Lensless Refractive Measurement System Based on Shack-Hartmann Wavefront Detection
Objective China currently has the highest myopia rate among youth in the world,with myopia in children and adolescents becoming the leading cause of visual impairment in the country.Myopia is a progressive condition,but early detection and treatment during the pre-myopia stage can help restore vision.Currently,most children and adolescents rely on traditional computerized optometry in hospitals and ophthalmology institutions for vision screening.However,the monitoring density is insufficient to keep up with the rapid progression of myopia,and if parents notice abnormal vision in their children,they may have missed the optimal intervention period.The objective of this study is to address the issues of bulky and expensive existing computerized optometry and vision-screening instruments.We aim to provide an experimental reference for the miniaturization and instrumentation of refractive measurement systems,enabling their application in scenarios that require portability and miniaturization.Methods In this paper,we first provide a detailed introduction to the measurement principles of Shack-Hartmann wavefront sensing technology,followed by the derivation of the wavefront reconstruction algorithm principles.Human eyes with different diopters were modeled using Zemax software,and a Shack-Hartmann wavefront sensor was used to simulate the diffuse reflection phenomenon of a laser spot used as a point light source at the fovea centralis of the human eye,which is located at the center of the retina.The human eye and Shack-Hartmann wavefront sensor were placed at different distances,capturing the outgoing wavefront of the human eye at the corresponding location and imaging it on the detector.This simulated the image acquisition optical path in the refractive measurement system.The collected refractive power images were fed into the algorithm to calculate and then analyze the relationship between the actual measured refractive power and true refractive power at different distances between the human eye and Shack-Hartmann wavefront sensor.Finally,we designed the optical-mechanical structure of an experimental prototype and constructed the system.Model eyes with different diopters were placed at different distances(55,60,and 65 mm)from the Shack-Hartmann wavefront sensor and measurements were repeated ten times.The actual measurement values were compared with the true values of the model eye to validate the accuracy of the measurements,and the coefficient of variation was used to assess the repeatability of the measurement results.Results and Discussions Measurements on model eyes with different diopters show that the stability of the measurement results is better for myopic eyes than for hyperopic eyes.Additionally,the maximum deviation between the measurement results of myopic eyes and the true values of the model eye is generally smaller than that of hyperopic eyes.This is because the wavefront of hyperopic eyes expands outward after exiting the eyeball,leading to fragmentation of the spot formed on the CMOS sensor by the received wavefront in the Shack-Hartmann wavefront sensor,thereby affecting the centroid-localization accuracy in the diopter calculation algorithm.A certain amount of astigmatism is observed in the measurement results for the diopter of cylinder on model eyes without astigmatism.This is due to the inability to strictly align the main optical axes of the human eye,Shack-Hartmann wavefront sensor,and central area of the CMOS during the device adjustment process,which subsequently affects the calculation of astigmatism values.However,overall,the coefficient of variation for repeated measurements of the diopter of sphere in the diopter measurement results remains below 3％,with a maximum error of 0.2 D.The coefficient of variation for repeated measurements of the diopter of cylinder is below 9％,with a maximum error not exceeding 0.25 D.The measurement accuracy meets the requirements of the"Verification Procedures for Ophthalmic Instruments"(JJG892-2022)of the People's Republic of China,which stipulates a maximum allowable error for the diopter of sphere within a range of-10 to+10 D with error of±0.25D,and a maximum allowable error for the diopter of cylinder within a range of 0 to 6 D with error of±0.25 D.Conclusions In this study,we design a compact diopter measurement system based on Shack-Hartmann wavefront sensing technology.The system is calibrated using a model eye provided by the National Institute of Metrology of China to observe the diopter measurement results.An analysis of the results shows that the system's measurement results are highly consistent with the true values of the model eye,with no significant differences and good repeatability.The system is capable of effectively measuring the diopter within a range of-10-+10 D,even at non-fixed distances along the z-axis.Furthermore,the system has a simple structure and low cost.It is expected that the size of the device can be further reduced with the future customization of key components,making it more suitable for scenarios requiring miniaturized instruments.Therefore,this system has broad prospects for applications.
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