Once a light is selected, a number of methods can be employed to modify light output, thereby increasing the efficiency of the inspection. Options include the use of filters for isolating a specific wavelength, strobing to stop motion or increase intensity, and color mixing to heighten contrast.
The effectiveness of a light is partly determined by the sensitivity of the camera (CCD) to specific wavelengths. A CCD with peak reception between 620-700nm will register more output from a 660nm (red) light than from a 470nm (blue) light.
Among other things, filters eliminate unwanted light, isolate specific colors, or decrease glare. The filters currently available offer a wide range of options for controlling or enhancing the quality of light that reaches the CCD.
Polarized Lighting: Polarizing filters are used to limit the amount of glare from an object. Light is projected through a polarizing filter, and then imaged by the camera through an "analyzer." Adjusting the orientation of the analyzer allows the viewer to modify the direction of the light collected by the camera.
Band Pass Filters: Allowing only a narrow range of wavelengths through to the camera, Band Pass Filters are useful in settings where ambient light creates a variable inspection envrionment. These filters remove all but a specific range of light, eliminating the need for a shroud around an inspection area. Band pass filters are also useful for isolating a single color or a range of colors within an inspection.
Cut off filters will similarly prevent light above or below a specific wavelength from being visible to the camera.
Choice of light color depends primarily on two elements in the inspection: the color of the object being viewed, and the sensitivity of the CCD. While a color CCD generally - though not always - requires the use of a white light source, black and white applications can sometimes benefit from the wise application of colored light.
Warm and Cool colors:
Colors can be divided into warm (red, yellow, and orange), and cool (green, blue, and violet). To create contrast, opposing types should be employed. For example, a warm colored light on a cool colored object will darken the object, while a warm colored light on a warm colored object will lighten it. The highest degree of contrast is achieved when using colors that are directly opposed on the wheel. (ie., green light on a red object will darken the red object to a greater degree than any other color.)
In the following example, the enhanced contrast in a black and white rendering of a stop sign is illustrated by showing the effects of red and green LED light.
Using an "all color" Red Green Blue light provides the opportunity to customize any color needed to obtain optimal CCD response. With all three colors at equal intensity, the light output is white; when any or all of the three colors are adjusted, the full color spectrum is available. In addition, an RGB light puts out approximately 2x the white light of currently available white LEDs.
Not visible to the human eye, Infra Red (IR) is often overlooked when considering illumination options. For application settings requiring frequent human interaction, IR eliminates problematic glare. Although IR sometimes causes unusual color readings, it has been proven useful in the inspection of plastic parts.
LEDs offer long life with a predictable intensity fall off. Manufacturers specify a lifespan of 100,000 hours for red (660nm) LEDs. Other colors, including blue (470nm), white, and green (520nm) offer approximately 10,000 hours of stable performance. Comparatively, Xenon and fluorescent light sources provide life spans in the hundreds or thousands of hours, with a considerable decrease in intensity after only a short time.
LEDs are versatile.
Available in both the visible spectrum and UV/IR, LEDs are small, energy efficient, and economical. More importantly, through Ai's patented Evenlite process, LEDs can be aimed to place light exactly where it is needed. LED light sources can also be placed remotely from power supplies and controllers.
LEDs can be strobed faster than any other light source, including xenon, and have very short rise times.
LEDs are stable, solid state devices, capable of functioning in a variety of environments.
LEDs are extremely efficient, and require significantly less current than other light sources.
LED technology continues to evolve.
Every advance in LED technology translates to more efficient and wider ranging lighting products for the machine vision industry. New 1 watt, 3 watt, and 5 watt LEDs provide opportunities for creating illumination sources offering more intensity and greater area coverage than was previously possible.
Ai was the first machine vision lighting company to combine red, green, and blue into a single light head.
Red, green, and blue LEDs, when combined in equal parts, create a white light approximately twice as bright as the white LEDs currently available. In addition to the increased intensity, the greater advantage of placing the RGB combination in a single unit is that the individual colors can be adjusted to create any hue in the visible spectrum. If this vast spectral range is difficult to imagine, consider a color television; each TV screen pixel is composed of red, green, and blue.
The ability to fine tune color can significantly increase the efficiency of an inspection, and provide a flexible and economical solution for manufacturing lines that produce the same product in a range of coors. As the product color changes, the light color can be adjusted to provide the maximum contrast. A single RGB light head can replace several different colored lights.
It's more than just strobing!
A strobed light source synchronized to the camera shutter provides the ability to stop motion on a high speed production line, giving the appearance that the object is standing still.
Turbo-charging is what Ai calls the process of driving short bursts of high current through an LED light that is precisely synchronized with the camera's open shutter.
The benefits of this process include increased intensity and longer LED life. An increase in intensity can reduce negative effects from ambient light.
Electrical engineering students generally learn that increments of 10ms can be forgotten. However, in turbo charging, discrepancies as short as 4 or 5ms can mean a significant difference in the amount of light gathered by the camera.
Greater Intensity: 10-12% of the current driven through an LED is converted into light, making the LED a highly efficient light source. When the current increases, light intensity increases. As long as a proper duty cycle is maintained, an increase in intensity of up to 40x can be achieved without compromising the LEDs.
Longer Life: LED life is determined by the cumulative number of photons produced at the die junction, as well as the junction operating temperature. Compared to an LED operating under standard conditions (20ma, 100% duty cycle), a strobed LED with a current of 600mA (an increase of 30 times) will benefit from an increase in life of approximately 30 times.
Diminish the effect of ambient light: When properly synchronized with a camera's shutter, the turbo charged output of an over-driven, strobed LED easily nullifies the effects of ambient light.
What is Duty Cycle? The duty cycle is the percentage of time that a light is on in a specified period of time. In machine vision lighting everything is related to 1 second. Therefore, if the light is on half of the time it is a 50% duty cycle. If the pulses are 1/16 second, and there are 4 per second, we have 1/16 second each mulitplied by 4 per second or mathematically 1/16 sec x 4 /sec. The seconds cancel out leaving 4/16 or 25% duty cycle. 100 micro second pulses at 30 per second results in .000100 seconds x 30 =.003 or .3% duty cycle.
The effectiveness of a illuminating source in an inspection is determined by the direction at which light strikes an object, and the direction of the reflected light into or away from the camera. Suitable illumination covers the required field of view, creates a consistently measurable degree of contrast, and does not cause reflected glare. Different light types are required to provide successful illumination for the broad range of products being inspected by automated systems.
Light is aimed directly at an object, often creating distinct shadows. This type of lighting is effective when used on objects requiring high degrees of contrast, but creates specular reflections when used with shiny or reflective materials.
Light is projected at an angle to the surface, causing any variations to deflect light up into the camera, creating bright spots on a dark background or field. Nothing is seen by the vision system if there are no aberrations on the surface.
An even field of illumination is projected from behind an object, which is seen as a silhouette by the camera. Backlighting is most commonly used for taking measurements or determining part orientation.
Reflected light, providing a non-directional, soft illumination free of harsh shadows that is well suited for highly specular objects. This illumination effect is similar to the type of light found on an overcast day.
A variation of diffuse light in which light is aimed at an angled beam splitter that reflects the light down. The object is viewed from above through the beam splitter. This light type is particularly helpful on highly reflective objects or in situations where the area of inspection is obscured by shadows from its surroundings.