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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