1.1 What is chromatic aberration
The chromatic aberration is caused by the difference in the transmissivity of the material. Natural light is composed of the visible light region with a wavelength range of 390 to 770 nm, and the rest are the spectrum that the human eye cannot see. Because the materials have different refractive indices for different wavelengths of colored light, each color light has a different imaging position and magnification, which results in chromatism of position.
1.2 How does chromatic aberration affect image quality
(1) Due to different wavelengths and refractive index of different colors of light, the object-point cannot be well focused into ONE perfect image-point, so the photo will be blurred.
(2) Also, due to the different magnification of different colors, there will be "rainbow lines” at the edge of the image-points.
1.3 How does chromatic aberration affect 3D model
When the image-points have “rainbow lines”, it will affect the 3D modeling software to match the same-point. For the same object, the matching of three colors may cause an error due to the “rainbow lines”. When this error accumulates large enough, it will cause “stratification”.
1.4 How to eliminate chromatic aberration
The use of different refractive index and different dispersion of glass combination can eliminate chromatic aberration. For example, use low refractive index and low dispersion glass as convex lenses, and high refractive index and high dispersion glass as concave lenses.
Such a combined lens has a shorter focal length at the middle wavelength and a longer focal length at the long and short wave rays. By adjusting the spherical curvature of the lens, the focal lengths of the blue and red light can be exactly equal, which basically eliminates the chromatic aberration.
But chromatic aberration cannot be completely eliminated. After using the combined lens,the remaining chromatic aberration is called "secondary spectrum". The longer the focal length of the lens, the more remaining chromatic aberration. Therefore, for aerial survey that require high-precise measurements, the secondary spectrum cannot be ignored.
In theory, if the light band can be divided into blue-green and green-red intervals, and achromatic techniques are applied to these two intervals, the secondary spectrum can be basically eliminated. However, it has been proved by calculation that if achromatic for green light and red light, the chromatic aberration of blue light becomes large; if achromatic for blue light and green light, the chromatic aberration of red light becomes large. It seems that this is a difficult problem and have no answer,the stubborn secondary spectrum cannot be completely eliminated.
Fortunately, theoretical calculations have found a way for APO, which is to find a special optical lens material whose relative dispersion of blue light to red light is very low and that of blue light to green light is very high.
Fluorite is such a special material, its dispersion is very low, and part of the relative dispersion is close to many optical glasses. Fluorite has a relatively low refractive index, is slightly soluble in water, and has poor process-ability and chemical stability, but due to its excellent achromatic properties, it becomes a precious optical material.
There are very few pure bulk fluorite that can be used for optical materials in nature, coupled with their high price and difficulty in processing, fluorite lenses have become synonymous with high-end lenses. Various lens Manufacturers have spared no effort to find substitutes for fluorite. Fluorine-crown glass is one of them, and AD glass, ED glass and UD glass are such substitutes.
Rainpoo oblique cameras use extremely low dispersion ED glass as the camera lens to make aberration and distortion to be very small. Not only reduces the probability of stratification, but also the 3D model effect has been greatly improved, which significantly improves the effect of the building corners and facade.
2.1 What is distortion
Lens distortion is actually a general term for perspective distortion, that is, distortion caused by perspective. This kind of distortion will have a very bad influence on the accuracy of photogrammetry. After all, the purpose of photogrammetry is to reproduce, not exaggerate, so it is required that photos should reflect the true scale information of the ground features as much as possible.
But because this is the inherent characteristic of the lens (convex lens converges light and concave lens diverges light), the relationship expressed in optical design is: the tangent condition for eliminating distortion and the sine condition for eliminating coma of the diaphragm cannot be satisfied at the same time, so distortion and optical chromatic aberration The same cannot be eliminated completely, only improved.
In the figure above, there is a proportional relationship between the image height and the object height, and the ratio between the two is the magnification.
In an ideal imaging system, the distance between the object plane and the lens is kept fixed, and the magnification is a certain value,so there is only a proportional relationship between the image and the object, no distortion at all.
However, in the actual imaging system, since the spherical aberration of the chief ray varies with the increase of the field angle, the magnification is no longer a constant on the image plane of a pair of conjugate objects, that is, the magnification in the center of the image and the magnification of the edge are inconsistent, the image loses its similarity to the object. This defect that deforms the image is called distortion.
2.2 How does distortion affect accuracy
First, the error of AT(Aerial Triangulation) will affect the error of the dense point cloud, and thus the relative error of the 3D model. Therefore, the root mean square (RMS of Reprojection Error) is one of the important indicators that objectively reflect the final modeling accuracy. By checking the RMS value , the accuracy of the 3D model can be simply judged. The smaller the RMS value, the higher the accuracy of the model.
