The light source

Machine vision system mainly consists of three parts: image acquisition, image processing and analysis, output or display. Image acquisition is the core of machine vision, image acquisition system is composed of light source, lens, camera three parts. The selection of light source and reasonable lighting can directly affect at least 30% of the imaging quality. So light source is a very important part of machine vision system.

role

Through the appropriate light source lighting design, the target information and background information in the image can be best separated, which can greatly reduce the difficulty of image processing algorithm segmentation and recognition, and improve the positioning and measurement accuracy of the system, so that the reliability and comprehensive performance of the system are improved. On the contrary, if the light source is not designed properly, it will lead to less effort in the design of image processing algorithm and imaging system. Therefore, the success or failure of light source and optical system design is the primary factor that determines the success or failure of the system.

Illuminate the target, improve the brightness of the target;

Highlight the measurement features and simplify the image processing algorithm;

To overcome the interference of ambient light, ensure the stability of the image and improve the signal to noise ratio of the image;

Improve the positioning, measurement, recognition accuracy of the vision system, and the speed of the system;

The complexity of the system design is reduced to form the most favorable imaging effect for image processing;

classification

1. Ring light source

The ring light source provides different illumination angles and different color combinations, which can better highlight the three-dimensional information of the object. High density LED array, high brightness; A variety of compact design, save installation space; Solve the problem of diagonally illuminated shadow; Optional diffuse plate light guide, uniform light diffusion. Application :PCB substrate testing, IC component testing, microscope lighting, liquid crystal correction, plastic container testing, IC printing inspection.

2. Back light source

High density LED array surface is used to provide high intensity backlight illumination, which can highlight the contour features of the object, and is especially suitable as a microscope payload. Red and white back light source, red and blue multi-purpose back light source, can be configured with different colors, to meet the multi-color requirements of different measured objects. Application: MEASUREMENT OF MECHANICAL PARTS size, electronic components, IC shape detection, film stain detection, transparent object scratch detection, etc.

3. Strip light source

The strip light source is the preferred light source for the object with large square structure. Color can be matched according to demand, free combination; The irradiation Angle and installation can be adjusted at will. Application: Metal surface inspection, image scanning, surface crack detection, LCD panel detection, etc.

4. Coaxial light source

The coaxial light source can eliminate the shadow caused by the uneven surface of the object, so as to reduce the interference. Part of the spectrometer design, reduce light loss, improve imaging clarity, uniform irradiation of the surface of the object. Application: THE SERIES LIGHT SOURCE is MOST suitable FOR highly reflective objects, such as metal, glass, film, wafer surface scratch detection, chip and silicon wafer damage detection, Mark point positioning, packaging bar code recognition.

5. Spherical integral light source

The inner wall of the hemispherical surface with integral effect uniformly reflects the light emitted 360 degrees from the bottom, so that the illuminance of the whole image is very uniform. Application: Suitable for curved surface, concave and convex surface, arc surface detection, or metal, glass surface reflective object surface detection.

6. Linear light source

Ultra high brightness, using cylindrical lens to collect light, suitable for all kinds of pipeline continuous detection occasions. Application: Special for array camera lighting and AOI.

7. Point light source

High power LED, small size, high luminous intensity; Fiber halogen lamp substitute, especially suitable for lens coaxial light source; Efficient heat dissipation device, greatly improve the service life of light source. Application: Suitable for telecentric lenses, chip testing, Mark point positioning, wafer and LCD glass substrate correction.

8. Combination of strip light source

Four sides are equipped with strip light, each side of the independent and controllable lighting; The lighting Angle can be adjusted according to the requirements of the object under test, with wide applicability. Application cases :CB substrate detection, IC element detection, solder inspection, Mark point positioning, microscope lighting, packaging bar code lighting, spherical object lighting, etc.

LED Light source features

At present, the main machine vision light source is LED (light-emitting diode), which is widely used in the industry due to its high degree of freedom in shape, long service life, fast response speed, good monochrome, diverse colors and high comprehensive cost performance ratio:

Freedom of shape

An LED light source is a combination of many individual leds, so it can be made into more shapes than other light sources, and it is easier to design the shape and size of the light source for the user's situation.

Long service life

In order for the image processing unit to obtain accurate and repeatable measurements, the lighting system must be able to provide stable image input for a considerable period of time. LED light source after 10,000 to 30,000 hours of continuous operation, but far better than other types of light source effect. In addition, with the control system to make it work intermittently, can inhibit the luminescent tube heating, life will be extended twice.

