Lighting and Optics
In the operation of machine vision systems, the objective of lighting is
to create a high-contrast image that clearly distinguishes the features
to be inspected. Proper lighting can reduce the processing time a system
requires, allowing more objects to be inspected in a shorter time, and improve
inspection results. Incorrect lighting produces shadows or glare that can
affect inspection results.
Light Sources and Configurations
Many light sources are available, such as incandescent, fluorescent, LED,
and quartz halogen (typically used as fiber-optic sources). Some, such as
fluorescent and LED, have a long life; others, such as quartz halogen, have
a shorter life span but experience little decay in light intensity over
time. All these sources are suitable for lighting static images (e.g., when
a part stops in front of the camera, is inspected, and is transferred to
the next station).
Many applications, however, require inspection of rapidly moving objects
along a conveyer. In these cases, strobe lighting, which produces short,
intense bursts of light, appears to stop motion at the time the image is
captured.
Light configurations can be designed to illuminate a large area or object
or to focus narrowly on one distinct feature of an object. Many lighting
configurations can be developed, depending on the features to be highlighted.
Two basic configurations include:
- Front lighting, in which objects are lit from the top. The angle at
which the lights are placed influences the effect produced by front lighting.
(See Figures 2 and 3.)
- Back lighting, in which the light is placed below the object to provide
an outline of the object.
Optics
Lenses, like lighting, are crucial to obtaining accurate inspection results.
A variety of lenses are available, offering a corresponding range of cost
and precision. Low-cost lenses designed for the surveillance industry are
popular in many machine vision applications. Although low-cost lenses can
be sufficient for simple presence/absence applications, they can prevent
accurate measurement. High-quality, high-resolution lenses are necessary
for applications requiring precise measurement and for detecting defects
on small parts.
Identifying the required field of view is important in determining which
lenses are best for your application. Do you have to inspect an entire printed
circuit board or just several components in the upper left-hand corner?
These are only some of the many lighting and optics variables that must
be considered when designing a vision system. Working closely with a systems
integrator and being aware of the importance of lighting and optics will
enable you to design the right machine vision system for your application.
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