Illumination Considerations Of Vision Sensor

Tuesday, May 1st, 2018 - Photoconductors, Transducer/Sensor

Illumination Considerations Of Vision Sensor

Object illumination is an important element of the visual data acquisition. Its purpose is to provide adequate distinction between the background and the target object (e.g. a car component on a conveyer belt) and help to bring out the relevant features of the object.

In the case of passive vision systems, such as those using cameras, object illumination can be very critical and ordinary ambient lighting, whether artificial or natural, is quite unsuitable since it may be either too low or produce misleading shadows and/or highlights.

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Figure 1. Different modes of illumination (courtesy of PERA)

Active vision sensors, such as direct 3-D rangefinders, produce their own object ‘illumination’, whether optical or ultrasonic, and are therefore not dependent on specially designed, and sometimes critically adjusted, artificial lighting systems.

In general, however, for passive vision sensors, two main techniques of illumination are employed:

  • reflected illumination
  • through illumination

Both these techniques present advantages and disadvantages and are used according to specific requirements in each case.

Reflected illumination yields details of the surface features of the object. However, to extract the required information from the picture, careful attention to lighting or powerful data processing software must be used to cope with the multigrey level image. Reflective illumination also includes diffused lighting to avoid shadow formation, and stroboscopic lighting to ‘freeze’ moving objects.

Through illumination allows easy distinction of the dark object on a light background (and vice versa). However, only the contour and ‘through­ going’ features of the object provide data for the subsequent processing and analysis. Furthermore through illumination produces only binary images (i.e. a pattern of black and white pixels) while in the case of reflected illumination the image can be digitized into any required number of grey levels thus yielding more information.

Comparison of two main illumination techniques

Table 1. Comparison of two main illumination techniques (Courtesy of PERA)

A comparison of the two illumination techniques is summarized in Table 1 while Figure 1 provides a pictorial description of the different modes of illumination (from PERA Report No. 366).

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