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Why
LED lighting is Preferred For Video Microscopy and Machine Vision
Applications
Illumination
system in an imaging system
Illumination is one of the most important issues in machine vision.
Most imaging systems usually include an optical system, illumination
system, camera sensor and data analyzing system. Any of these systems
can be a bottleneck in the imaging process. The best diffraction
limited optical system will not supply good image quality without the
correct illumination. The illumination system can either enhance or
diminish some features of a monitored object. Furthermore, poor
illumination can even create artifacts.
Fortunately,
many of these problems can be avoided with the correct lighting. When
considering illumination for an imaging system, the numerical aperture
of the optical system, camera sensitivity, light spectrum, light
intensity and angular distribution are the key parameters to keep in
mind.
Comparison
of LED and conventional light sources
a.
Spectrum
The spectrum of a conventional quartz halogen light source is very
similar to black body radiation. The effectiveness of the quartz
halogen lamps is low and further reduced by an additional 50% with
fiber optic coupling.
LEDs
are solid-state semiconductor devices that convert electrical
energy directly into light. The light generation efficacy is very high
because most of the energy radiates within the visible spectrum.
In addition to low efficacy. The light output of the QH lamps is very
much in infrared and red portions of the spectrum. The combination of
the "red" QH-light source from the machine vision system and the
"white-blue" light from the ambient fluorescent light makes it
impossible to properly adjust the white balance of the sensitive
camera. The combination of white LEDs and fluorescent light has very
good match.
b.
Lifetime
The lifetime of some QH lamps does not exceed 40 hours. This can be
very problematic for machine vision applications. For example, the
machine will need to be shut down to replace the bulb an average of
every two days. The lamp can also die in the middle of the operation
process, making it necessary to stop the machine, change the bulb and
recalibrate the machine before restarting it again.
Reliability
is the most important feature of the LED. The expected lifetime for
each LED is around 100,000 hours. This equals nearly 11 years of
continuous operation. However, it is important to consider the thermal
condition of the LED operation, which does change over the lifetime of
the light. Under normal operation conditions (25 degrees C and 20mA)
the light output drops off 2% for every 1000 hours of operation. The
results are much less promising for white LEDs where the light LED
intensity drops off 10% for every 1000 hours of operation. It is
possible to reduce the percentage drop off per 1000 hours to a very
small amount by operating at reduced power settings.
c.
Mechanical Stability
LEDs are mechanically very stable. If you look at an LED bulb closely
you will see that there are some wires and a little block of material
completely encapsulated in clear plastic. It is almost impossible for
the LED bulb to fail because of mechanical damage unless the plastic is
smashed with a hammer. The mechanical stability of the LED illumination
unit makes it the preferred choice for applications where lighting is
attached to moving parts. Fiber optics illumination, on the other hand,
is not as mechanically stable as LEDs and can break over time, reducing
illumination intensity.
References
Light Emitting Diode Illuminators (Author: Dr. Dmitry Gorelik)
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