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Accessory Range Vision Sensors and Systems
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| Why is the correct lighting so important for image processing? |
Why are there different light sources? |
Why are there different designs? |
To begin with, vision sensors require an image and are thus dependent on the light reflected by the object being inspected. If this image cannot be captured in a reproducible manner, the basic prerequisite for the reliable functioning of the entire solution is not given, even when using the latest algorithms and high processing power. What is lost during image acquisition can no longer be detected or compensated for during analysis.
Lighting and image acquisition must be set-up so as to ensure the capture of valid images. This is the essential condition for a robust operating application. |
Sensors with white light can be used for the widest range of applications as they contain the entire light spectrum and can achieve good contrast with many, different coloured objects.
Red light or infrared lighting on the other hand enables the highlighting or suppression of specific coloured object features. As light only reaches the receiver chip in a narrow transmitted spectrum via the band-pass filter, it is highly insensitive to extraneous light.
With infrared light (IR) there are no interfering flash effects at manual work stations, in contrast to visible light. |
Surface lights are often used for so-called transmitted light illumination, where the lighting is behind the object to be detected, seen from the sensor’s viewpoint. This provides a high-contrast black and white image, in which only the outer edges, e.g. the profile of the object, are visible, enabling e.g. their accurate measurement.
With surface or ring lights, so-called front illumination can be used, where the lighting is on the same side as the vision sensor, in relation to the object to be detected. Highly symmetrical illumination is achieved with ring lights, and diffuse lighting enables the illumination of very reflective and thus irregular surfaces ensuring the acquisition of valid images. |
The most important types of lighting: bright field, dark field, diffuse |
Bright field,
internal |
Bright field,
external |
Dark field,
internal |
Dark field,
external |
Diffuse, external |
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With bright field lighting, the lighting, sensor and object are arranged so that the object’s surface reflects the light directly into the sensor.
The smooth surface of the object appears as a bright area and each indentation, bump or defect, such as e.g. scratches, are a dark edge.
Attention: With bright field lighting, the angle of alignment between the lighting, object and sensor and the object’s surface is critical as direct reflection by the object’s surface only functions when the angle and surface characteristics (shiny, mat, oily ….) are constant!
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With dark field illumination, the lighting, sensor and object are arranged so that the smooth surface of the object does not reflect the light directly into the sensor.
Object edges (indentations and bumps) appear as bright areas, smooth object surfaces however are dark. This type of illumination functions with wide angle ranges and depends little on the object’s surface. |
Diffuse lighting is used everywhere where highly-reflective, curved and above all irregularly-shaped object surfaces are concerned (e.g. aluminium foil on blister packs etc.). Such objects cannot be illuminated with spot-shaped lighting, but only with Diffuse lighting (i.e. even lighting from all directions). Diffuse lighting is also known as "cloudy day" illumination, i.e. uniform light from behind the cover of clouds rather than from direct sunlight. |
| With bright field |
With dark field |
With bright field |
With dark field |
With bright field |
With dark field |
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Highly reflective
object clearly visible,
particularly in front
of a white
background |
Highly reflective
object does not
stand out from diffuse
white background. |
Edges and
background
not clearly distinguishable. |
Edges are clearly highlighted by dark
field illumination. |
Highly-reflective
object not clearly distinguishable
with spot lighting. |
Clear image of
highly-reflective,
irregularly-shaped
aluminium foil. |
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Lighting