Fei Li

Experimental research on radius of curvature measurement of spherical lenses based on laser differential confocal technique

A new approach based on laser differential confocal technique is potential to achieve high accuracy in radius of curvature (ROC) measurement. It utilizes two digital microscopes with virtual pinholes on the CCD detectors to precisely locate the cats-eye and the confocal positions, which can enhance the focus-identification resolution. An instrumental system was established and experimental research was carried out to determine how error sources contribute to the uncertainty of ROC measurement, such as optical axis misalignment, dead path of the interferometer, surface figure error of tested lenses and temperature fluctuation, etc. Suggestions were also proposed on how these factors could be avoided or suppressed. The system performance was tested by employing four pairs of template lenses with a serial of ROC values. The relative expanded uncertainty was analyzed and calculated based on theoretical analysis and experimental determination, which was smaller than 2x10-5 (k=2). The results were supported by comparison measurement between the differential confocal radius measurement (DCRM) system and an ultra-high accuracy three-dimensional profilometer, showing good consistency. It demonstrated that the DCRM system was capable of high-accuracy ROC measurement.

A method to enhance the measurement accuracy of Raman shift based on high precision calibration technique

Raman spectrometers are usually calibrated periodically to ensure their measurement accuracy of Raman shift. A combination of a piece of monocrystalline silicon chip and a low pressure discharge lamp is proposed as a candidate for the reference standard of Raman shift. A high precision calibration technique is developed to accurately determine the standard value of the silicons Raman shift around 520cm-1. The technique is described and illustrated by measuring a piece of silicon chip against three atomic spectral lines of a neon lamp. A commercial Raman spectrometer is employed and its error characteristics of Raman shift are investigated. Error sources are evaluated based on theoretical analysis and experiments, including the sample factor, the instrumental factor, the laser factor and random factors. Experimental results show that the expanded uncertainty of the silicons Raman shift around 520cm-1 can acheive 0.3 cm-1 (k=2), which is more accurate than most of currently used reference materials. The results are validated by comparison measurement between three Raman spectrometers. It is proved that the technique can remarkably enhance the accuracy of Raman shift, making it possible to use the silicon and the lamp to calibrate Raman spectrometers.

Research on Definition Evaluation Algorithm of the Test Equipments for Phoropters

Definition evaluation function of the defocused image is key to realize automatic focusing. Sensitivity of the definition evaluation function can directly affect detection accuracy of the test equipments. By analyzing and summarizing the definition evaluation functions which are commonly used currently, we will present a new image definition evaluation function based on wavelet transform in this paper. On the basis of theoretical analysis, we also establish and analyze the mathematical model of evaluation function, and make experiment and simulation with the series of images collected by the video image acquisition system. Compared with the traditional definition evaluation function, the definition evaluation function of defocused image defined by this algorithm is more sensitive to fuzzy degree of image and more consistent with human subjective vision.

A Novel Alignment Method for Collimator Based on Laser Differential Confocal Technique

A collimator is one of the most popular optical instruments used in optical workshop and parallelism of the beams is the key technical indicator of collimator. A great deal of efforts has been expended to ensure reticle locate at the focal plane. In this paper, a new laser differential confocal collimator alignment method is proposed for precision alignment of collimators. Based on this principle an experimental setup is built up and used to align two collimators whose focal lengths are 600mm and 1000mm and f-numbers are both 10.The wave-front parallelism error of the laser differential confocal system and the tested collimator is measured by the verified experiment based on scanning penta prism method. At last some influence factors alignment precision is analyzed. Theoretical analysis and experimental results show that the alighment precision based on laser differential confocal method can reach to 0.004mm.and can be a high-efficiency approach to align collimators, especially in industry production.

New approach to accuracy enhancement and traceability realization of radius of curvature measurement

High accuracy radius of curvature (ROC) measurement of optical surfaces is usually realized by techniques such as autocollimation, interferometry and profilometry, with theoretical accuracy as high as 10-6. In practical application, significant discrepancy may exist in results obtained by different methods owing to figure error of measured surfaces. In this paper, mathematical models are built up to characterize the relationship between the ROC and the figure error as well as the aperture angle. Based on the models, equations for calculating the ROC accuracy are derived and tested on several ROC measuring methods. Experiments are carried out on a set of high quality spheres whose diameters are from 11mm to 93mm and roundness is from 0.03μm to 0.07μm, measured by instruments with top level accuracy, which are a length measuring machine, a profilometer and a homemade differential confocal system. Uncertainties are calculated and analyzed against several factors. The reason for the discrepancy between different methods is explained. An approach is also proposed which could reduce the uncertainty of ROC by 1~2 scales, making it possible to trace the results of ROC measuring instruments to the primary standard of length via diameter and roundness measurement method.

Application of laser differential confocal technique in back vertex power measurement for phoropters

A phoropter is one of the most popular ophthalmic instruments used in optometry and the back vertex power (BVP) is one of the most important parameters to evaluate the refraction characteristics of a phoropter. In this paper, a new laser differential confocal vertex-power measurement method which takes advantage of outstanding focusing ability of laser differential confocal (LDC) system is proposed for measuring the BVP of phoropters. A vertex power measurement system is built up. Experimental results are presented and some influence factor is analyzed. It is demonstrated that the method based on LDC technique has higher measurement precision and stronger environmental anti-interference capability compared to existing methods. Theoretical analysis and experimental results indicate that the measurement error of the method is about 0.02m-1.