2.3 What are the factors that affect lens distortion
In general, the longer the focal length of a fixed-focus lens, the smaller the distortion; the shorter the focal length, the greater the distortion. Although the distortion of the ultra-long focal length lens(tele lens) is already very small, in fact, in order to take into account the flight height and other parameters, the focal length of the lens of the aerial-survey camera cannot be that long.For example, the following picture is a Sony 400mm tele lens. You can see that the lens distortion is very small, almost controlled within 0.5%. But the problem is that if you use this lens to collects photos at a resolution of 1cm, and the flight altitude is already 820m.let drone to fly at this altitude is completely unrealistic.
Lens processing is the most complex and highest precision step in the lens production process, involving at least 8 processes. The pre-process includes nitrate material-barrel folding-sand hanging-grinding, and the post-process takes core-coating-adhesion-ink coating. The processing accuracy and processing environment directly determine the final accuracy of optical lenses.
Low processing accuracy has a fatal effect on imaging distortion, which directly leads to uneven lens distortion, which cannot be parameterized or corrected, which will seriously affect the accuracy of the 3D model.
Figure 1 shows the lens tilt during the lens installation process;
Figure 2 shows that the lens is not concentric during the lens installation process;
Figure 3 shows the correct installation.
In the above three cases, the installation methods in the first two cases are all "wrong" assembly, which will destroy the corrected structure, resulting in various problems such as blurry, uneven screen and dispersion. Therefore, strict precision control is still required during processing and assembly.
Lens assembly process
The lens assembly process refers to the process of the overall lens module and the imaging sensor. The parameters such as the position of the main point of the orientation element and the tangential distortion in the camera calibration parameters describe the problems caused by the assembly error.
Generally speaking, a small range of assembly errors can be tolerated (of course, the higher the assembly accuracy, the better). As long as the calibration parameters are accurate, the image distortion can be calculated more accurately, and then the image distortion can be removed. Vibration can also cause the lens to move slightly and cause the lens distortion parameters to change. This is why the traditional aerial survey camera needs to be fixed and re-calibrated after a period of time .
2.3 Rainpoo’s oblique camera lens
Double Gauβ structure
Oblique photography has many requirements for the lens, to be small in size, light in weight, low in image distortion and chromatic aberration, high in color reproduction, and high in resolution. When designing the lens structure,Rainpoo's lens uses a double Gauβ structure, as shown in the figure:
The structure is divided into the front of the lens, the diaphragm, and the rear of the lens. The front and rear can appear to be "symmetrical" with respect to the diaphragm. Such a structure allows some of the chromatic aberrations generated in the front and rear to cancel each other out, so it has great advantages in calibration and lens size-control in the late stage.
For an oblique camera integrated with five lenses, if each lens doubles in weight, the camera will weigh five times; if each lens doubles in length, then the oblique camera will at least double in size. Therefore, when designing, in order to obtain a high level of picture quality while ensuring that the aberration and volume are as small as possible, aspheric lenses must be used.
Aspherical lenses can refocus the light scattered through the spherical surface back to the focus, not only can obtain higher resolution, make the color reproduction degree high, but also can complete aberration correction with a small number of lenses, reduce the number of lenses to make the camera lighter and smaller.
Distortion correction tech
The error in the assembly process will cause the lens tangential distortion to increase. Reducing this assembly error is the distortion correction process. The following figure shows the schematic diagram of the tangential distortion of a lens. In general, the distortion displacement is symmetrical with respect to the lower left——the upper right corner, indicating that the lens has a rotation angle perpendicular to the direction, which is caused by assembly errors.
Therefore, in order to ensure the high imaging accuracy and quality, Rainpoo has made a series of strict checks on design, processing and assembly:
In the early stage of design, in order to ensure the coaxiality of lens assembly, as far as possible to ensure that all lens installation planes are processed by one clamping;
②Using imported alloy turning tools on high-precision lathes to ensure that the machining accuracy reaches IT6 level, especially to ensure that the coaxiality tolerance is 0.01mm;
③Each lens is equipped with a set of high-precision tungsten steel plug gauges on the inner circular surface (each size contains at least 3 different tolerance standards),each part is strictly inspected, and position tolerances such as parallelism and perpendicularity are detected by a three-coordinate measuring instrument;
④After each lens is produced, it must be inspected, including projection resolution and chart tests, and various indicators such as the resolution and color reproduction of the lens.
RMS of Rainpoo’s lenses tec