DPI®: Freeform lens array master for imaging applications

By combining ultra-precision 3-axis diamond turning and DPI®, arrays of freeform lenses can be machined onto replication masters or mold inserts. With DPI®, extremely low surface form error and roughness can be obtained on a large number of lenses such as required for imaging applications like future AR/VR products.

By using these technologies, Wielandts UPMT has manufactured a lens array entirely composed of freeform lenses. Each lens of the array is a different freeform surface with a different orientation.

Figure 1: Freeform lens array master illustration.
Optics by Limbak.

Figure 2: Picture of DPI® machined freeform lens array master.
Optics by Limbak.
Based on the polynomial equation of each lens, a custom CAM algorithm was developed to generate all NC programs based on the chosen cutting parameters. The sample was manufactured in a brass disc and qualification metrology was performed internally.

To machine the brass master, DPI® took care of aligning each lens of the MLA with the diamond turning lathe’s spindle axis. Sequentially, the freeform lenses were turned in a 3-axis motion of the machine.

For this kind of master, lens surface roughness, lens surface form accuracy and lens position deviations are all of high importance and require accurate experimental determination.

The lenses’ roughness was measured using a laser scanning confocal microscope at 50X magnification. The measured roughness is between 3.0 and 4.2 nm Ra (with high-pass filters at 80 µm).

Given the required surface form error and the freeform shape of the lenses, the form deviation was measured using a tactile surface profiler with a very low probing force. Two cross-sections in X and Y directions of the master were measured. For each lens the theoretical profile was then RMS fitted (displacement and tilt) to evaluate the remaining form error. The described process rendered mean RMS form error values of 53nm and 81nm in the X and Y cross-sections, respectively. Read more about the DPI® technology.

Figure 3: Freeform lens 5X magnification inspection.

Figure 4: Freeform master residual form error in X cross-section.

Figure 5: Freeform master residual form error in Y cross-section.