Machine Vision series-light source selection method strategy

To determine the quality of Machine Vision lighting, you must first understand what the light source needs to do! Obviously, the light source should not only make the detection part "visible" by the camera ". Sometimes a complete machine vision system cannot support work, but simply optimizing the light source can make the system work normally. Contrast: contrast is very important for machine vision. The most important task of lighting for Machine Vision Applications is to generate the maximum contrast between the features to be observed and those to be ignored, so as to facilitate feature differentiation. Contrast is defined as there is a sufficient gray difference between a feature and its surrounding area. Good lighting should ensure that the features to be detected are prominent in other backgrounds. Brightness: When two light sources are selected, the best choice is the brighter one. When the light source is not bright enough, there may be three bad cases. First, the signal-to-noise ratio of the camera is not enough. Because the brightness of the light source is not enough, the contrast of the image is not enough, and the possibility of noise on the image increases immediately. Second, if the brightness of the light source is not enough, the aperture must be increased to reduce the depth of field. In addition, when the brightness of the light source is insufficient, random light such as natural light will have the greatest impact on the system. Robustness: another method to test the light source is to check whether the light source is least sensitive to the position of the component. When the light source is placed in different regions or different angles of the camera's field of view, the result image will not change accordingly. A light source with strong directionality increases the possibility of mirroring reflection in the highlighted area, which is not conducive to subsequent feature extraction. In many cases, a good light source must have the same effect as that in the lab in actual work. A good light source needs to make the features you need to find very obvious, except that the camera can capture components, A good light source should be able to produce the maximum contrast, brightness is sufficient, and is not sensitive to component position changes. After selecting the light source, the rest of the work will be much easier! Machine Vision Applications focus on reflected light (unless backlight is used ). The geometric shape, luster, and color of an object's surface determine how light is reflected on the object's surface. One of the secrets of light source control for Machine Vision Applications is how to control light source reflection. How can we control the reflection of the light source. Therefore, in Machine Vision Applications, when the light source is incident to the surface of a given object, the most important aspect of understanding the light source is to control the light source and its reflection. Predictable Light Source: when the light source is incident to the surface of the object, the reflection of the light source can be predicted. The light source may be absorbed or reflected. Light may be completely absorbed (black metal material, difficult to illuminate the surface) or partially absorbed (resulting in color changes and brightness differences ). Light that is not absorbed will be reflected, and the angle of the incident light is equal to the angle of the reflected light. This scientific law greatly simplifies the Machine Vision light source, the ideal effect can be achieved by controlling the light source. Object Surface: if the light source is transmitted in a predictable manner, what makes the design of the light source of machine vision so tricky? What complicate Machine Vision lighting is caused by changes in the surface of an object. If the surfaces of all objects are the same, there is no need to use different light source technologies when solving practical applications. However, because the surface of an object is different, we need to observe the surface of the object in the field of view and analyze the reflection of the incident of the light source. Control Reflection: As mentioned above, if the reflected light can be controlled, the image can be controlled. This is no more powerful than that. Therefore, in the light source design involving Machine Vision Applications, the most important principle is to control where the light source is reflected to the lens and the degree of reflection. The light source Design of Machine Vision is the research of reflection. In visual applications, when observing an object to determine what kind of light source is needed, you must first ask yourself the question: "How can I display an object ?" "How can I apply a light source to reflect the required light to the lens to obtain the appearance of an object ?" Factors that affect the reflection effect include the position of the light source, the texture of the object surface, the geometric shape of the object surface, and the uniformity of the light source. Position of the light source: Since the light source is reflected according to the incident angle, the position of the light source is very important for obtaining high-contrast images. The goal of a light source is to make the characteristics of interest different from the background reflection of the light source. Predicting how the light source is reflected on the surface of an object determines the position of the light source. Surface Texture: the object surface may be highly reflected (mirror reflection) or highly diffuse. The main factor determining whether an object is a mirror reflection or a diffuse reflection is the smoothness of the object surface. A diffuse reflection surface, such as a non-smooth paper, has a complex surface angle and is very bright when viewed with a microscope, this is because the surface angle of the object changes, causing the light source to shine on the surface of the object and be dispersed. A smooth paper has a smooth surface, which reduces the angle of the object surface. The light source shines on the surface of the light source and is reflected according to the incident angle. Surface Shape: a spherical surface reflection light source is not nearly the same as a plane object. The more complex the shape of an object is, the more complex the light source changes on its surface. Corresponding to a polished mirror surface, the light source needs to be illuminated at different