Lighting
Techniques
Lighting is dictated by the application
and depends on the object’s surface properties, color, background, and
the data needed to
extract. It may also be optimized for the image processing and analysis
techniques used.
Diffuse Backlighting – A diffuser and a source are placed
behind an object – with the diffuser positioned between the source
and the object. It creates a silhouette of the object, generating an
image with sharp contrast. If the object is thick, collimated light
or telecentric lighting may be required. It can also be used for
imaging translucent objects – e.g. glass containers.
Collimated Backlighting –
A collimating lens is used instead
of a diffuser.
Polarized Backlighting – Diffused backlight with a polarizer
in the front of it and a cross polarizer in front of the imaging lens.
Highlights birefringent defects but edges may be fuzzy.
Structured Lighting
– Projects a pattern of light
in order to gather three dimensional information about an object.
Diffuse Front Illumination –
A fluorescent lamp, a fiber-optic
light with diffuser, or an incandescent light with diffuser. It generates
very even illumination and reduces glare on specular surfaces, but
edges may be fuzzy.
Bright Field Illumination -
An incandescent lamp or fiber-optic
light illuminates an object from the top. Good for detecting
surface defects such as scratches, racks, pits, and rust spots –
which create shadows. It can create glare on specular surfaces or
unwanted shadows on uneven surfaces.
Dark Field Illumination – The surface is illuminated with
partly collimated light at a low angle. As the camera looks from
the top, the reflection from the regular surface is eliminated making
a field of view completely dark. Any deviation from flat surface (e.g. bump or depression) will cause a reflection visible by the sensor.
Directional Front Lighting –
Is similar to bright field
illumination except that the light is collimated with help of
collimating optics. It creates a greater shadowing for use in finding
small burrs on a flat surface or locating the edge of a hole.
Polarized Front Illumination – Is used for reducing
a specular glare from shiny or specular areas. It uses a point or
diffused illumination with a polarizer in the front of it and a
cross polarizer in the front of the lens. This method removes the
specular glare because the light reflected from a shiny surface
remains polarized and, therefore, cannot pass through the cross
polarizer. The diffuse surface, however, depolarizes the light so
that the light reflected from a diffused surface passes through the cross
polarizer and reaches the sensor.
Color Discrimination by Filters – Sometimes, it is possible
to achieve better image contrast by using a light source that does
not have some colors or wavelengths, or by filtering out unwanted
colors before they reach the camera. It is also possible to perform
multiple inspections at the same time using an illumination of a different
color or wavelength for each one of them. Commercial filters are
available that can filter or pass specific color bands and can be
used either at the light source or at the camera (lens).
For more information, please contact
High-Tech Digital Technical Support.
310-265-8203
support@high-techdigital.com.
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