Machine Vision Hardware study notes 3-- lens

This article documents the machine vision hardware learning content, Part III: Lens

The main content of this article for the optics of the lens, the basic parameters and selection principles. As engineers, we should focus on mastering the basic parameters of the lens and selection principles

1, the basic principle of the optical system

1.1 Main Glossary

Optical center: the optical center is the center of a convex lens, i.e., the center point of the lens.

Main optical axis: refers to a straight line perpendicular to the optical center of the lens, also called a spindle, and only one shaft.

Focus: parallel to the main optical axis of the light rays through the lens intersect at one point, this point of intersection is called the focus, front focus and the rear focus.

Focal plane: through focus and the plane perpendicular to the main optical axis, said focal plane of the system. Local installation photosensitive sheet is in the focal plane of the camera position.

Main points: the lens thickness , the presence of a pair of conjugate points on the main optical axis, i.e., P and P '. Respectively referred to as front principal point and a rear principal point.

Main plane: parallel to the main optical axis of an incident ray after refraction of light will pass through the lens focal point, an extension line of the two light (red dotted line) intersect at one point, all such intersections constitutes a curved surface, i.e. mainly plane (dashed black line)

Conjugate relationship: during imaging lens, between the object and the image correspond to each other, this relationship is in a conjugate relationship with the image of the object

Focal length: from the object side principal point P of the distance between primary focal point F to the front focal distance F (focal length of the object side), the image side principal point P 'to the image side focal point F' of the focal distance F ' (the image-side focal distance )

Object distance and image distance: object distance from the object side principal point P of the object plane to the image distance of the image side principal point P 'to an image plane.

1.2 Thin lens imaging principle

University of feeling physical optics experiment. . .

Machine vision lens is a thick lens, but generally may be calculated according to geometry and parameters of thin lens, as the basis for selection of the lens.

Refractive lens is imaged into the image may be inverted, reduced real image; inverted, and other large real image; inverted, enlarged real image; upright, enlarged virtual image. Following table:

Camera or camera lens is applied to the first row of the table, the object distance is greater than twice the focal length.

2, the basic parameters

The main parameters of the lens has a focal length, resolution, working distance, depth of field, the field of view, the amount of distortion and the like.

Resolution: refers to the influence of the lens configuration, materials, and other factors in processing accuracy at the image plane in the lens in millimeters can distinguish the number of black and white fringes.

Working distance: generally refers to the distance from the object end of the lens is less than the minimum working distance is greater than the maximum working distance of the system generally can not be imaged clearly.

Depth: to work best focus distance at the center of the lens, before and after the presence of a range within which range of the lens can be clearly imaged.

Field of view: the range of the image pickup apparatus that can be covered, i.e., the size of the object plane and the image corresponding to the target surface.

Focal length: focal length of the lens focal length convex lens concept is slightly different, since a plurality of convex lens combination . The focal length is the distance between the center point a clear image from the lens to the plane formed by the gel. The focal length determines the size of the size of the depth of field, the focal length the smaller the value, the greater the depth of field. According to the focus adjustment can be divided into fixed focus lens and a zoom lens into two categories.

The amount of distortion: imaging due to the inherent characteristics of the lens caused distortion can not be completely eliminated. It affects only the distortion aberration of the imaging geometry, without affecting the clarity of image

Relative aperture: refers to the diameter of the entrance pupil of the lens (denoted by D) and the focal length (indicated by f) ratio, i.e., D / f.

Aperture: reciprocal of the relative aperture is called the aperture factor.

For example: VS-LDA15 lens: The lens is a macro lens low distortion, five megapixels or less, 2/3, "corresponds to the chip, a focal length of 15mm, an infinite time imaging, approaches 0% distortion other parameters. the table below.