Image acquisition is the core of machine vision, and the image acquisition system consists of three parts: a light source, a lens, and a camera. The selection and proper lighting of the light source can directly affect at least 30% of the image quality. Therefore, the light source is a very important component of a machine vision system.
effect
By designing a suitable light source to optimally separate target information from background information in an image, the difficulty of image processing algorithms for segmentation and recognition can be greatly reduced, while simultaneously improving the system's positioning and measurement accuracy, thus enhancing its reliability and overall performance. Conversely, improper light source design can lead to significant inefficiencies in image processing algorithm design and imaging system design. Therefore, the success or failure of the light source and optical system design is a primary factor determining the success or failure of the entire system.
Illuminate the target and increase its brightness;
Highlight measurement features and simplify image processing algorithms;
Overcome ambient light interference, ensure image stability, and improve image signal-to-noise ratio;
Improve the positioning, measurement, and recognition accuracy of vision systems, as well as the system's operating speed;
Reduce the complexity of system design to achieve the best imaging effect for image processing;
Classification
1. Ring light source
Ring light sources offer different illumination angles and color combinations, better highlighting the three-dimensional information of objects; high-density LED arrays provide high brightness; various compact designs save installation space; they solve the problem of shadows from diagonal illumination; and optional diffuser plates ensure uniform light diffusion. Applications include: PCB substrate inspection, IC component inspection, microscope illumination, LCD calibration, plastic container inspection, and integrated circuit printing inspection.
2. Backlight
High-intensity backlighting using a high-density LED array effectively highlights the outline and contours of objects, making it particularly suitable as a microscope stage. Dual-use (red/white) and multi-use (red/blue) backlights can be configured to produce different colors to meet the multi-color requirements of various measured objects. Applications include: measurement of mechanical parts dimensions, inspection of electronic components and ICs, film stain detection, and scratch detection on transparent objects.
3. Bar light source
Bar light sources are the preferred light source for measuring large, square-shaped objects; colors can be customized and freely combined according to requirements; the illumination angle and installation are adjustable. Applications include: metal surface inspection, image scanning, surface crack detection, and LCD panel inspection.
4. Coaxial light source
Coaxial light sources can eliminate shadows caused by uneven object surfaces, thus reducing interference; some employ beam splitter designs to reduce light loss, improve imaging clarity, and uniformly illuminate object surfaces. Applications: This series of light sources is best suited for highly reflective objects, such as scratch detection on metal, glass, film, and wafer surfaces; damage detection of chips and silicon wafers; mark point positioning; and packaging barcode recognition.
5. AOI dedicated light source
Three-color light illumination at different angles highlights the three-dimensional information of the solder; an additional diffuser plate guides the light and reduces reflection; different angle combinations are available; application areas: used for solder inspection of circuit boards.
6. Spherical Integral Light Source
The hemispherical inner wall, with its integral effect, uniformly reflects light emitted from the bottom 360 degrees, resulting in highly uniform illumination across the entire image. Applications: Suitable for inspecting curved surfaces, uneven surfaces, arc-shaped surfaces, or highly reflective surfaces such as metals and glass.
7. Linear light source
Ultra-high brightness, using cylindrical lenses for light focusing, suitable for various continuous inspection applications on production lines. Applications: dedicated to illumination of array cameras, dedicated to AOI (Automated Optical Inspection) systems.
8. Point light source
High-power LEDs, small in size, and with high luminous intensity; a replacement for fiber optic halogen lamps, especially suitable as coaxial light sources for lenses; efficient heat dissipation device greatly improves the lifespan of the light source. Applications: Suitable for telecentric lenses, used for chip inspection, mark point positioning, and wafer and LCD glass substrate calibration.
9. Combined bar light source
The four sides are equipped with strip lights, and the illumination on each side is independently controllable; the required illumination angle can be adjusted according to the requirements of the object being tested, making it widely applicable. Applications include: CB substrate inspection, IC component inspection, solder inspection, mark point positioning, microscope illumination, packaging barcode illumination, and spherical object illumination.
10. Alignment light source
Fast alignment speed; large field of view; high precision; small size, easy to integrate for inspection; high brightness, with optional auxiliary ring light source. Application areas: The VA series light source is a dedicated light source for alignment in fully automatic circuit board printing machines.
LED light source characteristics
Currently, LEDs (light-emitting diodes) are the main light source for machine vision. They are widely used in the industry due to their high degree of shape freedom, long lifespan, fast response speed, good monochromaticity, diverse colors, and high overall cost-effectiveness.
• Degrees of freedom of shape
An LED light source is composed of many individual LEDs, which allows it to be made into more shapes compared to other light sources, making it easier to design the shape and size of the light source to suit the user's needs.
• Long service life
To ensure accurate and repeatable measurement results for the image processing unit, the lighting system must guarantee a stable image input over a considerable period. While LED light sources experience brightness decay after 10,000 to 30,000 hours of continuous operation, their performance remains significantly better than other types of light sources. Furthermore, intermittent operation via a control system can suppress LED heating and double their lifespan.
