# Double-Sided Freeform Lens for Light Collimation of Light Emitting Diodes
## 4 Experiment Results  
As described in the previous section, the designed double freeform lens was manufactured by injection molding in UV-grade PMMA polymer PLEXIGLAS(r) 8012 (Evonik Industries, Essen, Germany). A two-piece steel mold was first fabricated by a CNC machining system in accordance with the designed lens profile shown in Figure 5 for \(\xi\) = 0.3, \(\theta_{B}\) = 60\({}^{\circ}\), and \(r_{o}\) = 10.65 mm. The dividing point between the profiles of the two injection molds was set as point \(B\). Mass-production of the PMMA lens was then carried out using a standard injection molding system. Figure 7 shows a photograph of the double freeform lens produced in a typical molding cycle. Figure 8 presents a schematic illustration of the experimental setup used to measure the optical performance of the fabricated lens with the NCSU276A UV-LED illumination source. As shown, the system consisted mainly of the collimating lens and UV-LED with correct distance and positioning between them, a CCD camera (FLIR Systems Inc., CM3-U3-13S2M-CS, Wilsonville, OR, USA), and a  
Figure 6: Zemax simulation results for NCSU276A UV-LED and double freeform lens (\(\xi\) = 0.3, \(\theta_{B}\) = 60\({}^{\circ}\)and \(r_{o}\) = 10.65 mm): (**a**) 2D view of ray tracing results in Zemax, and (**b**) percentage of accumulated light intensity within collimating angle (\(\gamma\)). Figure 7: Photograph of double freeform lens produced by injection molding of UV-graded PMMA polymer. (**a**) Side view; (**b**) Front view; (**c**) Close side view  
pin-hole plate. The pin-hole had a diameter of 50 \(\upmu\)m and was placed at a distance of 4 mm from the sensing pixels of the CCD. Moreover, the assembled pin-hole/CCD unit was mounted such that it faced the lens/LED along its optical axis (z-axis). Consequently, following the lens collimation, the LED light passed through the pin-hole and was incident on the sensing pixels of the CCD. The intensity and distribution of the light incident on the pixel surface were determined using a calibration process with an optical power meter. Briefly, the distribution of the light intensity with respect to the incident angle was obtained using a simple geometry relationship between each sensing pixel and the pin-hole, while the overall light intensity and its distribution as a function of the collimating angle was determined by scanning the pin-hole/CCD or lens/LED along both \(x\)- and \(y\)- directions. The LED used in the experiment had a total emitting power of 348.5 mW under drive current of 500 mA, and the total light power received on the detector plane was just 152.6 mW. In other words, the double freeform collimating lens had an energy efficiency of 43.79%. Figure 8b shows the results obtained for the accumulated light intensity within a collimating angle of \(\gamma\). It is seen that around one half (50%) of the collimated UV-LED light intensity was obtained within a collimating angle of less than 1.88\({}^{\circ}\), while 90% of the intensity was obtained within a collimating angle of less than 4.84\({}^{\circ}\). In other words, the experimental results are in good qualitative agreement with the simulation results shown in Figure 6b. The discrepancy between the two sets of results arose most likely from machining errors of the lens profiles, experimental misalignments between the lens and the LED, and slight variations in the lens material constants. Nevertheless, the overall performance of the proposed lens in collimating the LED light was confirmed. When applying UV-LEDs to conventional proximity/contact photolithography, two of the most important requirements include a small collimating angle and a uniform distribution of the UV light intensity over a large area. The results presented in Figure 8 confirm the ability of the proposed double freeform lens to collimate the UV light from a UV-LED with a small collimating angle. In other words, the proposed lens satisfied the first requirement.