The previous two articles have already introduced the relevant technical parameters of the machine vision light source and the main classification of the light source. Today we will introduce how to choose the visual light source and build the most suitable optical system when faced with the inspection project.
For a specific application, choosing the best lighting system from among numerous solutions is the key to the stable operation of the entire image processing system. Unfortunately, there is no universal lighting system that can adapt to various situations. However, due to the multiple shapes and multi-color features of LED light sources, we still find some ways to select visual light sources. The main methods are the following:
1. Look and Experiment (most commonly used)
Try to illuminate objects in different positions with different types of light sources, then observe the image through the camera
2. Scientific Analysis (Scientific Analysis - the most effective)
Analyze the imaging environment and recommend the best solution.
Observation test
I. Preparation
1, the test sample should be rich, there must be different types of good samples and problem samples, as much as possible to make the sample all the problems, in particular, to have the most difficult to detect the actual problem of the sample.
2, there are a variety of spare test light source, LED light source common categories, and different colors must have.
Second, the environmental requirements analysis
1. Understand the requirements of the system structure and operation from the customer, and determine the spatial structure of the industrial camera, light source, and the measured object. Determined parameters are: field of view (FOV), working distance (WD);
2, the spatial structure: direct, side shot, back exposure:
Direct structure of the light source - part of the ring light source, coaxial light source, dome light source;
Side-projection light source - part of the ring light source, strip light source, line light source, point light source;
Back illumination - square backlight, strip backlight.
Third, the surface texture and color analysis
1. Is the surface of the object a curved surface or a flat surface? Is the surface of the object smooth and reflective? Is it strong?
For curved surface detection, dome light sources are preferred, smooth planes are suitable for coaxial light sources, and rough planes are suitable for bright field light sources.
2. How is the transparency of the object?
Light-transmissive objects can use IR light sources.
3. What color is the background (we don't need to detect) and what color is the foreground (we want to detect)?
A good light source has a good contrast - the background and the foreground are clear.
4, changes in the color of the foreground
It is appropriate to use colored light sources and white light sources;
Fourth, physical testing
We can initially select a light source to start testing. Problems that may be encountered (related to light sources):
1, the foreground brightness is not enough? What color is the foreground? It is recommended to change the light source test with the same color as the foreground.
2. Is the brightness of the entire image not sufficient? It is recommended to move the light source closer to the object, or use a strobe light, or change the light source with a shorter wavelength.
3, the image is white? It is recommended to reduce the brightness of the end of the light source, or reduce the aperture of the lens.
4, the brightness of the image is not uniform? Check the light source is not flat?
Scientific analysis
This method mainly analyzes the direction, angle, and color of the light source according to the surface characteristics of the measured object and the characteristics of the defect to be detected. The user can combine the related introductions in the previous two articles. Of course, even after the analysis, Need to be experimentally verified.
At the end of the day, we will summarize some of the techniques used in light source selection.
1. Need greater contrast between foreground and background? – Consider using a black and white camera with a colored light source
2. Problems with ambient light? – Try using a monochromatic light source with a filter
3, flash surface? - Try to use diffuse dome light
4, flash, flat, but rough surface? - Try to use coaxial light scattering
5. Look at the shape of the surface? – Consider using a dark field (low angle)
6. When Testing Plastics – Try UV or Infrared Light
7. Need to see features through reflective surfaces? – Try low angle line light sources (dark field)
8, combined light source can sometimes solve the problem
9, strobe light can produce 20 times stronger light than ever
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