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.

Read more:
Things to know about 50mm fixed focus lens/

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