Optical design and illumination simulation of Fresnel lenses for marine signal lanterns

Abstract

Providing marine signal lanterns, Fresnel lens has been adopted to transfer the beam from the lanterns up to 10 nautical miles (18.53 km). The Fresnel lens with diameters of 250 mm was designed by a ray tracing program and optimized by adjusting the groove parameters of the lens. Each optical sag element which is a part of a lens was independently designed by using the analytical method. The angular luminous intensity distributions (ALID) of this lens were calculated by the illumination analysis program considering the ALID of a light bulb. The ALID of a C-8 type bulb (24 W) was measured with a goniophotometer and its luminous flux was measured by an integrating sphere to be 397 lm. At the best alignment of the bulb, the maximum luminous intensity of the lantern was more than 1000 cd for the 250 mm lens. The ALID was investigated as a function of distance from the lens focus to determine the tolerance margin of the alignment. Horizontal deviations of the light bulb from the focus along the optical axis widened the angular FWHM of the vertical ALID. However, vertical deviations caused shifts of the vertical ALID without spreading the angular FWHM. The designed 250 mm aspherical lens of marine signal lantern was made by the injection molding with single peace acryl. We measured the luminous intensity distribution of acryl lens and found that the MLI of the lens was 827 cd. And the full width at half maximum of the diverging angle of the diverging beam was 3.5 deg. Although the measured MLI was 83% of the calculated result, it would be increased with surface polishing of prototype molding pattern.