Contrast is essential in machine vision applications. The primary function of lighting in machine vision is to create maximum contrast between the feature that needs to be observed and the image features that should be ignored in order to facilitate feature differentiation. Contrast is defined as having a sufficient amount of grayscale difference between the feature and its surrounding area. Good lighting should ensure that the feature to be detected stands out against the background effectively.
Brightness is also an important factor to consider when selecting light sources. When choosing between two light sources, it is recommended to choose the brighter one. When the light source is not bright enough, there are three challenges that may arise. Firstly, the camera's signal-to-noise ratio may not be adequate. Secondly, the image contrast may be insufficient due to the lower brightness of the light source, which may lead to the appearance of noise in the image. Thirdly, if the light source is not bright enough, the aperture will need to be increased, which reduces the depth of field. In addition, the impact of natural light and other random light sources on the system will be more significant if the light source is too dim.
Robustness is another critical factor in evaluating the suitability of a light source. A good light source should have the least sensitivity to the position of the parts being inspected. The resulting image should not change significantly when the light source is placed in different areas or at different angles within the camera's field of view. A highly directional light source increases the likelihood of specular reflections in bright areas, which can be detrimental to subsequent feature extraction.
In summary, selecting the right light source is crucial in machine vision applications. The light source should be able to create maximum contrast between the feature of interest and its surroundings, be bright enough to ensure optimal performance without impacting the signal-to-noise ratio, and be robust enough to maintain consistent image quality regardless of the position of the parts being inspected.