A brief discussion on the matching and selection of machine vision light sources

A Brief Discussion on Light Sources in Machine Vision

For the machine vision industry, the performance of the detection system lies in the processing and optimization of the algorithm. The basis of algorithm processing lies in the quality and difference of imaging, and the determining factor for the imaging effect lies in the matching and selection of the light source.

There are also various types of light sources:

1. According to color, there are red light, green light, warm white light, cold white light, etc.

2. According to the appearance, there are strip light, ring light, dome light, etc.

3. According to the characteristics, there are: point light source, line light source, shadowless light source, combined light source, etc.

From different industries to different products, everyone has their own selection criteria.

01For flexible packaging products

Mainly used to detect some printing defects, dome light is the best choice at this time, with high brightness and good uniformity, and the best imaging effect for printed products.

02For filter membrane or aluminized membrane products

The product is characterized by its strong reflective properties. You should pay more attention to the choice of light source. Due to the strong reflectiveness of the product, the image will be darker, and from the external appearance, it often gives people the judgment that the light source is not bright enough. In fact, the main reason is that the trajectory of the light source light path and the trajectory of the camera imaging are not on the same trajectory, which leads to the dark image.

If strip light is used, most of the light paths are outside the trajectory of the camera scan line, resulting in dark images, as shown in the following figure:

If dome light is used, since the light source is scattered inside the dome, a small part of the light trajectory will be in the camera's scan line trajectory, and the brightness will be slightly improved, as shown in the following figure:

However, in actual production activities, the above two methods cannot achieve good imaging effects. In actual production, the dome plus angle solution is often used. As shown in the following figure:

When there is a certain angle between the camera scanning line and the product feeding direction, a part of the light path of the dome light must overlap with the trajectory of the scanning line, thus obtaining a better image.

In fact, this solution is also a compromise solution, mainly for the mechanical design and installation convenience of the detection equipment. In this solution, many optical paths of the dome light are wasted.

Excluding the cost and mechanical design and installation difficulty issues, the imaging solution for products with aluminum-coated film or pure aluminum film should be as shown in the figure below.

This solution makes full use of the brightness of strip light, but has extremely high requirements on the size and design difficulty of the machinery.

03For glass products

The reflectivity of glass products has almost reached the state of full reflectivity, which places greater demands on the selection and solution of light sources. Currently, most solutions adopted are to use coaxial light sources or combined light sources.

There are also solutions that use a combination of dome light and coaxial light, which some manufacturers call a universal light source.

Light source plays a vital role in machine vision imaging. Choosing a good light source plays a decisive role in imaging.

A good imaging light source system is one of the keys to the success of the entire detection system and plays a very important role. It is not just about illuminating the object. First of all, the light source must be able to truly reproduce the color of the product without causing color distortion. This requires that the spectrum of the light source is close to that of daylight, while ensuring sufficient brightness and uniform brightness in the entire lighting area. Secondly, the design of the light source lighting system must avoid some high reflectivity areas on the product that cause a reduction in detection accuracy. Finally, the quality of the sampled image and the strength of the subsequent detection capabilities depend largely on the brightness, uniformity and stability of the light source. The reliability and life of the detection system also depend to a large extent on the reliability and life of the light source.