Vision System Cameras

More about vision system cameras

Types of sensors in vision system cameras

Vision sensors within vision system cameras use industrial image processing to analyse images captured and thus make decisions on process control based on the interpretation of the data. Typically, these application areas cover presence checking, optical character recognition, optical character verification, print inspection, measurement, surface checking, 2D and 3D code reading, robot guidance and location feedback. Following processing the vision system camera communicates with the production line to give results and statistical data.

Solid-state sensors are the electronic eyes of today’s cameras. The intensity of the incident light is converted into a charge or a digital value that is subsequently processed depending on the application. Basically, image sensors are categorized according to:

• The geometrical arrangement of the light-sensitive elements (pixels) in line or matrix sensors.
• The mode of operations and manufacturing technology in Charge Coupled Device (CCD) and Complementary Metal Oxide Semiconductor (CMOS) sensors.

Matrix sensors: Matrix sensors like common digital cameras for the consumer market, acquire an image consisting of several lines and columns during a single exposure time. This is the most commonly used type in machine vision by far in a vision system camera.

Line sensors: Line sensors acquire only one line at a time. In many line-scan camera applications, lines captured in sequence are assembled into a rectangular image.

CCD sensors: CCD sensors are continuing to decline in use for industrial image processing; the first specimens were developed in the early 1970s from analog bucket-brigade devices and quickly replaced the camera tubes. When they started conquering the consumer market in camcorders, the CCD sensors had reached an attractive price level accelerating their use in machine vision.

CMOS sensors: These are now the dominant sensors used in vision system cameras today. Research into CMOS sensors got impetus in the early 1990s. In 1993 the Jet Propulsion Laboratory (Pasadena, CA, U.S.A.) provided the US space agency NASA with a sensor having a resolution of 128 x 128 pixels being followed by the first one-megapixel sensor only three years later. Unlike the manufacture of CCD sensors, the manufacture of CMOS chips and sensors is based on wide-spread standard technology. Therefore, the production of CMOS sensors is in principle more cost-efficient. More important for the sensor properties are the different ways in which image information is read out within the vision system camera.

Basically all solid-state sensors operate according to the same principle:

1. The incident light in the form of a stream of photons generates free electrons in the light-sensitive part of the solid-state crystal.
2. The generated electrons are collected over a specific period of time, the exposure or integration time.
3. After the exposure time, the acquired charge is transferred pixel by pixel to the converter stage in the CCD sensor.
4. Finally, the acquired charge is converted into a proportional voltage that can be converted into a numeric value by an analog–digital converter. CCD and CMOS sensors use photodiodes to convert incident photons into charge and collect them in the potential well. There the charge is stored during exposure time. The storage capacity depends primarily on the geometric pixel size and the build-up of the solid-state structures. It is denoted as full well capacity of the pixel in electrons (e ). The rule of thumb for all sensors is important: the larger the pixel, the larger the full well capacity and thus the attainable range of dynamics and contrast.

A fundamental difference between CCD and CMOS sensors is in the read-out of the charges of the individual pixels. This has a significant effect on the sensor properties within the vision system camera.