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Overview: The focus meter mainly adopts the Hartmann diaphragm measurement method. The parallel light passes through the lens and the Hartmann diaphragm plate on the photodetector image. The spot image formed without the lens has a corresponding change in position, according to the position of the spot. The change in distance calculates the optical parameters of the lens' top power. This method can not only effectively reduce the error caused by human but also accurately measure the optical parameters of the lens. However, there is no complete error model for autofocus focal meters. Therefore, the relatively perfect error model analysis of the Hartmann measurement method is beneficial to the error correction of the subsequent focal meter measurement. The inaccuracy of the actual diopter measurement is caused by the dispersion error of the light source, the inaccurate of the photodetector's central positing, the tilt of lens, and the misalignment between the incidence axis and the main axis of the lens. Through the analysis of the error model, the inaccurate positioning of the CMOS photodetector centroid will lead to larger errors. Therefore, a dual bilinear method is proposed. Combining the interpolation method with the fitting method, one can find the center of mass, and it's the method is effectiveness and accuracy. The conclusion is obtained through analysis. The light source is the green LED light source. Ignoring the distance between the lens and the CMOS photodetector, the Abbe number correction can be eliminated and the complexity of the algorithm can be reduced, resulting in a small error in the final measurement result. For the errors caused by the lens tilt and the misalignment between the incidence axis and the main axis of the lens. The larger misalignment above the large diopter error, and it affects the accuracy of the experimental results. In the experiment, it is necessary to keep the lens and the incident light is perpendicular to each other. In practice, the error model described in this article can be used for correction. Moreover, if the location of the spot center of mass on the CMOS photodetector is inaccurate, a larger error will occur. Therefore, combining the fitting method in this article with the double bilinear interpolation can improve the accuracy of the spot center of mass and ensure the high positioning of the center of mass, ensuring the accuracy of the diopter measurement results.
Measuring model for the Hartmann method
Actual position diagram of the lens and diaphragm
Curves of diopter error caused by neglecting L1
Measurement model of lens tilt
Variation of diopter error caused by lens deflection
A lens model for an external light spot of spindle
Error curve caused by the deflection angle between the optical axis and lens axis
Diopter error curve caused by pixel difference
Flow chart of the whole algorithm
Gray image corresponding to the Gaussian spot
Binary image corresponding to the Gaussian spot
Schematic diagram of the bilinear interpolation algorithm
(a) Gauss surface figure before interpolation; (b) Gauss surface figure after interpolation
(a) Image edge information before interpolation; (b) Image edge information after interpolation