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UV Lighting in Machine Vision Applications is Coming of Age
by Matt Pinter, Design Engineer - Spectrum Illumination Posted 03/23/2006
Over the years, many applications tried to use UV lighting only to end up with failure or disappointing results. Advances in technology have helped to change UV vision applications into positive successful inspections.
Why use a Color Camera?
In the early years of machine vision monochrome cameras were prevalent while color cameras were scarce and rarely used. UV excitation inspections used a monochrome camera. A monochrome camera can only give gray scale information in pixel intensity. Each image contains only information on the intensity of a pixel from 0-100% or 8 bit 0-255 value. Inspection decisions are made only on the intensity of a sensor or collection of pixels. Pass/Fail is solely based on how bright or dark the pixels measure. A color camera adds to the formula by using wavelength in addition to intensity. Wavelength identification allows the UV inspection to make a decision on a certain color or wavelength. Figure1 shows a monochrome image with a UV mark on a large piece of timber. Figure 2 is the same timber with a color camera. Note the color of cyan can be easily identified by a color camera in Figure 2. The majority of UV excitation applications use an additive material that excites or emits a cyan color (505nm wavelength) when illuminated with 365nm or 395nm UV.
What’s with HB UV LED’s?
New High Brightness HB LED’s in UV have changed the usable distance and power of UV lights. UV HB LED’s in 1 watt packages are now available. Using UV HB LED’s in new lights increase the lights output by a factor 10x to 30x. UV HB LED’s can be strobed at an increased intensity for even brighter output for high speed inspections. Another advantage of UV HB LED’s is the ability to focus the light. Parabolic reflectors and lenses on the HB LED’s allow a more concentrated light pattern. The end result is more light in the area needed and longer working distances. Fluorescent black lights were the main source of UV in earlier years and the main disadvantage to these lights is the low output and wide angle of illumination. Fluorescent black lights needed to be very close to the inspected part for sufficient illumination. HB LED’s have changed this by providing a focused high output illumination at greater distances.
How can Short Pass or Band Pass Filters help?
A short pass or band pass filter can limit the unwanted wavelengths or colors entering the camera. A 505nm short pass filter will pass the light wavelength of cyan or a 505nm range and limit all other wavelengths. This will limit the unwanted colors and ambient light in the image and increase the favored color or wavelength.
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