This knowledgebase contains excerpts from National Instruments' white paper "A Practical Guide To Machine Vision".
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Vision Illumination Sources and Spectral Content
The lighting sources now commonly used in machine vision are fluorescent, quartz halogen, LED, metal halide (mercury), and xenon. Fluorescent, quartz halogen, and LED are the most widely used lighting types in machine vision, particularly for small- to medium-scale inspection stations. Metal halide, xenon, and high-pressure sodium are more typically used in large-scale applications or in areas requiring a very bright source. Metal halide, also known as mercury, is often used in microscopy because it has many discrete wavelength peaks, which complements the use of filters for fluorescence studies. A xenon source is useful for applications requiring a very bright strobe light. Figure 2 shows the advantages and disadvantages of fluorescent, quartz halogen, and LED lighting types and relevant selection criteria, as applied to machine vision. For example, whereas LED lighting has a longer life expectancy, quartz halogen lighting may be the choice for a particular inspection because it offers greater intensity.
Figure 2.Comparison of common vision lighting sources.
Historically, fluorescent and quartz halogen lighting sources have been used most commonly. In recent years, LED technology has improved in stability, intensity, and cost-effectiveness; however, it is still not as cost-effective for large area lighting, particularly compared with fluorescent sources. However, if application flexibility, output stability, and longevity are important parameters, then LED lighting might be more appropriate. Depending on the exact lighting requirements, oftentimes you can use more than one source type for a specific implementation, and most vision experts agree that one source type cannot adequately solve all lighting issues. Consider not only a source&#;s brightness but also its spectral content (Figure 3). Microscopy applications, for example, often use a full-spectrum quartz halogen, xenon, or mercury source, particularly when imaging in color; however, a monochrome LED source is also useful for a black and white CCD camera, and also now for color applications, with the advent of &#;all color&#;RGB&#; and white LED light heads. In those applications requiring high light intensity, such as high-speed inspections, it may be useful to match the source&#;s spectral output with the spectral sensitivity of your particular vision camera (Figure 4). For example, CMOS sensor-based cameras are more IR sensitive than their charge-coupled device (CCD) counterparts, imparting a significant sensitivity advantage in light-starved inspection settings when using IR LED or IR-rich Tungsten sources.
Figure 3.Light Source Relative Intensity Versus Spectral Content (The bar at the bottom denotes the approximate human visible wavelength range.)
Figure 4.Camera Sensor Absolute Quantum Efficiency Versus Wavelength (The bar at the bottom denotes approximate human visible wavelength range.)
Additionally, Figures 3 and 4 illustrate several other relevant points to consider when selecting a camera and light source:
Machine vision, the technology that enables machines to see and understand their surroundings, relies on various components to capture, process, and analyze images. Among these components, lighting plays a critical role. The color of the light used in machine vision is a key factor in ensuring high-quality and accurate imaging. In this article, we&#;ll explore the importance of machine vision lighting colors and discuss the specific colors that are used in particular applications to optimize image acquisition.
In machine vision, lighting serves as the source of illumination for cameras and other imaging devices along with other techniques like polarization. The color of the light used can significantly impact the quality of the images captured and the effectiveness of the overall system. Here are some key reasons why machine vision lighting colors are of paramount importance:
Different colors of light can enhance the contrast between objects and their backgrounds, making it easier for machine vision systems to detect and identify features, defects, or anomalies in the images.
Materials reflect and absorb light differently based on their properties. By using specific lighting colors, machine vision systems can differentiate between materials, aiding in quality control and inspection processes.
Machine vision systems often need to identify defects or irregularities on surfaces. The choice of lighting color can influence how clearly these defects are revealed in images.
Certain colors of light are better at minimizing glare and reflections, which can be a significant challenge in various applications, such as surface inspection and automotive manufacturing.
Different applications require tailored lighting solutions and different types of cameras used in vision systems. Machine vision lighting colors can be customized to match specific application requirements, ensuring optimal performance.
