Light
There are many types of light sources that can be illuminated, but in machine vision, halogen lamps, fluorescent lamps and LED lamps are the most widely used. In recent years, LED technology has developed rapidly, coupled with some of its inherent characteristics, such as: long life, brightness stability, can constitute different shapes and spectra, strobe and low power consumption, gradually in the machine vision used in the light source dominated. Of course, in color detection, fluorescent lamp with its good color reduction characteristics still have a large number of applications, in high-brightness applications, halogen lamps have their own advantages. Lighting in Machine vision The importance of lighting in machine vision has almost been recognized by all visual engineers, and we can often see the maxim, "better to light than write (software)", "avoid garbage in (bad lighting) causes garbage out [bad result], "create the best image First", "Lighting can make or break the job" and so on, the next question is how we choose the light source, design the lighting System to get the best image results.
To design a lighting system, you should first understand the characteristics of the light source. This includes: (1) The characteristics of the light source itself, such as the spectral distribution of the light source, the luminance distribution on the irradiated surface, the temperature characteristics and life expectancy in the use of the light source. (2) The interaction between the light source and the irradiated object. The light source shines on the object, can reflect, absorb and transmit three kinds of interaction, different material objects, three kinds of effects produced different phenomena, the same object on the defects and other parts, three effects of the phenomenon is also different. In the case of surface detection, we can roughly divide the target defects into two categories, such as: pits, scratches, cracks, burrs, bumps, abrasions, bumps, etc. the other is surface strength (or density) defects, such as: oxidation, rust, stains, stains, etc., the former so that the surface reflection changes, The latter causes the surface reflection and absorption to change. Only by carefully studying the characteristics of the interaction between the light source and the object, can we determine the light source and illumination mode. The color of an object is another example of light absorption. The whole band of white light shines on the surface of the object, some wavelengths of light are absorbed by the surface of the object, some wavelengths of light are reflected by the surface, the object shows the same spectral color as reflected light. Using this principle, we can use the black and white camera, select the specific wavelength of the light source, highlighting the surface of the object to be detected and other parts of the gray difference, to achieve reliable and stable detection. (3) The structure of the light source. Light sources can be made into a variety of shape structures, each of which emits light with different characteristics. such as: Diffuse light, you can obtain uniform illumination on the target plane, direct light, through reflection and shadow display edge and surface structure, but the light or target movement and tilt will significantly affect the image brightness and contrast. In addition, the light source can be placed above or below the illuminated object, can have a variety of angles and heights, through the combination of different shapes and lighting methods, such as the production of shadows, produce bright areas or non-reflective area of the method, so that the target area of interest and other areas with the largest possible difference.
After determining the light source and lighting mode, the final step in designing the lighting system is the most important step in the experiment. In the real world, the target surface characteristics are not single, the interaction between the light source and the target object is often a combination of various phenomena, some difficult to use theory to make accurate description. The experiment can verify the correctness of the design, and in the experiment, change the factors affecting the image, such as: the position of the target in the field of view and the angle of the light source, the brightness of the light source, can further verify the reliability and stability of the system.
In short, the design of the light source system, the ultimate goal is: To maximize the contrast of the characteristics of interest, to suppress and reduce the impact of other parts of the target, to suppress the impact of the external environment.
The following is a brief introduction to some basic concepts in machine vision lighting technology. Light and dark field illumination
Light and Dark field illumination describes the relative position of the light source and the camera, and is one of the common terminology in machine vision lighting technology.
In the case where the camera is perpendicular to the target being detected, the definition of light and dark field illumination is: Assume that the detection target has a flat, a smooth surface (mirror), the camera disposed above the target center, visible from the diagram, the "W" two "V" in the light emitted by the target surface reflection, all fall within the lens range, called the Light field illumination, and from the "W" two "V" out of the light, the target surface reflection, No light falls within the range of the lens, called dark-dark lighting.
When the target surface is defective, such as: And so on, light field illumination, the defective part of the reflected light no longer fall into the scope of the lens, the formation of a low gray value area, contrast with the background; Dark field illumination in contrast, the reflected light in the defective part enters the lens, producing a high gray value area.
The concept of light and dark field illumination can also be extended to situations where the camera is not perpendicular to the target surface:
front illumination and backlight illumination
The front and backlight illumination are the relative positions of the camera, light source and the target being detected. As the name implies, the front lighting is the camera and the light source on the side of the target being detected, backlight is the camera and light source on the side of the detected target.
The front illumination can be represented by various means, which we can use in a single hemisphere light source on the target being detected. ：
The light source (the luminous body) can be the whole hemisphere, either a ring with a radius of r in the hemisphere or a part of the ring, the position of the ring can be anywhere on the sphere, and the light source can also be distributed on the side of the camera. Some of the most common front lighting methods are: Top Light
The top light illumination is light field illumination, the light source can be direct light or diffuse reflection light.
Single Side illumination Low Angle Light
Low-angle illumination is a dark-spot illumination, and the light source can be direct or diffuse.
Coaxial Light
Coaxial light is light field illumination, ideally, the light that shines on the surface of the target object is parallel to the imagery of the lens. Coaxial light suitable for the smooth metal surface of micro pits, scratches, cracks, burrs, bumps and other defects detection, or surface by reflection, absorption characteristics of different materials composed of the detection of the target object. The coaxial light is made up of half-transparent and half-mirrors, and the actual efficiency is only 25%.
Dome
Light is emitted from every angle of the illuminated hemisphere, so the dome is a very homogeneous light source that produces a shadow-free image. The diagram on the left is the schematic of the dome, and the schematic diagram of the actual light source on the right.
Suitable for uneven smooth surface printing, or text detection.
Backlight Illumination
When measuring the target size using backlight illumination, if the target edge is a surface, the measurement results are smaller than the actual size. In order to avoid this phenomenon, we should use collimated light source, such as: coaxial light or parallel light as backlight, using telecentric lens.
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