In machine vision inspection systems, the choice of light source directly affects the quality of input data and application effectiveness, playing a crucial role in the overall product inspection level. Since there are no universal lighting devices for machine vision inspection, it is necessary to determine the appropriate light source model and type based on the customer-provided samples before proceeding with online debugging. The following characteristics should be considered when selecting a light source:
1. Brightness. When choosing between two light sources, the best choice is the brighter one.
When the light source is insufficiently bright, three undesirable situations may arise. First, the camera's signal-to-noise ratio will be inadequate; due to insufficient light intensity, image contrast will inevitably be insufficient, and the likelihood of noise appearing in the image will increase accordingly. Second, insufficient light intensity necessitates a larger aperture, thereby reducing the depth of field. Third, when the light source is insufficient, random light, such as natural light, will have the greatest impact on the system.
II. Light Source Uniformity. Uneven light will cause uneven reflection. Uniformity relates to three aspects.
1. Regarding the field of view, it should be uniform within the camera's field of view. Simply put, dark areas in the image indicate a lack of reflected light, while bright areas indicate excessive reflection.
2. Uneven light will cause some areas of the field of view to receive more light than others. This results in uneven reflection of light from the surface of an object (assuming that the reflection of light from the object's surface is the same).
3. A uniform light source compensates for changes in the angle of an object's surface. Even if the geometry of the object's surface is different, the reflection of the light source is uniform in all parts.
III. Spectral Characteristics. The color of the light source and the color of the object being measured determine the amount and wavelength of light reflected to the camera. White light or a specific spectrum may be a significant factor when extracting feature information for other colors. When analyzing multi-color features, color temperature is a crucial factor when selecting a light source.
IV. Contrast Ratio. Contrast ratio 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 the image features to be ignored, thereby facilitating feature differentiation. Contrast ratio 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.
V. Efficiency. Some light sources are highly efficient, emitting more light energy relative to their energy consumption, such as fluorescent lamps. Tungsten lamps, on the other hand, generate a considerable amount of heat and consume a lot of energy. Inefficient light sources cause localized overheating, resulting in significant energy waste. Generally, the higher the temperature of a light source, the shorter its lifespan and the higher its energy consumption.
VI. Lifespan Characteristics. Light sources generally need to be used continuously. To ensure consistent accuracy in image processing, the vision system must guarantee the acquisition of stable and consistent images over extended periods.
A suitable light source can provide a stable, high-contrast image for the image acquisition system, further improving detection accuracy and reducing false detection and false negative rates. Therefore, the selection of a light source needs to be taken very seriously, and its various characteristics should be fully considered to make a reasonable choice.
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