With advances in image sensors and their supporting interfaces, image-based measurement platforms will continue to offer higher performance and more programmable features. While the evolving technologies will simplify the physical connection and setting up of machine vision systems and open new application areas, the level of benefits will continue to depend on the collective ability of application engineers and system designers to quantify the characteristic parameters of their target applications. Although software can enhance a noisy, distorted, or defocused image, some of the measurement features embedded in the target scene may be lost in the process. Thus, a "good" source image, rather than a numerically enhanced image, is an essential building block of a successful machine vision application. Capturing an image is not difficult, but acquiring an image with the required characteristic features of the target requires insight into the imaging hardware.
This chapter starts with an outline description of video signals and their standards in the context of image display. This subject is followed by a description of the key components of framegrabbers and their performance. With the increasing use of images from moving targets for higher throughput and demands on measurement accuracy, latency and resolution have become important in the overall assessment of a machine vision system. This chapter concludes with some definitions and concepts associated with these topics and illustrative examples.
6.1 Image Display
As described in Chapter 2, the human visual system does not respond instantly for a given stimulus, nor does the sensation cease immediately when the stimulus is removed. Persistence of vision is a special feature of the eye that discriminates the intensity of a time-varying stimulus up to the CFF (see Sec. 2.7). Since the intensities of neighboring pixel brightness within an image are displayed as continuous streams of gray and dark patches on a 2D plane, the CFF is related to the brightness of the source as well as its 2D spatial variation. Early experimental work indicated that the human visual system has a CFF of around 50 cycles/sec. In traditional cinematic films, the actual projection rate is 24 picture frames/sec. To meet the above CFF, each frame is mechanically interrupted to present the same picture twice, thus giving an effective repetition rate of 48 picture frames/sec.