Fast response speed

LED light tube response time is very short, the real meaning of response time is to ensure that the work between multiple light sources or different areas of a light source can be switched according to the requirements. When the special controller is used to power the LED light source, the time to reach the maximum illumination is less than 10s

Various color

In addition to the shape of the light source, another aspect of getting a stable image input is choosing the color of the light source. Even the same shape of the light source, because of the color of different images will be very different. In fact, how to use the technical characteristics of light source color to get the best contrast image effect has been the main direction of light source development.

Low comprehensive operating cost

Choose cheap and performance is not guaranteed products, the initial investment savings will soon be offset by the cost of daily maintenance, repair. Other lights not only consume 2-10 times as much power as leds, but also need to be replaced almost every month, wasting a lot of valuable time for maintenance engineers. Moreover, the more light sources are put into use, the greater the cost of device replacement and labor, so it is economical to choose long-life LED light sources in the long run.

How to choose

Analysis of environmental requirements

1. Understand the requirements of system structure and operation from customers, and determine the spatial structure relationship between industrial cameras, light sources and measured objects.

The determined parameters are: field of view (FOV), working distance (WD);

2, space structure: direct, lateral, back irradiation;

Light source of direct beam structure -- partial annular light source, coaxial light source, dome light source;

The light source of lateral projection structure -- partial ring light source, bar light source, line light source, point light source;

Back light - square backlight, bar backlight.

Object surface texture and color analysis

1. Is the surface of the object curved or flat? Is the surface of the object smooth? Is it reflective?

Dome light source should be used for surface detection, coaxial light source should be used for smooth plane, and bright field light source should be used for rough plane.

2. How is the transmittance of the object?

Objects with good light transmission can use IR light source.

3, distinguish the background (we do not need to detect) is what color, foreground (we want to detect) is what color?

A good light source IS one that has a good contrast - the background and foreground are clear.

4, the foreground color changes

Color light source and white light source should be used.

Choose to summarize

1. Need more contrast between foreground and background? Consider using a black and white camera with a color light source

2. The problem of ambient light? - Try using a monochromatic light source and a filter

3. Flash surface? -- Try scattering dome light

4. Shiny, flat, but rough surface? - Try to scatter light with coaxial

5. Look at the shape of the surface? -- Consider using dark field of view (low Angle)

6. When testing plastic -- try ultraviolet or infrared light

7. Need to see features through reflective surfaces? -- Try using a low Angle line light source (dark field)

8. A combination of light sources can sometimes solve the problem

9, stroboscopic can produce light 20 times stronger than steady lighting

How to polishing

Lighting for general purposes:

Annular or point lighting is generally used for general lighting. The ring lamp is a commonly used general purpose lighting method that can be easily mounted on a lens to provide adequate illumination to diffuse surfaces.

Backlight lighting:

Backlighting is where the light source is placed on the back of the object relative to the camera. This is very different from other lighting methods because the image is not analyzing the incoming light but the incoming light. The backlighting creates a strong contrast. Surface features may be lost when backlighting is applied. For example, you can use backlighting to measure the diameter of a coin, but you can't tell which side it is.

Coaxial lighting:

Coaxial illumination is when light hits the surface of an object in the same direction as the axis of the camera. Coaxial lighting uses a special semi-reflective mirror to reflect the light source toward the lens axis of the camera. A semi-reflective mirror only allows light to pass that is reflected from the surface of the object perpendicular to the lens. Coaxial lighting is useful for achieving uniform illumination on flat surfaces with specular features. It is also possible to highlight parts of the surface that change in Angle, because light reflected from surfaces that are not perpendicular to the camera lens will not enter the lens, resulting in a darker surface. Continuous DIFFUSE ILLUMINATION: Continuous diffuse illumination is applied to objects where the surface is reflective or where the surface has complex angles. Continuous diffuse illumination is the uniform illumination of a hemispherical shape to minimize shadows and specular reflections. This lighting is useful for lighting fully assembled circuit boards. This light source can achieve uniform illumination in the range of 170 solid angles.

Dark field lighting:

Dark field illumination provides low Angle illumination relative to the surface of an object. Use the camera to shoot the mirror in its field of view. If the light source can be seen in the field of view, it is considered to be in the bright field of illumination. On the contrary, if the light source can not be seen in the field of view, it is dark field of illumination. Therefore, whether the light source is light or dark depends on the location of the light source. Typically, DARK FIELD LIGHTING IS USED TO ILLUMINATE AREAS OF A SURFACE THAT HAVE PROTRUSIONS OR CHANGES IN SURFACE TEXTURE.

Structure light:

Structural light is light with certain geometric shapes (e.g. lines, circles, squares) projected on the surface of an object. Typical structured light involves lasers or optical fibers. Structured light can be used to measure the distance from the camera to the light source. Multi-axis lighting: In many applications, multiple lighting techniques are required to achieve different contrast values for different features in the visual field.