angles. Illumination from different angles can reduce light and shade. Uniformity of Light Source: uneven light may cause uneven reflection. Uniformity is related to three aspects. First, the field of view should be even in the camera's field of view. Simply put, the dark area of the image is the lack of reflected light, And the highlight is that the reflection is too strong. Uneven light may make some areas in the field of view more light than other areas. As a result, the reflection on the object surface is uneven (assuming that the reflection on the object surface is the same ). A uniform light source will compensate for the angle change of the object surface. Even if the geometric shape of the object surface is different, the reflection of the light source in each part is even. Application of Light source technology: the light source technology is used to design the geometric and position of the light source to make the image have contrast. The light source will highlight areas of interest that require machine visual analysis. By selecting the light source technology, you should be concerned with how the object is illuminated and how the light source is reflected and scattered. The following are six lighting technologies: general lighting, backlight, coaxial (coaxial), continuous diffuse reflection, dark area and structured light. General purpose lighting: General Lighting generally uses ring or point lighting. Ring light is a common generic lighting method. It can be easily installed on the lens to provide sufficient illumination for the diffuse reflection surface. Backlight lighting: place the light source on the back of an object relative to the camera. This lighting method is very different from other lighting methods because the image analysis is not light emitting but light emitting. Backlight lighting produces a strong contrast. When backlight technology is applied, the surface features of objects may be lost. For example, the backlight technology can be used to measure the diameter of a coin, but the front and back of the coin cannot be determined. Coaxial lighting: coaxial lighting is the same direction as the axial direction of the camera. illumination is directed at the surface of the object. Coaxial lighting uses a special semi-reflective mirror reflection light source to the camera's lens axis direction. The semi-reflected mirror only allows the light source from the surface of the object to pass through the lens. Coaxial lighting technology is useful for even illumination of flat objects with a mirrored surface. In addition, this technology can highlight the surface angle variation part, because the light not reflected perpendicular to the surface of the camera lens does not enter the lens, resulting in a darker surface. Continuous diffuse reflection lighting: continuous diffuse reflection lighting is used to reflect the surface of an object or has a complex angle on the surface. Continuous diffuse reflection lighting uses semi-spherical Uniform Illumination to reduce shadow and mirror reflection. This lighting method is very useful for fully assembled circuit board lighting. This type of light source can achieve a uniform illumination of 170 in the three-dimensional angle range. Dark area lighting: the dark area lighting provides low angle lighting relative to the object surface. Use a camera to take a mirror so that the camera can see the light source in its field of view. If you can see the light source in the field of view, the light source is considered to enable the bright area lighting. On the contrary, the light source is invisible in the field of view. Therefore, whether the light source is in the bright area or in the dark area is related to the position of the light source. Typically, dark area lighting is applied to illumination of the protruding part of the surface or illumination of the surface texture change. Structured Light: a type of light projected on the surface of an object with a certain geometric shape (such as linear, circular, square ). Typical structured light involves laser or optical fiber. Structured Light can be used to measure the distance from the camera to the light source. Multi-axis lighting: in many applications, multiple lighting technologies are required to make different features in the field of view show different contrast. Select the Light Source: Once the lighting technology is selected, the next step is to select the light source. The light source must have sufficient uniformity and good stability. In the application of machine vision, the following characteristics of light source should be taken into account: spectral characteristics: The color of the light source and the color of the surface of the measurement object determine the size and wavelength of light energy reflected to the camera. White light or a special spectrum may be an important factor in extracting feature information from other colors. When multiple color features are analyzed, color temperature is an important factor when selecting a light source. For example, halogen lamps are more yellow than xenon lamps. Efficiency: some light sources are very efficient. Compared with energy consumption, they emit more light energy, such as fluorescent lamps. Tungsten lamps produce a considerable amount of heat and consume a lot of energy. Low-efficiency light sources cause local overheating and waste a lot. Generally, the higher the temperature of the light source, the shorter its lifetime and the higher energy consumption. Life characteristics: the light source generally needs to be used for multiple hours. A light source with a life cycle of 1000 hours can only last for about one week in two shifts. Replacement of light bulb is required. The LED light source is a popular light source. It can work continuously for a long time and can be operated continuously for about 100,100 hours. For most light sources, with the aging of the light source, the energy released by the light source will decrease. Depending on the type of the light source, the decrease of light energy may be full or very fast. Changes in solar energy output may also affect spectral characteristics. When the aging speed of the light source affects the image processing result, you can pay attention to the changes in the light source. Cost: Many light sources need to be replaced during use of the visual system. If the light source is expensive, the later cost may be increased during the use of machine vision. In addition, it is easier to purchase light sources in the market.