Numerous vision applications have attempted to use UV lighting over the years, only to end up with disappointing or failed results. UV inspection applications are not carried out in the UV but instead, in the visible region.
To excite a material under inspection, UV wavelengths are utilized. The material fluoresces and emits light somewhere in the visible spectrum and this visible wavelength is captured by a camera for examination.
Technological advances have helped to change UV vision applications into successful inspections. Color cameras can help solve challenges that monochrome cameras could not, and filters can make or break UV applications.
Using Filters to Separate and Enhance the Wavelengths – How Band Pass, Short Pass, or UV Blocking Filters Can Help?
Depending on a dark or light background, a vision system may need a filter to darken or brighten the visible luminescent emission and block the unwanted visible light and UV light. The majority of inspection cameras possess significant near-UV sensitivity, which creates interference between the visible fluorescence and the UV Light source.
A UV blocking filter can be utilized to prevent the UV light from interfering with the desired wavelength for camera imaging. A blocking, band pass or short pass filter can stop the unwanted colors or wavelengths from appearing in the image.
Cyan is the color emitted in a typical fluorescence application and a 470 nm or 505 nm band pass filter will pass the light or wavelength of cyan and block all other wavelengths. This will restrict the ambient light and unwanted colors in the image and enhance the desired wavelength or color.
UV 395 nm LED light with no filter
UV 395 nm LED light with UV filter (blocking UV wavelengths)
Color Cameras Can Be Beneficial When Inspecting UV
Monochrome cameras were hugely popular in the early years of machine vision, whilst color cameras were rare and barely used. UV excitation inspections were carried out using a monochrome camera. A monochrome camera can only supply grayscale information in pixel intensity. Each of the images only holds information on the intensity of a pixel from 0-100% or 8 bit 0-255 value.
Inspection decisions are done on the basis of the sensor’s intensity or collection of pixels. Pass/Fail is based solely on how dark or bright the pixels are. By utilizing wavelength in addition to intensity, a color camera can add to the formula. Wavelength identification permits the UV inspection to decide on a certain wavelength or color. A monochrome image with a UV mark on a large piece of timber can be observed in Figure 1.
Figure 1. Monochrome camera
Figure 2 contains the same timber but using a color camera. It is worth noting that the cyan color can be identified easily in Figure 2 by using a color camera. Most UV excitation applications employ an additive material that emits or excites a cyan color (505 nm wavelength) when illuminated with 365 nm or 395 nm UV.
Figure 2. Color camera
Plastic support is not visible in image on left with monochrome camera while image on right with color camera shows presence of support (Cyan color).
Advances in High Current UV LED’s
New high current, high brightness, LED’s in UV have extended the usable power and distance of UV lights. UV High Current LED’s in up to 10 watt packages are now available. Employing UV High Current LED’s in new lights enhances the lights output by a factor of somewhere between 10x-30x. For even brighter output for high speed inspections, UV High Current LED’s can be strobed at an increased intensity.
A further benefit of UV High Current LED’s is the capability to focus the light. Parabolic reflectors and lenses on the High Current LED’s permit a more highly concentrated pattern of light. The outcome is longer working distances and more light in the area required. In earlier years, fluorescent black lights were the main source of UV. The main drawbacks to these lights are the wide angle of illumination and low output.
Fluorescent black lights are required to be extremely near to the inspected part for sufficient illumination. High Current LED’s have changed this by supplying a focused high output illumination at greater working distances.
This information has been sourced, reviewed and adapted from materials provided by Smart Vision Lights.
For more information on this source, please visit Smart Vision Lights.