The image quality of different industrial lenses in visual inspection varies greatly, even for the same type of industrial lenses. This is mainly due to different factors such as material, processing accuracy and lens structure. At the same time, the price of industrial lenses of different grades varies from several hundred to tens of thousands of yuan. Differences. The more famous ones are four three-group sky plug lenses and six four-group double Gauss lenses. Optical Transfer Function (OTF) is generally used to evaluate the imaging quality of lens for lens design and manufacturers. Optical system transmits the information of brightness distribution along space. Optical system transmits sinusoidal wave signals of various spatial frequencies, their modulation degree and position when transmitting the information of the subject. The change of phase in real image is a function of spatial frequency, which is called optical transfer function. OTF is generally composed of MTF (Modulation Transfer Function) and PTF (Phase Transfer Function).
Aberration is an important aspect of image quality. There are six common aberrations:
Spherical aberration: A monochrome conical beam emitted from a point on the main axis to an optical system, refracted by the optical series, is formed if the rays of the original beam with different aperture angles cannot intersect at the same position on the main axis, or even form a diffuse spot (commonly known as a blur circle) at the ideal image plane on the main axis. Image errors are called spherical aberrations.
Coma aberration: A monochrome conical beam emitted to an optical system from an external object point on an axis outside the main axis, refracted by the optical series, is called coma aberration if it can not form a clear point at the ideal image plane, but a comet-shaped spot with a bright tail.
Astigmatism: An oblique monochrome conical beam emitted from an external object point on an axis outside the principal axis to an optical system. After refraction by the optical series, it can not form a clear image point, but can only form a diffuse spot. The imaging error of the optical system is called astigmatism.
Field Curve: A clear image of a plane object perpendicular to the principal axis formed by an optical system. If the image plane is not perpendicular to the principal axis, but on a curved surface symmetrical to the principal axis, that is, the best image plane is a surface, the imaging error of the optical system is called field curvature. When the image is focused to the center of the picture, the image around the picture is blurred; when the image around the picture is clear, the image at the center of the picture begins to blur again.
Chromatic aberration: A white beam of light is emitted from a white object to an optical system. After refraction by the optical system, the colored light can not converge at one point and form a color image spot, which is called chromatic aberration. The reason for the color difference is that the refractive index of the same optical glass is different for different wavelengths of light. The refractive index of short wave light is higher than that of long wave light.
Distortion: The imaging error of the optical system is called distortion when the line outside the principal axis in the plane of the object is transformed into a curve after imaging by the optical system. Distortion aberration only affects the geometric shape of the image, but does not affect the clarity of the image. This is the fundamental difference between distortion and spherical aberration, coma aberration, astigmatism and field curve.
In evaluating the quality of industrial lens, several practical parameters such as resolution, sharpness and depth of field are usually used to judge:
1. Resolution: Also known as discrimination, resolution, refers to the ability of the lens to clearly distinguish the details of the subject's fiber. The reason that restricts the resolution of industrial lens is the diffraction phenomenon of light, that is, the diffraction spot. The unit of resolution is "line pair/mm" (lp/mm).
2. Acutance: Also known as contrast, refers to the contrast of the brightest and darkest parts of an image.
3. Depth of field (DOF): In the scene space, the scene located within a certain distance before and after the focus plane can also form relatively clear images. The above-mentioned depth range between the scenes that can form relatively clear images before and after the focus plane, that is, the depth range of the scenery space that can obtain relatively clear images on the actual image plane, is called depth of field.
4. Maximum relative aperture and aperture coefficient: relative aperture refers to the ratio of incident aperture diameter (expressed in D) to focal length (expressed in f), i.e. relative aperture = D/f. The reciprocal of relative aperture is called aperture scale, or f/aperture coefficient or aperture number. The relative aperture of general lens is adjustable, and its maximum relative aperture or aperture coefficient is often marked on industrial lens, such as 1:1.2 or f/1.2. If the light on the scene is darker or the exposure time is short, the industrial lens with the largest relative aperture should be selected as far as possible.
The choice of industrial lens in visual inspection is very important.
