Machine vision solutions essentially automate manual visual inspection applications. Unfortunately, computers are not as good as humans at reading contextual information that tells whether something is “good” or “bad.” As a result, machine vision systems are highly dependent on effective visual inspection lighting techniques to generate consistent images of manufactured products. An example of poor visual inspection lighting involves the inspection of car brake pads. No customer wants an inspection system to categorize good brake pads as defective based on the effects of changing light inside a plant caused by the sun’s path across the sky outside the plant. So visual inspection lighting systems need to be carefully designed and consistent to enable successful automated inspection.
Visual Inspection Lighting
High-Intensity Inspection Lighting
Not only is the effectiveness of a machine vision system highly dependent on lighting quality, but the speed at which the machine vision system can operate is also highly dependent on lighting. For example, a CMOS industrial camera operating at 200 frames per second has only a few milliseconds to absorb photons and convert them to electrons before the next frame. If the machine vision system does not have high-intensity inspection lighting, the sensor will not collect sufficient photons and the image will lack contrast and brightness, severely limiting the vision system algorithm’s ability to compare “good” product images against “bad” product images as part of a quality assurance system. In short, high-intensity inspection lighting may not be critical for slow applications running at dozens of operations per minute, but for high-speed applications, high-intensity inspection lighting is essential.
Surface Inspection Lighting
Unless the machine vision system is using X-ray illumination or infrared illumination on organic materials, the solution is focused on features on the surface of the object under test. Surface inspection lighting techniques such as diffuse and structured light can solve problems caused by reflective surfaces and 3D surfaces in particular. For example, polished metal parts will reflect a surface inspection lighting source, creating a hot spot or bloom in the image by overpowering the quantum wells of the pixels in that area of the sensor or image. Diffuse lighting spreads the light evenly across the object surface, reducing hot spots and improving contrast. Structured light patterns can also create texture on the surface. Image processing algorithms can use this texture for measurements when no other visual cues are present.
Paint Inspection Lighting
Paint inspection lighting is one of the most challenging areas of machine vision lighting for a number or reasons. Most painted surfaces are highly polished, reducing the availability of visual features for an image processing algorithm. This may suggest that diffuse or structured lighting techniques would be the best for paint inspection lighting applications. But can projected light reveal color? If so, system designers may need to use multiple lighting techniques, including multispectral and white light inspection to adequately create a paint inspection lighting application.
Glass Inspection Lighting
Glass inspection lighting solutions are some of the most challenging machine vision applications. Glass inspection lighting systems need to detect defects on translucent, highly reflective material. Therefore glass inspection lighting solutions are highly dependent on the specific needs of the application. Is the glass being inspected for surface pitting? If so, darkfield indirect lighting would highlight invisible defects. Is it a color inspection, where color matching with the right filter will provide a cost-effective, high-throughput solution, or does the customer need to use infrared (IR) solutions to locate pixel defects in glass panels used in monitors? Call us to learn how to solve your most complex glass inspection lighting applications.
Machine vision designers have multiple ways to measure and inspect the color of an object. Monochrome cameras with filters can detect specific colors but cannot make objective measurements. Color cameras with broadband white light can detect more colors than monochrome filters, while only multispectral, hyperspectral, or other specialized measurement systems may be required for NIST calibration of monitor performance. Metaphase’s LED inspection lights include white, red, green, blue, infrared (IR), ultraviolet (UV), multicolor RGB, and other custom configurations. Many of these color bands offer unique advantages for common machine vision applications.
UV LED Inspection UV light is used for disinfecting surfaces, curing glues (and inspecting the curing process), fluorescence, and other life science applications. Invisible UV Barcode area and Line Scan applications.
White LED inspection is important for color camera packaging and general inspection applications. Color Verification, object detection/location, Artificial Intelligence illumination assistance.
Red LED inspection is centered around the 630 nm band of the visible spectrum and is widely used due to silicon sensors’ increased sensitivity to light closer to the red end of the visible spectrum.
Blue LED inspection is centered around 470 nm and is highly effective for metal part inspection.
Green LED inspection, at 530 nm, lies between the red and blue LED inspection bands and is used for monochromatic inspection of purple objects. Also used for Manual operator Glass inspection Systems.
Infrared LED inspection, centered around 850 or (from 850-940) 940 nm, is widely used because IR LEDS are high output, long lived, and low cost due to their use in telecom, laser disc, and many other applications. Also used to eliminate operator eye irritation of high-power strobing visible lights.
UV LED inspection is being deployed more often as these LEDs gain in power and life expectancy while dropping in cost. Traditional UV LEDs are expensive compared to other LED colors due to low yields from semiconductor manufacturers.
RGB LED inspection is used with packaging inspection systems that employ different colors of illumination, enabling automated color measurement that costs less than spectroscopic techniques. Blending of colors also provides an almost unlimited variety of methods to achieve the best contrast against multicolored backgrounds such as MasterCard, Visa, Bottle, Shrink Wrap, and so on.