Research On Precision Displacement Measurement With Digital Single Lens Reflex Camera

SLR Digital Cameras are use to deformation observation now, and in a currently instance the measurement accuracy of SLR Digital Camera is not as good as some routine measuring instruments, thereby there are some restrictions on applications received in deformation measurment field for it. In the imaging process of SLR Digital Cameras, many factors will being caused by measurement errors, some errors are taken to eliminate a certain way, Moore Effect is caused by neatly arranged of its photosensitive pixels, and using Moore Stripes can acquire precision displacement measurement.For producing color, most of the SLR Digital Cameras's image sensor are covered with a layer of bayer filter, this filter makes red or blue pixels existence of de-interlacing. When red or blue image edge falls on signs of non-sense of red or blue will produce the error sensor location of signs, Sensor error 0～1 pixel, because each row (column) has a sense of the green pixel, if does not considering the noise, there are little green signs sensor error, panchromatic mode images marked with red, green, blue, each pixel can be induced, there is no position sensor error. Color mark sensor position error is not only relative with the content ratio of red and green and blue in logo images, but also with the algorithm for converting RAW images to black and white images, color mark sensor error is small when Multi-ingredient green or red and blue color content is considerable, by color-related adaptive color interpolation algorithm and a simple algorithm for the average black and white color to be black and white images, the highest positioning accuracy. It should be noted that commonly used in the weighted average algorithm for color to black and white not only lost a large number of blue image information, sometimes affect the positioning accuracy of the full color logo. Image sensor in front of a mirror will consider low-pass mark to the left and up the edge of a pixel expansion, the square green signs and panchromatic, their image centers offset to the upper left corner 0.71 pixel, to the red and blue logo, as a result of taking into account the existence of mirror bayer, their image centers depending on the situation of migration 0～1.41 pixel. Pixel gap existed, When the mark falls on the edge of the gap may also have a position sensor error. Gamma images are often non-linear transform, making some of the image gray-level lost, marked impact on positioning accuracy. Camera shutter caused high frequency vibration, high-speed shutter can not eliminate the impact of high-frequency vibration. The photosensitive pixel performance is closely related to positioning accuracy, when images of the line marks the location of the changes occurred in small and pixel sensor can cause a greater fluctuation in value and high positioning accuracy, this principle can be derived from the physical characteristics and the relationship of pixel positioning errors, the relationship between the display:under normal circumstances, square full color logo positioning accuracy higher than the Green Label positioning accuracy; The shape is closely related with the accuracy, marked the higher the positioning accuracy of the greater area, square than circular logo sign high positioning accuracy, elliptical than circular high positioning accuracy; Small changes in location of signs occur, the positioning accuracy will also change.SLR digital camera to study the source of errors allows us not only to a better understanding of the measurement error SLR, but also can take the following ways to improve the accuracy of measurement of single-lens reflex digital cameras:1 To make full use of RAW format images, in black and white into color images should be related to the use of adaptive color interpolation algorithm and a simple algorithm for the average color to black and white, and not to non-linear transform of the gamma.2 Control points selected as the preferred choice to be full color signs, control points as the shape of signs should be square or round,but avoid to strip; 3 As a large number of control points can reduce the impact of the above-mentioned error.There will be moire fringes when taking photos of intensive pinstripes using digital cameras. The moire fringes have been always considerated harmful for a long time and people are trying to clear them up through the design of cameras.For the modern digital camera,the moire fringes almost can not be seen as a result of low-pass filter.However,they still exist and they can be extracted from images through the superposition of some kind of CCD grating.As a matter of fact, it is found that there will be moire fringes no matter how width the stripes are. When taking photos of a mark in the form of long grating,the metrologic moire fringes will be generated,whose changes in gray are similar to a sinusoid.When the mark moves by a grid distance,the moire fringes move by a groove spacing and the corresponding sinusoid will move by a period.Based on this,if the