The unwanted light transmitting through the gap spaces of low fill factor lens array will deteriorate the signal contrast detected by the sensor. The high cost photolithographic techniques and optical coating technology have to be used to deposit a metallic film precisely filling these gap regions, which is only suitable for the lens array on flat substrate. In this paper, we will propose an efficient way for the fabrication of gap-masked lens array on a curved plastic substrate by using the direct diamond turning and spray coating process. The semi-finished lens array (SFLA) on a convex surface is first fabricated by rough cutting with fast tool servo. A paint film layer with mirror surface effect is then formed uniformly by spray coating on the SFLA which has a rotation speed controlled by the computer-numerically controlled (CNC) machine. At last, the lenslet surfaces are uncovered and regenerated by finishing cutting process. Without any reloading or re-position error, this cost-effective method can generate various gap-masked array optics on curved substrate with customized period and grid pattern.
Wavefront matching, i.e. phase control of an arbitrary input wavefront (IW) to produce target output wavefront (OW), can be realized using an optical freeform surface (OFS). The accuracy of wavefront matching is dependent on the discrete sampling data of OFS, categorized as feature point coordinates and surface slopes, especially for surface generation with modal Zernike method. To find out the optimum sampling data type, first we propose an exact ray mapping process ruled by Snell’ law and the constancy of optical path length from IW to OW to derive the feature point coordinates and surface slopes of OFS simultaneously. Then, wavefront error is numerically calculated by ray tracing for OFS generated from different sampling data types to demonstrate the effectiveness and accuracy of wavefront matching. We find that OFS generation method based on feature point coordinates has superior performance in wavefront matching than that based on surface slops under the condition of high density sampling of IW regardless of complexity of IW. Additionally, in the case of complex IW, the accuracy of wavefront matching benefits from heavy weight of low-order aberrations especially defocus and astigmatism in IW.
The conventional wavefront reconstruction of the Shack-Hartmann method is based on the measured slopes by using the least-square fitting method. However, this reconstruction method suffers from the low order Zernike terms in wavefront, especially defocus and astigmatism, due to the discrepancy of coordinate systems between the lenslet array and the original wavefront caused by the propagation of subaperture wavefront. In this paper, a ray tracing method was used to calculate the slope error between an input wavefront and the reconstructed wavefront. Then, an iteration procedure including slope compensation and coordination compensation was constructed and implemented to minimize the wavefront reconstruction error. The numerical simulation was performed for a variety of defocus-dominated input wavefront and high-order-dominated input wavefront, and verified that high accuracy wavefront reconstruction can be achieved by the proposed iterative compensation method.
We used the explicit expression of Zernike polynomials in Cartesian coordinates to fit and describe the freeform surface of progressive addition lens (PAL). The derivatives of Zernike polynomials can easily be calculated from the explicit expression and used to calculate the principal curvatures of freeform surface based on differential geometry. The surface spherical power and surface astigmatism of the freeform surface were successfully derived from the principal curvatures. By comparing with the traditional analytical method, Zernike polynomials with order of 20 is sufficient to represent the freeform surface with nanometer accuracy if dense sampling of the original surface is achieved. Therefore, the data files which contain the massive sampling points of the freeform surface for the generation of the trajectory of diamond tool tip required by diamond machine for PAL manufacture can be simplified by using a few Zernike coefficients.
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