A complete vision inspection system mainly consists of an image acquisition section and an image analysis section. The image acquisition section is mainly supported by industrial cameras, industrial lenses, and machine vision light sources. Today, we will mainly introduce the relevant basic knowledge and selection techniques of machine vision light sources.
First, we need to understand the role of light sources in machine vision.
Illuminate the target and increase brightness;
To create imaging effects that are conducive to image processing, and reduce the complexity of the system and the requirements for image processing algorithms;
Overcome ambient light interference, ensure image stability, and improve system accuracy and efficiency;
With proper lighting design, the target information and background information in an image can be optimally separated. This not only greatly reduces the difficulty of image processing algorithms but also improves the accuracy and reliability of the system. Unfortunately, there is currently no universal machine vision lighting system that can handle different detection requirements. Therefore, for each specific case, it is necessary to design an appropriate lighting device to achieve the best results. Inappropriate lighting will cause many problems. Machine vision light sources are so important, yet they are often overlooked by many people.
Currently, LEDs (light-emitting diodes) are the main light source for machine vision. They are widely used in the industry due to their high degree of shape freedom, long lifespan, fast response speed, good monochromaticity, diverse colors, and high overall cost-effectiveness.
Degrees of freedom of shape
An LED light source is composed of many individual LEDs, which allows it to be made into more shapes compared to other light sources, making it easier to design the shape and size of the light source to suit the user's needs.
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.
Fast response time
LEDs have a very short response time. The true significance of response time lies in its ability to ensure smooth switching between multiple light sources or different areas of a single light source. When a dedicated controller powers the LED light source, the time to reach maximum illuminance is less than 10 seconds.
Various 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.
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.
Basic knowledge of machine vision lighting technology
01 Irradiation Method
Choosing different light sources and controlling and adjusting the direction of incident light illuminating the object are the most basic parameters in machine vision system design. It depends on the type of light source and the position of the object. Generally speaking, there are two basic methods: direct light and diffused light. All other methods are derived from these two methods.
Direct light: The incident light comes from basically one direction and has a small angle of incidence; it can cast shadows on objects.
Diffuse light: Incident light comes from multiple directions, or even all directions, and it does not cast obvious shadows.
02 Reflection Method
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 can be very useful at times, but it can also produce extremely strong glare. Specular reflection should be avoided in most cases.
Diffuse light: Light that strikes an object diffuses outwards in all directions. In most practical cases, diffuse light is formed within a certain angular range and depends on the angle of the incident light.
03 Color
A large portion 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.
04 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.
05 Light Source Classification
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 quite common, while ultraviolet light is used less frequently for various reasons.
b) Appearance
Manufacturers categorize light sources based on their shape and 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.
