Machine Vision Light Source
What is a machine vision light source
In the machine vision system, the target information of the measured object can be distinguished from the background information by appropriate light source illumination to obtain a high-quality, high-contrast image, which can reduce the difficulty of the image processing algorithm, while improving the accuracy and reliability of the system Sex.
In actual projects, the actual imaging effect of the image is closely related to the lighting conditions. It is no exaggeration to say that it directly affects the quality of the input data and at least 30% of the application effect, because good lighting conditions can achieve a good imaging effect, thereby effectively distinguishing the target object from the background, reducing the difficulty of recognition.
Three tips for light source selection
Tip 1 Use the characteristics of wavelength
Infrared light has a long wavelength and strong penetration characteristics. Infrared light can filter the interference of organic paint on the surface of the product, detect surface scratches, and can also penetrate dark oral liquid to detect internal impurities.
The short wavelength of ultraviolet light has the characteristics of high diffusivity and excited fluorescence. It is suitable for the positioning of Mark points on the surface of transparent objects; router character detection, the ink has low reflectivity for short wavelength ultraviolet; UV colloid detection; invisible code reading.
Technique 2 Superposition of colors (complementary colors, adjacent colors)
Complementary colors: Symmetrical colors in the color circle are superimposed on the black and white camera to present dark colors
Adjacent colors: adjacent or the same color in the color circle is superimposed on the black and white camera to present a light color
Tip 3 Different materials have different reflectivity for light sources in different wavelength bands
Copper and gold have weak reflections for light sources with short wavelengths
Silver and aluminum have the largest reflective difference at a wavelength of about 850nm
Blue light source can better make the difference between copper, gold and aluminum
A complete visual inspection system mainly includes an image acquisition part and an image analysis part. The image acquisition part mainly includes industrial cameras, industrial lenses, and machine vision light sources. Today we mainly introduce the related basic knowledge and selection techniques of machine vision light sources.
First of all we need to understand what role the light source in machine vision plays
Illuminate the target and increase the brightness;
Form an imaging effect conducive to image processing, reduce the complexity of the system and the requirements for image processing algorithms;
Overcome ambient light interference, ensure image stability, improve system accuracy and efficiency;
Through proper light source lighting design, the target information and background information in the image can be optimally separated, which not only greatly reduces the difficulty of image processing algorithms, but also improves the accuracy and reliability of the system, but unfortunately, there is currently no universal The machine vision lighting system can cope with different detection requirements. Therefore, for each specific case, it is necessary to design an adaptive lighting device to achieve the best results. Unsuitable lighting will cause many problems, such as machine vision light sources. Important, but often overlooked by many people.
At present, machine vision light sources mainly use LEDs (light emitting diodes). Due to their high degree of freedom in shape, long service life, fast response speed, good monochromaticity, diverse colors, and high comprehensive cost performance, etc., they are widely used in the industry:
Freedom of shape
An LED light source is composed of many single LEDs, so compared with other light sources, it can be made into more shapes, and it is easier to design the shape and size of the light source according to the user's situation.
long lasting
In order for the image processing unit to obtain accurate and reproducible measurement results, the lighting system must ensure that a stable image input can be provided for a relatively long time. After 10,000 to 30,000 hours of continuous operation, the brightness of the LED light source decreases, but it is far better than other types of light sources. In addition, using the control system to make it work intermittently can suppress the heating of the light emitting tube, and the service life will be doubled.
quick response
The response time of the LED light tube is very short. The real meaning of the response time is to ensure the work switching between multiple light sources or different areas of a light source as required. When a special controller is used to power the LED light source, the time to reach the maximum illumination is less than 10s
Various colors
In addition to the shape of the light source, another aspect of obtaining stable image input is to choose the color of the light source. Even light sources of the same shape will have very different images due to different colors. In fact, how to use the technical characteristics of the color of the light source to obtain the best contrast image effect has always been the main direction of light source development.
Low integrated operating costs
Choose low-cost products without guaranteed performance, and the initial investment savings will soon be offset by daily maintenance and repair costs. Not only do other light sources consume 2-10 times the power of LED light sources, they are replaced almost every month, wasting a lot of precious time for maintenance engineers. Moreover, the more light sources that are put into use, the greater the cost of device replacement and labor. Therefore, it is very economical to choose an LED light source with a long life in the long run.
Basic knowledge of machine vision lighting technology——
Irradiation
Choosing different light sources, controlling and adjusting the direction of the incident light irradiating the object is the most basic parameter of machine vision system design, it depends on the type of light source and the position relative to the object, there are generally two kinds of The way: direct light and diffuse light, all other ways are extended from these two methods.
Direct light: The incident light basically comes from one direction, the angle of incidence is small, it can project the shadow of the object;
Diffuse light: The incident light comes from multiple directions, even in all directions, it will not cast a significant shadow.
Reflection method
Objects reflect light with two different reflection characteristics: direct reflection and diffuse reflection
Straight reflection: The reflection angle of light is equal to the angle of incidence. Direct reflection is sometimes very versatile, and sometimes it can produce extremely strong glare. In most cases, specular reflections should be avoided.
Diffuse reflection: Light shining on an object diffuses from all directions. In most practical cases, diffuse light is formed within a certain angle range and depends on the angle of incident light.
colour
A large part of the electromagnetic spectrum in the spectrum is visible to the human eye. Electromagnetic radiation in this wavelength range is called visible light, and the range is between 400nm and 760nm (some people can observe 380nm-780nm), that is, from purple to 380nm To red 780nm.
The color circle is to sort the colors in the visible light spectrum to form a purple connected to the other end of the red. Machine vision species are usually applied to the color circle and include 6 different colors, which are divided into two categories: warm and cool colors. The cold color comes from the blue tone. Usually the light with the opposite color temperature is irradiated. The image can reach the highest level of contrast. The light with the same color temperature can be effectively filtered out. Therefore, the flexible use of color temperature characteristics is very helpful for us to choose the light source.
Bright field and dark field
The bright field is the most commonly used lighting scheme, which is formed by direct light irradiation, and the dark field is mainly formed by low-angle or backlight lighting. For different project detection needs, choose different types of lighting methods. Generally speaking, the dark field will be Makes the background appear dark, and the detected object appears bright.
