Ultimate guide for visual inspection

The basis of visual inspection is “visual inspection”, in which parts and products are checked by human eyes and quality judgment is made. Here, we will explain the basics of visual inspection, advantages and disadvantages, the limits of visual inspection in visual inspection, and the advantages of automating with image sensors.

What is a visual inspection?

A visual inspection is literally an inspection performed by the human eye. It is the most familiar inspection method and can be said to be the basis for judging the quality of parts and products. In addition, the inspection method that judges the quality by the five human senses (visual, auditory, taste, smell, touch) including the visual inspection is called “sensory inspection”, and the visual inspection is the most common inspection method.

What is a sensory test?

Sensory test is a test method in which the inspector judges the quality by regarding the five human senses (sight, hearing, taste, smell, and touch) as an instrument. For example, shape defects and scratches on automobile parts, missing characters and bleeding on printed matter, texture and surface gloss for cloth products, odor and taste for food, sound quality and volume for musical instruments, etc. Pass / fail judgment is made by human sense while comparing with.

Advantages and disadvantages of visual inspection

I will explain the advantages and disadvantages of visual inspection. The advantage is that no special equipment is required, so there are no initial costs such as equipment costs and development costs. As long as you have an inspector, you can start a visual inspection from that day.

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The disadvantage is that human error is likely to occur because the pass / fail judgment is made by human senses, and the judgment criteria for defective products vary depending on the inspector. The inspection may be affected by the environment such as the lighting of the inspection table and the inspection room, and the work accuracy and speed may differ due to mental and physical fatigue as a person. It is difficult to eliminate human error and variability in judgment criteria even with thorough education and tools such as Pokayoke and free samples.

In addition, the initial cost can be kept low compared to the automated optical inspection using image sensors, but it is difficult to secure human resources in Japan today, and if you hire an inspector, labor costs will be incurred. Another disadvantage is that it takes time to prepare tools such as education and free samples.

Limitations of visual inspection and the flow to automation

While visual inspection can be carried out easily, it is difficult to secure experienced human resources, and there is a problem that it takes time and effort such as education to become a full-fledged person. In Japan, where the population is declining and the population is aging, the number of experienced inspectors is decreasing, and young people are not growing up.

In addition, in recent years, manufacturing has become smaller and more integrated with parts and products, and the required quality is also increasing. Furthermore, in order to meet the needs of users, high-mix production is increasing and the product cycle is shortened, so the burden of inspection is increasing.

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Therefore, attention is being paid to the automated optical inspection using image sensors. For example, if the inspection process is automated using an image sensor, problems such as human error, variation in judgment criteria, securing personnel, labor for education, and labor costs can be solved. Automation also enables high-speed in-line inspections, ensuring quality and increasing production efficiency.


AOI means automated optical inspection. For 2D type AOI, the inspection method using the line scan method using a line sensor camera is adopted. In the line scan method, the entire board is imaged while switching the lighting at high speed, enabling high-speed inspection. You can also use UV lighting to inspect for uncoated or scattered coatings, and to automatically detect defects such as surplus parts and dust in unexpected boards.

The 3D type AOI can detect the quantitative height information of the parts in the board and recognize the parts position. By visualizing in 3D, it is possible to inspect the inclination and floating of parts, which was difficult with the 2D type, and it is possible to improve the accuracy of solder paste inspection.