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The ancient Egyptians and Mesopotamians had long known to polish quartz crystals into lenses that could be used to focus sunlight or magnify images. The oldest of these lenses is more than 3,000 years old, such as the Nimrud lens, which is the earliest known optical lens.
In the history of optical development, during the period of more than 200 years from the 17th century to the 19th century, the scientific community discovered the phenomenon of light color difference in the process of exploring the nature of light. Today, optics has been widely used in industrial equipment and machine vision detection, the influence of lens on imaging quality is not limited to resolution, relative illumination, distortion and other common parameters, color difference has become an important imaging index.
01 What is color difference?
Color difference (CA) refers to the phenomenon that the lens cannot focus light of different wavelengths on the same point due to the different refractive index of light of different wavelengths in the lens (low refractive index for red light, high refractive index for blue and violet light).
It is manifested in the image as the edge of the measured object (in the case of high contrast) appears fuzzy or obvious color edge, there is an obvious “rainbow band” phenomenon.
< Color difference – Image from Internet >
02 Color difference classification
Color difference can be roughly divided into two categories, one is axial color difference, the other is transverse color difference.
Axial color difference
Axial chromatic aberration, the phenomenon manifested in the direction of the optical axis. Is due to the different wavelengths of color and focal length, so that they focus on the lens at different points far and near, showing the phenomenon of outward radiation from the center, so it is also known as position color difference.
< Axial color difference – Image from Internet >
The main factors affecting the axial color difference are as follows:
① Related to the focal length of the lens
② It is related to the refractive index and dispersion rate of optical materials
③ It is related to the aperture Angle and increases with the increase of the aperture Angle
< Axial color difference (different position, different color) – Photo from Internet >
Transverse color difference
Transverse chromatic aberration refers to the phenomenon generated in the direction perpendicular to the optical axis. Because the magnification of the installed lens is related to the refractive index, different colors of light will be focused on different positions in the focal plane, so it is also called magnification chromatic aberration.
< Horizontal color difference – Image from Internet >
The more the edge of the field of view, the more obvious the lateral color difference appears as a phenomenon similar to drag shadow. The main factors affecting the transverse color difference mainly include the following three points:
① Related to the position of the diaphragm;
② It is related to the refractive index and dispersion rate of optical materials
③ It is related to the field of view and increases with the increase of the field of view
< Horizontal color difference (green above, purple below) – Photo from Internet >
03 Color difference correction method
For axial and transverse color difference, the following methods are generally used for color difference correction:
① Replace glass
② Adjust the focal distribution
③ Change the lens spacing
For optical color difference
It is generally difficult to completely correct and eliminate it
But because of the human eye and other light receivers
There are certain defects in the reception of light
So as long as the optical color difference
Just keep it to the tolerable limit