Optical Transform Limitations in Headlamp Photometric Performance

Abstract

Automotive lamps are essentially the optical transform devices. A light intensity angular distribution from a given light source (filament, HID arc, LED, etc.) is transformed to a desired new light intensity angular distribution namely beam pattern by means of an optical system such as a reflector or lens optics, or a projector module system. There are fundamentally five types of optical transformations occurring in a headlamp optical design: A). Light intensity angular distribution transforms from a light source to a beam pattern that is another fashion of angular distribution via a reflector-optics device. This transform device, sometimes, is referred to as the free-form reflector design; B). Light intensity angular distribution from a light source is transformed to a spatial distribution on a focal plane of an ellipsoidal (or similar) reflector; C). Light intensity spatial distribution on the focal plane of the ellipsoidal reflector is transformed to a beam pattern (angular distribution) using a projection lens. The combination of these transforms, B and C, is referred to a projector module system; D) Light intensity angular distribution is transformed to a quasi-parallel beam (uniform angle) via a parabolic reflector; E) The quasi-parallel beam is transformed to a beam pattern (angular distribution) using a traditional lens array. The combination of the transforms of D and E is referred to the lens optical design. In an optical system, both luminous flux and etendue is conserved. These conservations dominate optical design limits for a headlamp photometric performance. This paper discusses these limitations: luminous intensity and size for a headlamp using studies of the above mentioned optical transformations. Furthermore, the paper will show that these limitations are related to the light source and the uniqueness of LEDs. This is the continuation of the studies published in the last year's SAE paper written by the same authors.