Focus on challenging optical components

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The VR headsets price gap is very big, from just a few dollars Google Cardboard, the expected to cost $350 O

by:ChangHui     2020-05-20
Are most of the manufacturers of FOV in about 100 °, but the author thinks that the real value of the parameter is not very big. First of all, don't like the size of the memory or hard disk, FOV without a clear measurement method, vendor will empty mark the user can not be sure; Second, people in the center of the watch is not in view of the target, the more inclined to turn, rather than squinting. Refer to baidu a human eye view diagram 2 shows, the human eye level perspective can even more than 180 degrees, and monocular comfortable view only about 60 degrees, the position inside the eye is most sensitive, more than the range people will instinctively turn. So you need not too obsessed with the size of the parameters, but more should pay attention to 60 degrees center area of the image clear degree, and the actual use wearing feeling. In general, the size of the lens, the greater the human eye can not easily noticed on the edge of the lens, immersive is better, the user can more intuitive to compare the parameters, at present, most of the VR headsets lens size is around 50 mm. Clarity as the VR world, benchmarking DK1 in lens definition of the edge image is often the early users, this is due to off-axis aberration in optical design. In order to accurately test DK1 lens optical design quality, I have been commissioned the teacher of the department of zhejiang university light in the special equipment used for measuring lens imaging quality, the transfer function ( MTF) Tester is quantitatively measured on its imaging quality, look from the analysis results, leave the central area is a bit far away, the imaging has fallen sharply. It's like when using quality general magnifying glass, we found that the edge of the image will become fuzzy, the reason for this is that under the influence of off-axis aberration. At present, most of the company processing method is designed aspherical lens before and after the two sides, as shown in the figure below, as far as possible to reduce off-axis aberration, improve the edge image of the image. Molding technology of plastic lenses are very mature, can suppress the plastic lenses optical quality is very high, so have adopted such as DK2 double-sided aspherical design, the user can focus on VR helmet lenses if design adopted the aspheric surface, usually much higher than aspheric in clarity. , of course, it has excellent optical design is not enough, manufacturing industry has a high influence to the stability of the lens quality, therefore, if the VR manufacturers can seek big contract for processing the lens, one is the guarantee of the quality of product. Color difference now most of the VR headsets in the use of time will be in the edge of the area ( Edge location) In red, green and blue color edge, that is, the dispersion phenomenon, it in the use of high refractive index material, it is easy to appear when after like white light through a prism can be divided into the colorful light. In DK1, this phenomenon is especially obvious. From the Angle of the optical design, two or more material is needed to eliminate the dispersion, so in principle, the single lens ( A material) Is unable to solve, DK2 in order to solve the problem, before the image display, using software to do a first instead of color compensation, as shown in figure 4, basically solved the problem. This method is only in the aspect of software to do the correction, but can cause certain influence to the image resolution, and each pixel on the image needs to be done once the processing of backward dispersion, increased the burden on hardware, reduces the frame rate of the image. Best method is to adopt more sets of materials of different lenses achromatic lens group, using optical methods to eliminate color edge. This has been very mature in lens design, such as apochromatic objective lens ( 复消色差的目的) To eliminate the image color difference. Distortion distortion in popular terms is image distortion, give a person with the middle protrude ( Barrel distortion) (or sag Negative distortion) , this also is a kind of aberration, is due to the pupil ( The human eye, that is) In the optical system in different position, before and after for spherical lens, the aberration is inevitable, as shown in the following figure on the left side of the figure, and with the increase of FOV, edge image distortion will be more obvious. In order to solve the problem, the common method is to use the design of the aspheric surface, as shown in the diagram below right, it can reduce the distortion of the image, but also can greatly reduce the weight of the mirror, it is also the high number of spectacle lens is no longer so thick as the bottom of the bottle. But because the lens can be used to optimize the parameters of monolithic extremely limited, in meet improve clarity, under the condition of increasing the FOV is hard to meet again at the same time to eliminate the purpose of the distortion. How to compromise between the gain and loss became the three lens designers need to focus on. Given the VR headsets solutions are copied from DK2, from its scheme shows that it is similar to eliminate the dispersion methods, the image is presented in the user before, want to take a barrel distortion is used to offset the lens pincushion distortion, as shown in figure 6, which makes the user feel caused by the lens distortion is not reality, through the analysis of DK1 SDK, the internal set up four distortion coefficient ( k0 ~ k3) , form a distortion compensation equation, by controlling the four different amount to fit the size of the distortion degree of the lens, so as to achieve the goal of the SDK general different lenses.
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