Selection of the appropriate camera
The image generation is a complex interaction of the material and surface characteristics of the test object and the lighting used. The central role, however, plays the camera system as "detector unit". In this step, an image is captured on the sensor, processed and digitalised.
The required accuracy and speed of the inspection as well as the requirements of the application for the image sensor determine the camera selection.
Typical types of industrial cameras
- Area scan cameras: by means of the matrix-shaped sensor a two-dimensional image is captured at the same time which is then sent to a PC and evaluated there.
- Line scan cameras: one single linear sensor line captures a great number of individual lines in very quick succession. By means of the forward speed of conveyor or camera unit, a two-dimensional image is created and sent to a PC. This process is ideal to capture continuous materials, etc.
- Intelligent cameras are mainly area scan cameras, very rarely also line scan cameras. The image acquisition, but also the complete evaluation of the image takes place within the camera. Results and measured data are communicated directly to a PLC, robot control, etc. via different interfaces and protocols. A PC is only required to set up the system.
Selection of the appropriate camera
The camera selection is determined by the inspection task. The features to be detected must be depicted in such a way that the software can evaluate them in a reasonable manner. But what does that mean?
- Area scan camera or line scan camera:do the parts arrive individually or is the material transported? In the simplest case images are captured using an area scan camera which works in continuous or triggered mode. A line scan camera allows for the seamless scanning of objects which are transported past the camera. Long components, continuous reel material or cylindrical objects which can be rotated are suitable for the image acquisition using a line scan camera.
- Resolution and sensor size : which finest features and object structures on the component must be detected with which attention to detail? Particularly in case of large components, high camera resolution is required. How large must the sensor be and which optics is to be used?
- Speed: how many images per second or how many lines per second must the camera capture? Especially fast cameras require fast transmission interfaces. The sensors are often produced as CMOS which can directly read out each pixel and provide a higher frame rate than CCD sensors.
- Image quality: how good must the camera image be? CCD cameras tend to produce a more homogenous image, they are more light-sensitive but tend to smearing and blooming in case of strong overexposure. CMOS cameras usually require more light and must be well calibrated (dark image, bright image), as they produce a more inhomogeneous image by nature. However, they are more tolerant towards extreme lighting conditions and do not display any smearing and blooming.
- Monochrome or colour sensor : can the application be solved with a monochrome or colour camera? Monochrome cameras are clearly more light-sensitive and the transmitted image data are typically only coded with 8 bits. Colour images with 24-bit signal information allow for additional image evaluations in order to separate different colours of the same brightness (= shade of grey), but the volume of data is much larger. In case of 1-chip colour cameras detailed information is lost due to the Bayer colour interpolation, the measuring accuracy is reduced.
- Transmission interface: if you multiply bit depth * frame rate * image size, the required bandwidth per second can be calculated. The interface must be capable of transporting the image to the evaluation unit (= PC) in a quick and reliable way. It is also important that the software used is capable of supporting the transmission interface with the connected camera.
- Trigger functions , camera electronics and processing: how should the image be captured, with how many bits of signal depth (e.g. 8, 10, or 12 bits monochrome, etc.), should the image be calibrated already in the camera, etc.?
- Mechanic dimensions and form factor: does the camera housing fit into my system? There are many different cameras on the market so that the user has a wide choice.
- Completeness of the product portfolio: a good supplier is supposed to have a complete range of hardware so that they can cover many applications with one camera family.
- Ability to supply, reliability, support: look for a good, reliable partner. This will help you to quickly integrate the camera and even in case of problems you are not left out in the cold.
Only the right camera as "detector system" allows for the acquisition of the right inspection characteristics of the application with the required accuracy. It is especially worthwhile to make a good choice!