Dimensional Measurement using Vision Systems - NPL Publications ...

Measurement Good Practice Guide No. 39
3.1.2 Xenon strobe lamps
These lamps are most associated with high speed imaging, perhaps on a high volume
production lines. They are long life lamps, producing high intensity, daylight colour
temperature illumination. They can be used with fibre optic light guides and the more
expensive types can be synchronised with the camera shutter. Xenon arc lamps are also
available, providing a constant high intensity output, but there are few applications
requiring this type of illumination.
3.1.3 Fluorescent tubes
Fluorescent tubes are the most common source of illumination for macro vision systems.
They give a very bright, homogenous illumination, but do show periodic intensity
variations, as they are normally powered by an AC supply. The use of high frequency
fluorescent tubes with long persistence phosphors can reduce this intensity variation.
Fluorescent tubes are ideally used as large area backlights and for front flood lighting.
They are also commonly used in line scan applications.
3.1.4 LED lighting
Perhaps the most versatile illumination source, Light Emitting Diodes (LED’s) are very
reliable and can be switched, strobed or operated continuously. The illumination intensity
can equal that of halogen lamps, when strobed. White light and monochromatic LED’s are
available and the configurations include ring lights, rod lights, line lights, backlights and
on-axis diffuse lighting. A disadvantage is that the high current requirements and circuitry
add significantly to their cost.
3.1.5 Lasers
Whilst not generally used for dimensional measurement, laser sources offer a low power,
monochromatic and high reliability illumination. They are particularly suited to
structured illumination requirements, where high intensity grids, lines, spots or concentric
circles can be created with additional optics. A typical gauging application would be
dimensional measurement using laser line illumination on parts with little grey level
contrast.
3.2 ILLUMINATION CONFIGURATIONS
As with microscopy, large area vision systems use transmitted or reflected illumination,
although on the macro scale they are known as back lighting or front lighting,
respectively. The following sections describe the most common illumination
configurations.
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