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How to select a good lens for visual inspection?

How to select a good lens for visual inspection?

Cameras, lenses, and lighting. These three items are as important as the main body of the inspection device when performing “image processing inspection”. First of all, the camera is absolutely indispensable as an input device. Lenses and lighting contribute to achieving the same results in a constant environment. In this column, I focused on the lens. We will introduce simple evaluation methods and tips for practical use, not academically, so please use them as criteria for selecting a good lens.

Basic knowledge of lenses

First, I will briefly introduce the structure of the lens used for image processing inspection. Cameras usually consist of multiple lenses and have features such as aperture and focus adjustment. Lenses used in image processing inspection are usually classified into two types: CCTV and telecentric lenses.

Next, I would like to introduce some basic terms that you should know when talking about lenses.

Focus: The point at which light coming from infinity is focused through the lens.

Optical magnification: Magnification for forming an image on a solid-state image sensor such as a CCD or CMOS. The actual field of view varies depending on the size of the image sensor

Real field of view: The actual range that can be captured by the camera. Even if the optical magnification is the same, the actual field of view will change if the size of the image sensor changes.

WD (Working Distance): The distance from the tip of the lens on the object side to the work.

Depth of field: Range in which the lens is in focus.

Distortion: Distortion. A phenomenon in which the image shrinks or stretches around both sides of the image, and the originally straight line looks distorted.

Chromatic aberration: The position and magnification of the image differ depending on the wavelength (color) of light.
* When using a color camera, chromatic aberration is likely to occur because images with different wavelengths of RGB are combined with one lens.

Telecentric lens: A lens designed to allow the main ray to pass through the focal point of the optical system. There is no angle of view and distortion is reduced.

What are the points and methods for selecting lenses in image processing inspection?

The following 7 items are important points when selecting a lens for image processing inspection.

  • Distortion
  • Brightness
  • Peripheral illumination difference
  • Peripheral resolution
  • Telecentricity
  • Depth of field
  • Chromatic aberration

It is difficult to verify each of these items and manage them numerically, but by capturing and comparing images of the “lattice chart”, it is difficult to understand from the specifications notation in the catalog etc., which is important for practical use. You can see the “real ability” and easily understand the quality of the lens.

Next, We will introduce the lens selection method for image processing inspection.

Items to prepare

Camera, grid chart (preferably 1mm pitch with high dimensional accuracy), depth of view chart, LED lighting (red / blue 2 color backlight, blue bar lighting total 3 types)

The adjustment

Aperture of the lens is set to “open” and evaluated. (Only telecentricity evaluation is narrowed down) If there is focus adjustment, adjust to the farthest (FAR, infinity). The brightness of the image is adjusted according to the exposure time of the camera.

In fact, a telecentric lens with great merit

Telecentric lenses that are often pointed out only for their price and have a sense of restraint among users. However, considering the merits of telecentric lenses, it is more than enough to make up for the price. In a word, it can be summarized as “there is no angle of view”, but this can be characterized by the following three aspects.

This is an indispensable function for connector pin inspection, which is often published in lens manufacturer’s catalogs, etc., which allows a three-dimensional object to be imaged from the same direction even around the image. It can be said that the importance of this function is especially increasing in these days when the number of pixels is increasing and the field of view is expanding more and more. It is expected that this will advance the support for complicated shapes that could not be inspected until now.

There is no need to use unnecessarily large size coaxial lighting. This is a surprising blind spot, but large coaxial lighting is required to cover the field of view of ordinary lenses. Compensating for this with a telecentric lens can be said to enable significant cost savings in this area.

This is an important function that leads to stable operation without changing the scale factor (lens magnification) even if the WD changes. When the lens is tilted, the telecentric lens only looks like a rectangle, but the non-telecentric lens is trapezoidal. Therefore, the checker pattern cannot be placed on the actual inspection stage, and there may be cases where the optical axis cannot be aligned.

The advantage that the optical axis adjustment is not severe is useful in this way. Simply changing these endpoints to a better lens can dramatically improve image processing performance.

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