Optical Sensor Technology

Keywords

• optical sensors
• multispectral imagers
• hyperspectral imagers.

Introduction

Optical imaging sensors are a key technology in the field of remote sensing. Nearly all applications of remote sensing rely on optical imagery as a primary data source for analysis. The interpretation of visible images is as intuitive for remote sensing analysts as looking at a photograph. This comfort and reliance on optical imagery comes naturally as the very first remote sensing instrument was the human visual system. The human eye collects light in the visible optical spectrum and records the intensity with rods and cones on the retina. Optical remote sensing systems in use today operate with many similar components to the human visual system but provide dramatically richer information through higher resolution and additional phenomenology available by sensing in spectral regions beyond what the human eye can see.

The main function of electro-optical (EO) imaging sensors is to collect incident electromagnetic (EM) radiation and convert it to a stored representation useful for remote sensing analysis. These sensors operate in the optical region of the EM spectrum traditionally defined as radiation with wavelengths between 0.4 and 15 μm. This range spans the visible (VIS, 0.4–0.7 μm), the near infrared (NIR, 0.7–1.1 μm), the shortwave infrared (SWIR, 1.1–2.5 μm), the midwave infrared (MWIR, 2.5–7.5 μm), and the long wave infrared (LWIR, 7.5–15 μm) spectral regions. While it is possible to build a single sensor spanning this entire range, it is more common to find sensors (or at least sensor subsystems) limited to one or two of these regions.

Optical imaging sensors resolve and sample the incident EM field spatially, spectrally, radiometrically, and temporally producing an image of the radiance emanating from the scene. The resolution and accuracy with which these four domains are sampled determine the image characteristics and ultimately the quality and utility of the recorded image. While many applications still rely primarily on the visual interpretation of the imagery, modern remote sensing systems make precise radiometric measurements of the incident radiance and quantitative applications are on the rise.

This chapter provides a general introduction to optical imaging sensors. The sensor system elements which convert the scene radiance field to a digital image are first described. Metrics are then presented which quantify the performance of sensors in several domains. The chapter concludes with a discussion of remaining challenges and new directions in optical sensor technology.

Sensor Systems

All optical imaging sensors have in common the basic components illustrated in Figure 7.1. [Page 96]While other chapters in this handbook describe details for specific satellite or airborne sensors, this chapter describes these elements generically and provides references for the interested reader to pursue. The following subsections trace the flow of the incident electromagnetic waves as they are focused by the optical imaging subsystem, spatially sampled by the spatial scanning subsystem creating a two-dimensional image, spectrally filtered by the spectral selection devices, converted to an electrical signal by the detectors and then processed to a final image by the electronics and the calibration algorithms to an accurate representation of the scene with proper radiometric units and geometric orientations. Further details on sensor systems may be found in the texts by Wolfe and Zissis (1985), Wyatt (1987), Hobbs (2000), and Schott (2007).

...