The Basic Structure of Machine Vision Applications - Part 2

Please Note: This blog post is part of a series of four posts altogether. The posts include: Intro, Part 1, Part 2 and Part 3.

Camera

As already discussed in the overview, the camera does not create the image but only transforms optical signals (light) into electrical ones (voltage) and digitizes them (raw digital image). Thus, a camera is not able to compensate for incorrectly chosen illumination and/or optics. On the other hand, it is important to select an appropriate camera to avoid a malfunction of the system and/or unnecessary costs. Please find an overview of the most important decision criterion below:

Monochrome / color Rule of thumb: color cameras are only used, if the different colors of an image "carry" information. For all other applications, it is recommended to use monochrome cameras since color cameras bring along some disadvantages:

• They are less sensitive in comparison to monochrome cameras.
• Assuming the same number of pixels, the effective resolution of a color CCD is lower than that of a monochrome CCD. Every second pixel of a color CCD is sensitive to green, while blue and red share the remaining pixels.
• Since we expect a green, blue and red value for every pixel the raw digital image of a color CCD has to undergo a color interpolation. This interpolation requires extra processing power and bandwidth during the data transfer.

We will describe in detail how color cameras work in one of the next articles.

IR cut filter In contrast to the human eye, CCDs are also sensitive to the near infrared (IR). To approximate the human eye, cameras are equipped with IR cut filters. However, cameras without IR cut filters offer more flexibility, since the user is able to adapt them to his or her requirements using custom filters.

Therefore, usually the manufacturers of modern machine vision cameras do not equip their monochrome cameras with IR cut filters and also offer a variation of color cameras without IR cut filters.

Format The format describes the CCD's size. It is an important parameter when deciding upon which lens to choose. In one of the next articles we will offer some general advice on how to select and setup lenses.

Resolution To avoid unnecessarily high costs, as well as a large amount of data, the resolution should be as small as possible. For two typical application areas of machine vision there are the following rules of thumb: the measurement of a distance requires at least 10 pixels, while checking the presence of an object requires at least 2 or 3 pixels. Please note that these rules of thumb serve only as a basic indicator.

Frame rate (fps) For visualization purposes, as well as for the setup of illumination, optics and a frame rate of 15 fps (frames per second) are usually sufficient. Automatic image analysis often requires a much lower frame rate.

I/O and trigger Digital I/Os (Input/Output) and appropriate software allow the camera to control external devices and to respond to signals from external devices. The "Trigger" is a special input, which is comparable to the shutter button of a photo camera. A pulse appearing at this input starts the exposure of an image. After the output of this image the camera waits for a new trigger pulse.

Interface The machine vision pioneers had to make their first steps with pick-up tube cameras. They were based on the analog video standards NTSC and PAL. One of the principal issues was the digitalization of the analog video signals.

Meanwhile, pick-up tubes have almost been completely replaced by CCD chips. These chips already provide a digital signal. Additionally, today we have PCs with fast and easy to handle digital connectors, such as FireWire. Therefore, new projects in the domains of industry, medicine and science are mainly based on FireWire cameras.

To be continued...