Now, let&#;s delve into specific machine vision lighting colors and their applications:
White light is a versatile choice in machine vision applications. It contains a broad spectrum of colors and can be further customized by adjusting the color temperature. White light is often used in applications where a general and evenly distributed illumination is required. Some common applications include:
White light is suitable for general inspection tasks where no specific color enhancement or material differentiation is needed. It provides a neutral background for capturing images.
When reading barcodes, white light is used to illuminate the code for cameras to capture the pattern. It ensures the barcode is well-contrasted against the background.
In packaging lines, white light is employed to inspect labels, packaging materials, and product contents. It offers good overall illumination to detect defects and ensure product quality.
Red light is often used in machine vision for its unique properties, particularly its ability to minimize reflections and glare. Red light is absorbed by many materials, making it suitable for applications where surface reflection needs to be controlled. Applications for red light in machine vision include:
Red light is effective in reducing reflections when inspecting shiny or reflective surfaces, such as metal, glass, or plastic. This ensures that defects or imperfections are clearly visible.
In the textile industry, red light is used to enhance the visibility of fabric patterns and defects, ensuring the quality of textile products.
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Red light is employed for inspecting food products to detect contaminants, such as foreign objects or discolorations, without altering the appearance of the food.
Green light is another color frequently used in machine vision applications. It offers good contrast for certain materials and surfaces and is often chosen for applications like:
In electronics manufacturing, green light is utilized for solder joint inspection, PCB inspection, and component placement verification. It provides good contrast for soldering defects
Green light can be used in pharmaceutical applications to inspect pill coatings, print quality on tablets, and vial filling processes. It highlights subtle imperfections effectively.
In the agricultural sector, green light is employed for the inspection of fruits and vegetables. It helps identify defects, bruises, and ripeness levels while preserving the natural appearance of the produce.
Blue light has unique characteristics that make it suitable for certain machine vision applications, especially those that involve fluorescence and specialized inspection needs. Applications for blue light include:
Blue light is used in conjunction with fluorescent dyes or markers to identify specific features or substances, such as DNA, biological samples, and chemicals.
In forensic science, blue light is used to reveal hidden details and substances at crime scenes, making it easier to capture evidence.
Blue light is employed in dental and medical imaging to enhance contrast and reveal details in tissues, bones, and dental materials.
Infrared (IR) light is beyond the visible spectrum, and its use in machine vision is particularly beneficial for applications that require imaging in low-light or no-light conditions. Infrared lighting is employed in various applications, including:
In military and surveillance applications, infrared lighting is used for night vision, enabling cameras to capture images in complete darkness.
In precision agriculture, infrared light is utilized to analyze plant health, moisture content, and disease outbreaks. It can reveal information that is not visible in visible light.
IR lighting is essential for security cameras, allowing them to monitor and capture footage in low-light environments.
Ultraviolet (UV) light is used for machine vision applications that require the detection of fluorescent materials or the identification of features not visible in visible light. UV lighting applications include:
UV lighting is employed to detect and inspect fluorescent materials in various industries, such as quality control in the semiconductor and automotive sectors.
In the banking and document verification industry, UV light is used to detect security features on banknotes and official documents.
UV light helps conservators identify aging, restoration, and hidden details in artworks, manuscripts, and historical artifacts.
RGB (Red, Green, Blue) lighting involves using multiple colors simultaneously to achieve specific lighting effects and enhance image contrast. Applications of RGB lighting include:
RGB lighting is essential in applications where color recognition and inspection are critical, such as in the food industry to ensure product consistency and quality.
RGB lighting is used to detect patterns, codes, and markings on products, packaging, and materials, facilitating their identification and sorting.
In robotics and automation, RGB lighting is used to segment objects based on color, allowing robots to handle and sort items accurately.
Machine vision lighting colors are not chosen arbitrarily but are carefully selected to meet the specific requirements of diverse applications. The choice of lighting color can significantly influence the quality and accuracy of imaging, making it a critical factor in the success of machine vision systems.
As technology continues to advance, machine vision lighting solutions are becoming increasingly sophisticated and customizable, contact Sciotex to learn more about Vision Systems solutions to fit your application.
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