In machine vision system applications, the choice of light source is crucial, directly impacting the quality of input data and application effectiveness. A suitable light source provides stable, high-contrast images for the image acquisition mechanism. So how do you choose a suitable machine vision light source, and how do you evaluate the quality of a light source? The following will answer these questions one by one.
1. Contrast
Contrast is crucial for machine vision . The most important task of illumination in machine vision applications is to create maximum contrast between the features to be observed and those to be ignored in the image, thus facilitating feature differentiation. Contrast is defined as a sufficient grayscale difference between a feature and its surrounding area. Good illumination should ensure that the features to be detected stand out from the background.
2. Robustness
Robustness means good adaptability to the environment. A good light source needs to have the same effect in actual work as it does in the laboratory.
3. Brightness
When choosing between two light sources, the best choice is the brighter one. Insufficient light can lead to three problems. First, the camera's signal-to-noise ratio will be low; insufficient light will result in insufficient image contrast and a higher likelihood of noise. Second, insufficient light necessitates a larger aperture, reducing depth of field. Third, when the light source is insufficient, random light, such as natural light, will have the greatest impact on the system.
4. Predictable light source
When a light source strikes an object's surface, its response is predictable. The light source may be absorbed or reflected. Light may be completely absorbed (as in black metal materials where the surface is difficult to illuminate) or partially absorbed (causing changes in color and brightness). Unabsorbed light will be reflected, and the angle of the incident light equals the angle of the reflected light. This scientific law greatly simplifies machine vision lighting, as desired effects can be achieved by controlling the light source.
5. Uniformity
Uniformity is a very important technical parameter of a light source. A light source with good uniformity makes the system work stably.
6. Maintainability
Maintainability mainly refers to the ease of installation and replacement of the light source.
7. Lifespan and heat generation
The brightness of a light source should not decay too quickly, as this will affect system stability and increase maintenance costs. Lamps that generate a lot of heat will experience rapid brightness decay, and the lifespan of the light source will also be greatly affected.
How can we enhance image contrast using light sources?
Illuminating the area with light of the same or similar color can make the illuminated part brighter.
Illuminating a part with light of opposite or similar color can darken the illuminated area.
Different wavelengths have different penetrating power (penetration rate) on matter. The longer the wavelength, the stronger the penetrating power on the object, and the shorter the wavelength, the greater the diffusion rate on the surface of the matter.
Tips for choosing a light source
If you need greater contrast between the foreground and background, you can consider using a monochrome camera with colored light sources.
To address the ambient light issue, try using a single-color light source and a filter.
For a shimmering curved surface, consider using diffused dome lighting;
Flashes of light, flat but rough surfaces, try using coaxially scattered light;
Consider using dark field (low angle) to assess the surface shape.
When testing plastics, try using ultraviolet or infrared light;
To see the features through the reflected surface, try using a low-angle line light source (dark field).
When a single light source cannot effectively solve the problem, consider using a combination of light sources;
Strobe light can produce light 20 times brighter than constant illumination;
