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Ring illumination around 4 cameras

Grey-level distribution of 1 section

Ring illumination around 1 camera

Grey-level distribution of 1 camera

23 11 4 -1 -3 -3 0

21 10 3 -2 -5 -5 -2

21 10 3 -1 -2 -2 0

26 15 8 4 2 3 6

34 24 16 13 11 12 15

45 31 21 19 21 34 47

38 22 9 6 9 22 40

36 18 4 2 5 19 37

32 16 2 -3 3 16 33

44 28 16 14 18 44 30

Figure 24. Problem of inhomogeneous grey-level by ring-illumination types

Fig. 7 illustrates two types of camera-illumination systems. The first system contains 4 cameras,

the second only 1 camera. Both have LED-ring illumination modules. The grey-level distribu‐

tion maps shown above have been measured with the same type of illumination and grey-

reference flat. The green areas indicate the valuable field of the camera. This example clearly

points out the importance of the homogeneity. Of course this phenomenon is also present when

the illumination system is multi-coloured.

Most optical inspection / control appliance decisions are based on image-processing methods

that have been set experientially. The stress is on the word “experientially”. Most of the AOI

machines make some kind of template matching. These sample-templates can be colour or

greyscale, stand from parts/windows or form a complete pattern. The machine can be ‘self-

learning or directed by means of an “external trainer”. Due to the fact that the overall reliability

of these machines is not 100%, the defined limits between good and bad classified patterns are

not strict. In some cases it could be that just two pixels differ between the data provided. If the

phenomenon which the system needs to detect is not so unambiguous, then it should search

for another method to make the gap wider between the 2 classes.

9. Software questions

One of the most wide-spread criticisms against the principles and methods of automated

optical inspection systems stems from a very interesting paradox. As we have mentioned

earlier, the introduction of AOI devices in the manufacturing lines was a result of the growth

in manufacturing process complexity. These inspection and control devices have to fulfill

certain reliability criteria which need to be validated. But unfortunately, these validation

Materials Science - Advanced Topics

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D-MAX DMC-20SEC Manual de usuario Pagina 32