6.1 Digital Sensors in the Imaging Chain
The sensor of a camera "senses" the light shaped by the optics to create a record of the image. We will focus on modeling digital sensors used in the imaging chain (Fig. 6.1) to record panchromatic visible light. The three main impacts to image quality that we will look at are signal integrity (the signal and noise), blurring (the sensor transfer function), and sampling.
6.2 Focal Plane Arrays
The most common sensor used for imaging visible light in a digital camera is a focal plane array (FPA), which is composed of solid state detectors that convert the incident light into electrons through the photoelectric effect. The optics focus the light onto the array of detectors, and the photons incident on each detector are converted into electrons that are then accumulated in the detector well during the exposure time (Fig. 6.2). Dark areas in the scene will generate fewer electrons than bright areas in the scene, so the number of electrons accumulated in the well will be proportional to the local scene brightness. The number of electrons stored in each well is then converted to a digital count value that is displayed as a gray level for each pixel in the digital image (Fig. 6.3).
Figure 6.4 shows the basic layout for a charge-coupled device (CCD) sensor that collects, transfers, and then converts the charge to a voltage, which is then converted to digital counts using the analog-to-digital (A/D) converter.1 Another common imaging sensor is the complementary metal-oxide semiconductor (CMOS) that also converts light into electronic signals using an array of detectors, but with the electron-to-voltage conversion occurring at each detector.