Unlike high-volume consumer and automotive applications that demand consumer-level pricing, machine vision applications in manufacturing are not nearly as large. However, these applications require performance and precision and take advantage of the developments and advances in image sensing for the larger markets.
Two technologies provide the vision sensing capability for image capturing: a charge-coupled device (CCD) and CMOS-based imaging systems.
Using semiconductor processing and design methodology developed specifically for imaging applications, CCD technology has historically been the choice for high-performance applications. In a CCD sensor, light received at a photoactive region is stored as an electrical charge and then converted to voltage, buffered and output as an analog signal using a limited number of output nodes. However, the process used to make a CCD image sensors does not have logic in it. As a result, a CCD sensor requires additional ICs to drive it and to receive and format the signal for system usage.
Additional criteria for the design decision is shown in Table 1. Some of these performance factors provide an edge to one technology over the other. However, ongoing improvements have made the differences small in some areas.
In contrast, in a CMOS sensor, each pixel has a charge-to-voltage conversion. The CMOS sensor delivers a digital output thanks to integrated amplifiers, noise-correction, and digitization circuitry. In addition to taking advantage of increasing CMOS technology integration, the CMOS imaging sensor also consumes less energy and generates less heat. With their digital output, CMOS sensors can be controlled at the pixel level in more ways than CCDs. However, noise performance and sensitivity are lower for CMOS sensors than CCD sensors.