• Current Optics and Photonics
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• v.8 no.1
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• pp.97-104
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• 2024

In aerial cameras, image quality is easily affected by weather, temperature, and the attitude of the aircraft. Aiming at this phenomenon, based on the theory of two-step zoom optical systems, a dual-band optical-despun two-step zoom optical system is designed. The system has a small field of view of 2.00° × 1.60°, and a large field of view of 4.00° × 3.20°. In the zoom process, the wavelength range is 0.45-0.70 ㎛ and 0.75-1.10 ㎛, and the size of the optical system is 168 mm (L) × 90 mm (W) × 60 mm (H). The overall lens weight is only 170.8 g, which has advantages for miniaturization and light weight. At the Nyquist frequency of 104 lp/mm, the modulation transfer function of the visible-light optical system is more than 0.44, and that of the near-infrared optical system is more than 0.30, both of which have good imaging quality and tolerance characteristics in the range of -45 to 60 ℃.

• Current Optics and Photonics
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• v.2 no.3
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• pp.241-249
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• 2018

We report a midinfrared dual-field-of-view (FOV) optical system design for an airborne electro-optical targeting system. To achieve miniaturization and weight reduction of the system, it has a common aperture and fore-optics for three different spectral wavelength bands: an electro-optic (EO) band ($0.6{\sim}0.9{\mu}m$), a midinfrared (IR) band ($3.6{\sim}4.9{\mu}m$), and a designation laser wavelength ($1.064{\mu}m$). It is free to steer the line of sight by rotating the pitch and roll axes. Our design co-aligns the roll axis, and the line of sight therefore has a fixed entrance pupil position for all optical paths, unlike previously reported dual-FOV designs, which dispenses with image coregistration that is otherwise required. The fore-optics is essentially an achromatized, collimated beam reducer for all bands. Following the fore-optics, the bands are split into the dual-FOV IR path and the EO/laser path by a beam splitter. The subsequent dual-FOV IR path design consists of a zoom lens group and a relay lens group. The IR path with the fore-optics provides two stepwise FOVs ($1.50^{\circ}{\times}1.20^{\circ}$ to $5.40^{\circ}{\times}4.32^{\circ}$), due to the insertion of two Si lenses into the zoom lens group. The IR optical system is designed in such a way that the location and f-number (f/5.3) of the cold stop internally provided by the IR detector are maintained when changing the zoom. The design also satisfies several important performance requirements, including an on-axis modulation transfer function (MTF) that exceeds 10% at the Nyquist frequency of the IR detector pitch, with distortion of less than 2%.

• Korean Journal of Optics and Photonics
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• v.33 no.4
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• pp.146-158
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• 2022

The detector of a focal-plane-array mid-wave infrared (MWIR) camera has different response characteristics for each detector pixel, resulting in nonuniformity between detector pixels. In addition, image nonuniformity occurs due to heat generation inside the camera during operation. To solve this problem, in the process of camera manufacturing it is common to use a gain-and-offset table generated from a blackbody to correct the difference between detector pixels. One method of correcting nonuniformity due to internal heat generation during the operation of the camera generates a new offset value based on input frame images. This paper proposes a technique for dividing an input image into block image patches and generating offset values using only homogeneous patches, to correct the nonuniformity that occurs during camera operation. The proposed technique may not only generate a nonuniformity-correction offset that can prevent motion marks due to camera-gaze movement of the acquired image, but may also improve nonuniformity-correction performance with a small number of input images. Experimental results show that distortion such as flow marks does not occur, and good correction performance can be confirmed even with half the number of input images or fewer, compared to the traditional method.

• KSII Transactions on Internet and Information Systems (TIIS)
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• v.14 no.8
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• pp.3295-3311
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• 2020

With the wide application of hyperspectral images, it becomes more and more important to compress hyperspectral images. Conventional recursive least squares (CRLS) algorithm has great potentiality in lossless compression for hyperspectral images. The prediction accuracy of CRLS is closely related to the correlations between the reference bands and the current band, and the similarity between pixels in prediction context. According to this characteristic, we present an improved CRLS with adaptive band selection and adaptive predictor selection (CRLS-ABS-APS). Firstly, a spectral vector correlation coefficient-based k-means clustering algorithm is employed to generate clustering map. Afterwards, an adaptive band selection strategy based on inter-spectral correlation coefficient is adopted to select the reference bands for each band. Then, an adaptive predictor selection strategy based on clustering map is adopted to select the optimal CRLS predictor for each pixel. In addition, a double snake scan mode is used to further improve the similarity of prediction context, and a recursive average estimation method is used to accelerate the local average calculation. Finally, the prediction residuals are entropy encoded by arithmetic encoder. Experiments on the Airborne Visible Infrared Imaging Spectrometer (AVIRIS) 2006 data set show that the CRLS-ABS-APS achieves average bit rates of 3.28 bpp, 5.55 bpp and 2.39 bpp on the three subsets, respectively. The results indicate that the CRLS-ABS-APS effectively improves the compression effect with lower computation complexity, and outperforms to the current state-of-the-art methods.

• Korean Journal of Optics and Photonics
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• v.29 no.3
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• pp.110-118
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• 2018

We have designed a mid-infrared optical system for an airborne electro-optical targeting system. The mid-IR optical system is a dual-field-of-view (FOV) optics for an airborne electro-optical targeting system. The optics consists of a beam-reducer, a zoom lens group, a relay lens group, a cold stop conjugation optics, and an IR detector. The IR detector is an f/5.3 cooled detector with a resolution of $1280{\times}1024$ square pixels, with a pixel size of $15{\times}15{\mu}m$. The optics provides two stepwise FOVs ($1.50^{\circ}{\times}1.20^{\circ}$ and $5.40^{\circ}{\times}4.23^{\circ}$) by the insertion of two lenses into the zoom lens group. The IR optical system was designed in such a way that the working f-number (f/5.3) of the cold stop internally provided by the IR detector is maintained over the entire FOV when changing the zoom. We performed two analyses to investigate thermal effects on the image quality: athermalization analysis and Narcissus analysis. Athermalization analysis investigated the image focus shift and residual high-order wavefront aberrations as the working temperature changes from $-55^{\circ}C$ to $50^{\circ}C$. We first identified the best compensator for the thermal focus drift, using the Zernike polynomial decomposition method. With the selected compensator, the optics was shown to maintain the on-axis MTF at the Nyquist frequency of the detector over 10%, throughout the temperature range. Narcissus analysis investigated the existence of the thermal ghost images of the cold detector formed by the optics itself, which is quantified by the Narcissus Induced Temperature Difference (NITD). The reported design was shown to have an NITD of less than $1.5^{\circ}C$.