• Fast response time
LEDs have a very short response time. The true significance of response time lies in its ability to ensure smooth switching between multiple light sources or different areas of a single light source. When a dedicated controller powers the LED light source, the time to reach maximum illuminance is less than 10 seconds.
• Diverse colors
Besides the shape of the light source, another crucial aspect of obtaining stable image input is selecting the color of the light source. Even light sources of the same shape can produce significantly different images depending on their color. In fact, utilizing the technical characteristics of light source color to achieve optimal contrast in image quality has always been a major focus in light source development.
• Low overall operating costs
Choosing cheap, low-performance products means that initial savings will quickly be offset by ongoing maintenance and repair costs. Other light sources not only consume 2-10 times more power than LEDs, but also require replacement almost monthly, wasting valuable time for maintenance engineers. Furthermore, the more light sources in use, the greater the costs for component replacement and labor. Therefore, choosing long-life LEDs is economical in the long run.
How to choose
• Environmental Requirements Analysis
1. Understand the customer's requirements for system structure and operation, and determine the spatial structural relationship between the industrial camera, light source, and the object being measured.
The defined parameters are: field of view (FOV) and working distance (WD);
2. Spatial structures include: direct sunlight, side sunlight, and backlighting;
Direct-type light sources include: partial ring light sources, coaxial light sources, and dome light sources;
Side-emitting light sources—partially ring light sources, strip light sources, line light sources, point light sources;
Backlighting—square backlight, strip backlight.
• Surface texture and color analysis of objects
1. Is the object's surface curved or flat? Is the surface smooth? Is the reflection very strong?
Dome-shaped light sources are suitable for inspecting curved surfaces, coaxial light sources are suitable for smooth planes, and bright-field light sources are suitable for rough planes.
2. How transparent is the object?
Objects with good light transmission can use IR light sources.
3. Distinguish between the background (which we don't need to detect) and the foreground (which we do need to detect).
A good light source provides good contrast—the background and foreground are clearly distinguishable.
4. The foreground colors are varied.
Colored light sources and white light sources are recommended;
• Select Summary
1. Need greater contrast between foreground and background? – Consider using a monochrome camera with colored light sources.
2. Ambient lighting issues? – Try using a single-color light source with a filter.
3. Flashing curved surface? – Try using diffused dome light.
4. A flash, flat, but rough surface? – Try using coaxial scattered light.
5. Observe the shape of the surface? – Consider using dark field (low angle) viewing.
6. When testing plastics – try using ultraviolet or infrared light.
7. Need to see features through a reflected surface? – Try using a low-angle line light source (dark field).
8. Combining light sources can sometimes solve the problem.
9. Flickering can produce light 20 times stronger than constant illumination.
How to set up lighting
• General purpose lighting:
General lighting typically uses ring or spot lighting. Ring lights are a common type of general lighting; they are easy to mount on lenses and can provide sufficient illumination to diffuse surfaces.
• Backlighting:
Backlighting involves placing a light source behind the object relative to the camera. This method differs significantly from other lighting techniques because image analysis focuses on incident light rather than emitted light. Backlighting produces strong contrast. However, when using backlighting, surface features of the object may be lost. For example, backlighting can be used to measure the diameter of a coin, but it cannot determine whether the coin is heads or tails.
• Coaxial lighting:
Coaxial illumination projects light onto the surface of an object in the same direction as the camera's axis. It uses a special semi-reflective mirror to reflect the light source towards the lens axis of the camera. The semi-reflective mirror only allows light reflected from the object's surface perpendicular to the lens to pass through. Coaxial illumination is useful for achieving uniform illumination of flat objects with mirror-like surfaces. Furthermore, this technique can highlight areas of varying surface angles, as light reflected from surfaces not perpendicular to the camera lens will not enter the lens, resulting in a darker surface. Continuous diffuse illumination is used for reflective surfaces or surfaces with complex angles. It employs hemispherical uniform illumination to reduce shadows and specular reflections. This method is very useful for illuminating fully assembled circuit boards. This light source can achieve uniform illumination over a 170° solid angle.
• Dark area lighting:
Dark-field lighting provides low-angle illumination relative to the surface of an object. To determine whether a light source is bright-field lighting, photograph a mirror with the light source within its field of view. If the light source is visible within the field of view, it is considered bright-field lighting; conversely, if the light source is not visible, it is dark-field lighting. Therefore, whether a light source is considered bright-field or dark-field lighting depends on its position. Typically, dark-field lighting is used to illuminate protruding parts of a surface or to illuminate areas with varying surface textures.
• Structured light:
Structured light is a type of light projected onto the surface of an object with a specific geometric shape (such as lines, circles, or squares). Typical structured light involves lasers or optical fibers. Structured light can be used to measure the distance from a camera to a light source. Multi-axis illumination: In many applications, multiple illumination techniques are needed to achieve different contrasts for different features within the field of view.
