The organic light emitting diode (OLED) has recently evolved into an alternative for LED in vehicle rear lamps. This fact is highlighted by the OLED taillight applications of Audi TT RS and A8, BMW M4 GTS or Mercedes S Class Coupé. Aspects of light source technology and especially lighting technology with regard to an efficient usage are rarely discussed in research as well as questions about the perception of brightness.
Methods for an efficient development of OLED taillights are presented in this work. Especially photometric and colorimetric aspects are discussed. As shown in examples, an angle-dependent light distribution of an OLED can cause perceptible differences of luminance and color coordinates in rear lamps. In case of 2,5D Designs, various light areas can appear at different angles for one perspective. Hence, from this perspective, different luminances and color coordinates appear within the design. For the examined case, the perceptible differences are considered to be higher than the variations within the light areas.
Several OLED architectures can be used for taillight applications. Beside bottom and top emitters on glass substrate, bottom emitters on plastic substrate were investigated. With the focus on photometric and colorimetric characterization, a technological trade-off has to be considered: A directed light distribution, as typical for a top emitter, helps to efficiently meet the regulatory requirements, but causes large angular dependencies of luminance and color coordinates. The bottom emitter is the counterexample. Hence, depending on the application and visible angle, one has to choose the appropriate architecture.
Furthermore, three investigations were carried out to research the optimum brightness of homogeneous taillights. Within these studies, the influence of the observation distance, the size and geometry of the light area as well as the speed of the signal in case of dynamic taillight functions was considered. To minimize interfering influences and to guarantee reproducible conditions, the investigations took place in a totally darkened light channel.
The results suggest the existence of a close range and a far range, which begins at ~30m. Within the close range, the optimum brightness increases with the observation distance. In fare range, a constant value between 864cd/m² and 1430cd/m² is recommended. The values of the dynamic signal are almost entirely below the static ones. One exception could be found at an angular velocity of 18°/s. This range should be avoided in future dynamic taillight applications. | English |