time during dawn and dusk is brief yet does show significant range performance reduction for SWIR and eSWIR. Under heavy cloud cover, eSWIR suffers the most at range due to a low signal-to-noise ratio. In cases of severe reduction in illumination, we propose utilizing active illumination or the emissive component of eSWIR to improve the signal-to-noise ratio for various discrimination tasks.