Intensity diffusion:a concealed cause of fringe distortion in fringe projection profilometry

Fringe projection profilometry(FPP)is widely used in optical three-dimensional(3D)measurements because of its high stability.In FPP,fringe distortion is an inevitable and highly complex systematic error that significantly reduces the 3D measurement accuracy.At this point,the existing causes of fringe distortion represented by gamma distortion,high-order harmonics,and image saturation have been effectively analyzed and compensated to restore high-quality fringe images.In this paper,we innovatively reveal a concealed cause of fringe distortion,i.e.,intensity diffusion across pixels,which is induced by photocarrier diffusion between photodiodes.To the best of our knowledge,intensity diffusion has not been studied in the field of fringe restoration.Based on the motion of photocarrier diffusion,we theoretically analyze the mechanism of how the intensity diffusion affects FPP.Subsequently,an intensity diffusion model is established for quantifying the diffused intensity in each pixel,and an intensity diffusion correction algorithm is presented to remove the diffused intensity from the fringe images and correct the fringe distortion.Experiments demonstrate the impact of intensity diffusion on FPP,and the 3D measurement results prove the effectiveness of the proposed methods on improving the 3D measurement accuracy by correcting the fringe distortion.

Quantitative research on higher order harmonics in metrology beamline

The synchrotron radiation spectra of the spherical grating monochromator(SGM)working in the soft X-ray and VUV region are often contaminated by significant amounts of higher order harmonics.They cannot be suppressed completely by suitable filters.Higher order contributions in the spectral radiation standard and metrology beamline were researched using transmission grating(made in-house)and IRD AXUV100G(USA)photodiode detector.The exit beam was dispersed with the transmission grating behind the exit slit of the monochromator,and the contributions of the different orders were analyzed.The higher order distributions were quantitatively determined for three gratings with line densities of 1800,600 and 200 l/mm.Experiment results show that in wavelengths between 5 nm and 15 nm the contributions of the higher orders to the detector signal are restricted to less than 7%even without the use of filters.In wavelength regions between 5 nm and 34 nm,the contributions of the higher orders to the detector signal are less than 14%with proper Al,Si_(3)N_(4) and Zr filters,and after being modified by quantum efficiency of the detector,the higher order contributions are restricted to less than 6.5%.The study also shows that higher orders are almost totally suppressed by MgF_(2) filter when the wavelength ranges between 115–140 nm.