In machine vision inspection systems, the selection of light sources directly affects the quality of input data and application effectiveness, playing a crucial role in the overall level of product inspection. Due to the lack of universal lighting equipment for machine vision inspection, it is necessary to perform lighting based on the samples provided by the customer, determine the type and model of the light source, and proceed with the next step of online debugging. The selection of light sources should consider the following characteristics:
1、 Brightness. When choosing between two light sources, the best choice is the brighter one.
When the light source is not bright enough, three bad situations may occur. Firstly, the signal-to-noise ratio of the camera is insufficient; Due to insufficient brightness of the light source, the contrast of the image is inevitably insufficient, and the likelihood of noise appearing on the image increases accordingly. Secondly, if the brightness of the light source is not sufficient, it is necessary to increase the aperture, thereby reducing the depth of field. Thirdly, when the brightness of the light source is insufficient, random light such as natural light will have the greatest impact on the system.
2、 Uniformity of light source. Uneven light can cause uneven reflection. Uniformity is related to three aspects.
1. For the field of view, the camera's field of view should be uniform. Simply put, the dark areas in the image lack reflected light, while the highlight is that the reflection is too strong here.
2. Uneven light can cause some areas within the field of view to have more light than others. This causes uneven reflection on the surface of the object (assuming that the reflection of light on the surface of the object is the same).
3. A uniform light source will compensate for the angular changes on the surface of the object, and even if the geometric shape of the object surface is different, the reflection of the light source on each part is uniform.
3、 Spectral characteristics. The color of the light source and the color of the surface of the measured object determine the size and wavelength of the light energy reflected onto the camera. White light or a specific spectrum may be an important factor in extracting feature information from other colors. When analyzing multi color features and selecting light sources, color temperature is a relatively important factor.
4、 Contrast. Contrast is very important for machine vision. The most important task of lighting in machine vision applications is to generate maximum contrast between features that need to be observed and image features that need to be ignored, making it easy to distinguish features. Contrast is defined as having sufficient grayscale differences between a feature and its surrounding area. Good lighting should ensure that the features to be detected protrude from other backgrounds.
5、 Efficiency. Some light sources have high efficiency and emit more light energy compared to energy consumption, such as fluorescent lamps. Tungsten lamps generate a considerable amount of heat and consume a significant amount of energy. Low efficiency light sources generate local overheating and waste a lot. The higher the temperature of a general light source, the shorter its lifespan and the higher its energy consumption.
6、 Lifetime characteristics. Light sources generally require continuous use. In order to maintain consistent and accurate image processing, the visual system must ensure that stable and consistent images are obtained for a long time.
A suitable light source can provide a stable and high contrast image for the image acquisition system, further improving detection accuracy, reducing false positives and missed detections. Therefore, great attention should be paid to the selection of light sources, fully considering their various characteristics, and making a reasonable choice.
