First, we need to understand the role of light sources in machine vision:
1. Illuminate the target and increase brightness; 2. Create an imaging effect that is conducive to image processing, reducing system complexity and the requirements for image processing algorithms; 3. Overcome ambient light interference, ensure image stability, and improve system accuracy and efficiency;
Proper lighting design can optimize the separation of target and background information in an image, significantly reducing the algorithmic complexity of image processing and improving system accuracy and reliability. However, unfortunately, there is currently no universal machine vision lighting system that can handle diverse inspection requirements. Therefore, a customized lighting device must be designed for each specific case to achieve the best results. Inappropriate lighting can cause numerous problems. Despite its crucial importance, the importance of machine vision light sources is often overlooked. Currently, LEDs (Light Emitting Diodes) are the primary light sources used in machine vision. Their high degree of shape freedom, long lifespan, fast response speed, good monochromaticity, diverse colors, and high overall cost-effectiveness have led to their widespread application in the industry. I. Shape Freedom: An LED light source is composed of many individual LEDs, allowing for a wider range of shapes compared to other light sources. This makes it easier to design the shape and size of the light source to suit specific user needs.
II. Long service life
To ensure accurate and repeatable measurement results for the image processing unit, the lighting system must guarantee a stable image input over a considerable period. While LED light sources experience brightness decay after 10,000 to 30,000 hours of continuous operation, their performance remains significantly better than other types of light sources. Furthermore, intermittent operation via a control system can suppress LED heating and double their lifespan.
3. Fast response speed
LED light-emitting diodes have a very short response time. The true significance of response time is that it can ensure the switching between multiple light sources or different areas of a single light source as required. When a dedicated controller is used to power the LED light source, the time to reach maximum illuminance is less than 10 seconds.
IV. Diverse Colors
Besides the shape of the light source, another crucial aspect of obtaining stable image input is selecting the color of the light source. Even light sources of the same shape can produce significantly different images depending on their color. In fact, utilizing the technical characteristics of light source color to achieve optimal contrast in image quality has always been a major focus in light source development.
V. Low overall operating costs
Choosing cheap, low-performance products means that initial savings will quickly be offset by ongoing maintenance and repair costs. Other light sources not only consume 2-10 times more power than LEDs, but also require replacement almost monthly, wasting valuable time for maintenance engineers. Furthermore, the more light sources in use, the greater the costs for component replacement and labor. Therefore, choosing long-life LEDs is economical in the long run.
01、
1) Illumination Method: Selecting different light sources and controlling and adjusting the direction of the incident light illuminating the object are the most basic parameters in machine vision system design. This depends on the type of light source and the position of the object. Generally, there are two basic methods: direct light and diffused light. All other methods are derived from these two.
Direct light: The incident light comes from basically one direction with a small angle of incidence, and it can cast the shadow of the object; Diffuse light: The incident light comes from multiple directions, or even all directions, and it does not cast obvious shadows; 2) Reflection mode There are two different reflection characteristics of light reflected by an object: direct reflection and diffuse reflection.
Direct reflection: The angle of reflection of light is equal to the angle of incidence. Direct reflection is sometimes very useful, but can sometimes produce extremely strong glare. Specular reflection should be avoided in most cases. Diffuse reflection: Light that strikes an object diffuses in all directions. In most practical cases, diffuse light forms within a certain angular range and depends on the angle of incidence. 3) Color A large part of the electromagnetic spectrum is visible to the human eye. Electromagnetic radiation in this wavelength range is called visible light, ranging from 400nm to 760nm (some people can observe 380nm-780nm), that is, from violet 380nm to red 780nm.
A color wheel is a sequence of colors in the visible light spectrum, connecting red to violet at one end. In machine vision, the color wheel typically includes six different colors, divided into two main categories: warm colors and cool colors. Warm colors are composed of red tones, while cool colors come from blue tones. Usually, when illuminated with light of opposite color temperatures, the image can achieve the highest level of contrast. Illumination with light of the same color temperature can effectively filter out certain colors. Therefore, making flexible use of color temperature characteristics is very helpful in selecting light sources.
4) Bright field and dark field
Bright field is the most commonly used lighting scheme, which is formed by direct frontal illumination, while dark field is mainly formed by low-angle or backlighting. Different types of lighting methods are selected for different project inspection needs. Generally speaking, dark field will make the background appear dark, while the inspected object will appear bright.
5) Classification of light sources
Currently, the main classification methods are as follows:
a) Color: Commonly used light source colors are concentrated in the visible light range, mainly white light (composite light), red, blue, and green. Infrared light is also relatively common, while ultraviolet light is used less frequently for various reasons.
b) Shape: Manufacturers classify light sources according to their shape characteristics, which is also the current mainstream classification. Examples include ring light sources, low-angle ring light sources, strip light sources, dome light sources (bowl light sources/dome light sources), and surface light sources.
c) Working Principle/Characteristics: Based on different application methods or principles, light sources can be classified into shadowless light sources, coaxial light sources, point light sources, line light sources, backlight sources, combined light sources, and structured light sources, etc.
02、
1. General purpose lighting (direct lighting): Light shines directly onto the object to produce a clear image.
This type of light is effective when we need to obtain high-contrast images of objects. However, when we use it to illuminate shiny or reflective materials, it can cause mirror-like reflections. General lighting typically uses ring or spot lighting. Ring lighting is a common general lighting method that is easy to mount on a lens and can provide sufficient illumination to diffuse surfaces.
2. Dark field lighting: Dark field lighting provides low-angle illumination relative to the surface of an object.
An industrial camera is used to photograph a mirror within its field of view. If a light source is visible within the field of view, it is considered bright-field illumination; conversely, if the light source is not visible, it is dark-field illumination. Therefore, whether a light source is considered bright-field or dark-field illumination depends on its position. Typically, dark-field illumination is used to illuminate protruding parts of a surface or to illuminate variations in surface texture.
3. Backlighting: Light with a uniform field of view shines from behind the object, allowing the camera to see the side profile of the object.
Backlighting is commonly used to measure the size of objects and determine their orientation. Backlighting produces strong contrast. However, when using backlighting, surface features of an object may be lost. For example, backlighting can be used to measure the diameter of a coin, but it cannot determine whether the coin is heads or tails.
4. Diffuse lighting: Continuous diffuse lighting is used on the reflectivity of an object's surface or on surfaces with complex angles.
Continuous diffuse lighting uses a hemispherical, uniform illumination to reduce shadows and specular reflections. This lighting method is very useful for illuminating fully assembled circuit boards. This light source can achieve uniform illumination over a 170-degree solid angle.
5. Coaxial lighting: The formation of coaxial light – diverging light from a vertical wall is directed onto a beam splitter that points the light downwards, and the camera views the object from above through the beam splitter.
This type of light source is particularly helpful for detecting highly reflective objects and is also suitable for detecting objects whose area is not obvious due to shadows cast by the surrounding environment.
6. Polarizer: Only light energy with a vibration direction parallel to its permitted direction is allowed to pass through; the perpendicular component is blocked.
For a specific application, selecting the best lighting system from among many options is crucial for the stable operation of the entire image processing system.
