INTRODUCTION

Imaging systems have numerous applications in industrial, military, consumer, and medical settings. Assembling a complete imaging system requires the integration of optics, sensing, image processing, and display rendering. This feature issue is aimed at scientists, engineers, and practitioners interested in understanding how different materials and components combine with image processing to determine and influence image system performance. The design of optical systems must factor the system as an integrated unit and optimize the performance for a given application. There are numerous disciplines that are needed for the design and advancement of an optical system. These disciplines include imaging optics, optical detection, computational, adaptive, and compressive imaging, displays, and usability of information, which all contribute to defining the system. Scientists and engineers from commercial, academic, and military disciplines came together to share advances in imaging systems at the virtual Optica (formerly OSA) Imaging Systems and Applications Topical Meeting in 2021. Following on the spirit of this meeting the chair of this topical meeting joined forces to work on a special issue at Applied Optics.

This issue contains contributions from the wider imaging systems’ community covering a broad spectrum of investigations in imaging systems and applications. In this collection of papers you will find a wide range of research from microscopic to radar imaging scale with varied applications such as pulsed thermographic imaging to visualize underdrawing visualization in paintings and spectral classification of images using a single pixel camera. There is an improved algorithm for millimeter-wave near-field one-side stationary bistatic synthetic aperture radar, a cost-effective device capable of quantitative measurement of the embryo and endosperm areas of brown rice, and resolution improvement of a microsphere-assisted microscope with arrays of plasmonic structures.

Melada et al. in “Pulsed thermographic and infrared reflectography: comparative results for underdrawing visualization in paintings” describe their method of using reflectographic analyses applied on paintings [1]. They use cameras equipped with different detectors with varying abilities in detecting and visualizing underdrawings, re-paintings, restorations and other non-visible information. This is followed by statistical imaging post-processing and resulting images that are compared with results obtained with traditional reflectographic methods in the near-infrared and short wave infrared ranges by studding the thermal sequence after a single pulse of light with different spectrum of ad hoc mockups. The results showed that signal-to-noise ratio seems to be more relevant in obtaining reliable images of underdrawings respect to the effect of optical absorption of visible light by patenting layers.

Hinojosa et al. in “C-3SPCD: Coded aperture similarity constrained design for spatio-spectral classification of single-pixel measurements” demonstrate a faster method to spectrally classify images using a single-pixel camera (SPC) [2]. The key innovation is preforming the spectral classification without reconstructing an image of the scene. The image reconstruction is avoided by collecting a single RGB image of the scene. The collected RGB image allows the authors to use similarity information between pixels to efficiently design SPC coding patterns. The proposed method was demonstrated using a visible through near-IR spectrometer as the single pixel camera. Both simulation and experiments obtained high classification accuracy with up to 50 spectral bands. This work may impact remote sensing, natural resource exploration, manufacturing quality control, food assessment, as well as biomedical imaging.

Chen et al. describe an improved algorithm for millimeter-wave near-field one-side stationary bistatic synthetic aperture radar in “Non-interpolated frequency-domain imaging algorithm for near-field OS-BiSAR in the millimeter-wave band” [3]. The proposed algorithm only requires multiplication and fast Fourier transform operations increasing the image reconstruction speed compared to previous approaches. In addition, the proposed algorithm has better imaging quality at longer ranges because propagation attenuation is taken into consideration. This work on mmW imaging may lead to improved detection capabilities at security checkpoints.

“Mobile-device-based two-dimensional measurement for estimating the embryo and endosperm areas of brown rice,” by Chaitavon et al., proposes a simple and cost-effective device capable of quantitative measurement of the embryo and endosperm areas of brown rice. The device is built on a smartphone equipped with a lens module specifically designed for cross-polarization imaging [4]. Image acquisition with a white-light illumination and subsequent image analysis result in the sizes of embryo and endosperm areas. The authors evaluate the validity of the method with 1400 rice samples from 14 varieties of Thai brown rice, and present promising results with a measurement error $ \lt 9\%$, compared with the off-of-sheff image software. The proposed method may find a great utility for the non-destructive evaluation of a rice grain in precision agriculture and on-site food quality assessment.

In their article “Super-resolution imaging of plasmic nanostrucutres by microsphere-assisted microscopy, ” Cao et al. report on numerical and experimental studies about the resolution improvement of a microsphere-assisted microscope with arrays of plasmonic structures [5]. Triangularly and circularly shaped Au, Ag, and Cu nanoparticle arrays are fabricated and imaged by a ${\rm BaTiO}_3$ glass (BTG) microsphere-assisted microscope. The authors show that when an immersed BTG microsphere is dispersed on the surface of a plasmonic nanostructure, an enhanced electric field is generated in the vicinity of the nanostructures, especially at the gap of the microsphere and the nanostructure, due to the focusing effect of the microsphere and the excitation of localized surface plasmon resonance in the plasmonic nanostructure. The microsphere then collects, amplifies, and propagates the enhanced near-field information to the far field, resulting in the improvement of resolution and contrast. The authors find that the field enhancement in Ag nanostructures is significantly stronger than those in Au and Cr; for an array of circular Ag nanoparticles, a gap resolution of $\sim \lambda / 7.7$ can be achieved.