Components of an Image Processing System

This section briefl y outlines the capabilities of modern image processing systems. A general purpose image acquisition and processing system typically consists of four essential components:

1. An image acquisition system. In the simplest case, this could be a CCDcamera, a fl atbed scanner, or a video recorder.

2. A device known as a frame grabber to convert the electrical signal (normally an analog video signal) of the image acquisition system into a digital image that can be stored.

3. A personal computer or a workstation that provides the processing power.

4. Image processing software that provides the tools to manipulate and analyze the images.

 

Image Sensors

Digital processing requires images to be obtained in the form of electrical signals. These signals can be digitized into sequences of numbers which then can be processed by a computer. There are many ways to convert images into digital numbers. Here, we will focus on video technology, as it is the most common and aff ordable approach.

The milestone in image sensing technology was the invention of semi- conductor photodetector arrays. There are many types of such sensors, the most common being the charge coupled device or CCD. Such a sensor consists of a large number of photosensitive elements. During the accu- mulation phase, each element collects electrical charges, which are gen- erated by absorbed photons. Thus the collected charge is proportional

22                                                                                      1 Applications and Tools

 

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Figure 1.17: Modern semiconductor cameras: a Complete CMOS camera on a chip with digital and analog output (image courtesy, K. Meier, Kirchhoff -Institute for Physics, University of Heidelberg), [114]). b High-end digital 12-bit CCD cam- era, Pixelfl y (image courtesy of PCO GmbH, Germany).

 

to the illumination. In the read-out phase, these charges are sequentially transported across the chip from sensor to sensor and fi nally converted to an electric voltage.

For quite some time, CMOS image sensors have been available. But only recently have these devices attracted signifi cant attention because the image quality, especially the uniformity of the sensitivities of the individual sensor elements, now approaches the quality of CCD image sensors. CMOS imagers still do not reach up to the standards of CCD imagers in some features, especially at low illumination levels (higher dark current). They have, however, a number of signifi cant advantages over CCDimagers. They consume signifi cantly less power, subareas can be accessed quickly, and can be added to circuits for image preprocess- ing and signal conversion. Indeed, it is possible to put a whole camera on a single chip (Fig. 1.17a). Last but not least, CMOS sensors can be manufactured more cheaply and thus open new application areas.

Generally, semiconductor imaging sensors are versatile and powerful devices:

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Precise and stable geometry. The individual sensor elements are pre- cisely located on a regular grid. Geometric distortion is virtually ab- sent. Moreover, the sensor is thermally stable in size due to the low linear thermal expansion coeffi cient of silicon (2 10− 6/K). These fea- tures allow precise size and position measurements.

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Small and rugged. The sensors are small and insensitive to external infl uences such as magnetic fi elds and vibrations.

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High sensitivity. The quantum effi ciency, i. e., the fraction of elemen- tary charges generated per photon, can be close to one ( R1 and R2). However, commercial CCDs at room temperature cannot be used at low light levels because of the thermally generated electrons.

1.7 Components of an Image Processing System                            23

 

But if CCDdevices are cooled down to low temperatures, they can be exposed for hours. Such devices are commonly used in astronomy and are about one hundred times more sensitive than photographic material.

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Wide variety. Imaging sensors are available in a wide variety of reso- lutions and frame rates ( R1 and R2). The largest built CCD sen- sor as of 2001 originates from Philips. In a modular design with 1k 1k sensor blocks, they built a 7k 9k sensor with 12 12 µm pixels [60]. Among the fastest high-resolution imagers available is the 1280 1024 active-pixel CMOS sensor from Photobit with a peak frame rate of 500 Hz (660 MB/s data rate) [137].

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Imaging beyond the visible. Semiconductor imagers are not limited to the visible range of the electromagnetic spectrum. Standard sili- con imagers can be made sensitive far beyond the visible wavelength range (400–700 nm) from 200 nm in the ultraviolet to 1100 nm in the near infrared. In the infrared range beyond 1100 nm, other semicon- ductors such an GaAs, InSb, HgCdTe are used ( R3) since silicon be- comes transparent. Towards shorter wavelengths, specially designed silicon imagers can be made sensitive well into the x-ray wavelength region.

 

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