Calibration Method of Gamma Value in Phase Measurement System

Abstract

The invention belongs to the field of precise three-dimensional measurement, and particularly relates to a method for calibrating a gamma value in a phase measurement system. On the basis of an ideal gamma distortion model, a gamma non-linear response model considering projector defocuses is created, and on the basis, a novel gamma value calibration method easy to implement is provided. The method includes 1), projecting two groups of raster images onto a to-be-tested target respectively, wherein the phase shift L and gamma' of the raster images are valued as 1 or positive integers greater than 1 respectively, and shooting the two groups of raster images by an industrial camera; 2), as to each group of raster images, calculating amplitude of kth harmonic wave k and then acquiring 2 / 1 and 2' / 1' after respective calculation; 3) calculating gamma and sigma of each pixel in each camera image respectively; 4), averaging the gamma of pixels in camera images to take the average value as a gamma value calibrated by the phase measurement system. By the gamma value calibration method, phase errors caused by system gamma nonlinearity can be effectively eliminated.

Description

technical field [0001] The invention belongs to the field of precision three-dimensional measurement, and in particular relates to a gamma value calibration method in a phase measurement system (PMP system). Background technique [0002] Phase measurement profilometry (PMP) is widely used in 3D shape measurement due to its low cost, high flexibility and simple hardware. In an actual PMP system, the sensor noise and the gamma nonlinear response of the projector-camera system are the main factors affecting its measurement accuracy. The gamma nonlinear response distorts the image of an ideal sinusoidal grating compared to the random errors caused by sensor noise, resulting in periodic phase errors. In ordinary PMP systems, this periodic error caused by gamma nonlinear response can be suppressed by increasing the number of phase shifts, but in fast PMP systems, it is required that the number of phase shifts should be as small as possible, and a three-step phase shift is usually...

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Problems solved by technology

The above methods can improve the measurement accuracy to a certain extent, but still cannot completely eliminate the phase error

[0004] In 2004, Guo et al [Guo, H., He H., Chen M.Gamma correction for digital fringe projection profilometry.Applied optics, 2004.43(14): 2906-2914.] proposed a static analysis of raster images The method of obtaining the system gamma value eliminates the phase error, which can reduce the phase error, but because the actual gamma value of the projector is very complicated, it is not a single value, and this method can only use a single gamma The value compensates the phase error, and the gamma value of the projector is required to be fixed, so there are certain limitations

In 2009, Pan et al [Pan, B., Qian K., Lei H. et al. Phase error analysis and compensation for nonsinusoidal waveforms in phase-shifting digital fringe projection profilometry. Optics Letters, 2009.34(4): 416-418. ] On the basis of the theoretical analysis of the phase error of the grating image, a phase error compensation algorithm based on the idea of ​​iteration is proposed. less efficient

[0008] Although these methods can reduce the phase error to a certain extent, usually digital optical projectors use a larger aperture in order to obtain greater brightness, resulting in a smaller depth of field of the projector. Therefore, in the PMP system, only a small Part of the measurement space is in the best focus area of ​​the projector, and most of the measurement space is in different degrees of defocus

Although the defocusing phenomenon of the projector can suppress the high-frequency harmonics and reduce the phase error, the distribution of the phase error in the entire measurement space is unstable due to the different degrees of defocusing in different regions

The existing phase error compensation and gamma correction methods do not consider the influence of projector defocus, so there is still a lack of a complete system nonlinear response model and response gamma calibration method

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