original gray values are taken as observations,then the sinusoid function can be used to fit to these values to calculate both the former and the latter phase for the further calculation of displacement using phase difference.Furthermore,because the length of a sinusoid in one period is much more larger than a grid distance,the moire fringes have the characteristics of displacement amplification.That is exactly why higher accuracy will be gotten when the moire fringes are used in the displacement measurement.In the images of moire fringes,two sinusoids with phase difference of 180 degrees can be fit to the pixels in each line.Considering the total number of sinusoid is generally more than 20,the average of the corresponding displacement could be used to reduced greatly the noise impact on the accuracy of displacement measurement.Besides,the displacement could also be gotten using the normal processing methods of images of moire fringes.However,there is a big difference in the frequency domain between the images by taking photos of moire fringe mark and the images from the raster mark. And the main signals in the former images are low-frequency,while for the latter ones they are formed by the low-frequency signals superimposed on the strong high-frequency signals.When low-pass filter is used to the latter signals through normal filtering methods,the strength of the moire signals will be greatly reduces.So,based on the point that the processing ways used for the former moire fringes can not be used directly to the latter ones, the new moire fringes deprived of high-frequency signal must be extracted before they could be processed using the normal methods.When the actual measurement, grating mark with the image sensor (CCD grating) the relative positions can not be achieved by conventional measurement requirements Moire fringe. Grating and the CCD in accordance with signs of the light intensity grating distribution function, as well as their relative position, signs can be derived from the existence of grating and CCD grating angle when space equation Moire fringe, if the two pictures on the Moire fringe changes, and based on the changing value of Moire fringe under the sign inversion grating equation and the relative position of image sensor, when the location of fixed cameras, change is the grating space marked displacement. Moire fringe equation can be used to prove, whenφ<7°(ω,Kopen), sine curve fitting using the model error can be ignored, so, when measuring the angle between the two grating space requirements do not strictly. Sine curve fitting using least square adjustment is generally calculated, phase is unknown in an adjustment calculation, so, factor from the HS phase array can be co-factor, Association factor multiplied by the unit weight variance is Phase error. Co-factor is the function of the phase sinusoid, including angular frequencyω, the initial phase f,amplitude A, and observations, function can be analyzed throughω,f,A,A,, and the number of observations on the effects of phase-precision, to determine the best film program. Lens distortion will result in sinusoid deformation, in a small area is a certain degree of deformation, so, two sinusoids in the calculation of the phase, can eliminate the effects of lens distortion. Signs of uneven illumination of the Moire pattern will also have a deformed sinusoid, and seriously affect the accuracy of phase difference, the solution is to redesign the fitting function, adding quadratic polynomial in the sine curve fitting function, fitting function with new amendments can be good low-frequency error resulting from uneven illumination. Camera shutter high-frequency vibration caused non-uniform pixel displacement in images, affect the accuracy of displacement measurement marks, so, it Should be the use of heavy camera tripod and mirror pre-start up function. Electronic shutter does not shock, so, using the the camera with electronic shutter is the best choice.Moire pattern of signs as a result of the displacement is very sensitive, directly in the calculation and the use of RAW format images, each of the three primary colors in order to displacement, imaging algorithm does not require, Bayer not consider low-pass mirror and consider the effects of mirror, also basically pixel not be impacted by the the gap, adjustment or average or Difference calculation of data processing computing, the elimination of the majority of accidental error (noise), and the elimination of some systematic errors, so, Moore photographic effect of the high precision measurement. Many experiments show that the Photo from the 100m (the longest distance test), that is, the use of cheap low-end single-lens reflex digital camera, in the general outdoor environment, as the displacement of a measurement accuracy has reached the 1/0.5 million (from the displacement of error than the material), and anti-interference ability is robust, less demanding on the environment, operating process is not complicated. If using more than a piece of images to calculate the displacement, accuracy will surpass 1/1000000.