Designing an LED luminaire with balance between uniformity of luminance and illuminance for non-Lambertian road surfaces

We present a method of designing a light-emitting diode (LED) luminaire for non-Lambertian road surfaces, in which we optimized the overlap between the lighting areas of a luminaire and its adjacent luminaires to attain balance between uniformity of luminance and illuminance. The goal of the balance is to approach highly uniform luminance and raise illuminance uniformity as high as possible. The optimal illuminance rendered by a single luminaire and the corresponding far-field intensity were derived from the optimal overlap condition, and, thus, the optical model was established. The performance of the model is as follows: overall uniformity of luminance (U_O) is 0.91, longitudinal uniformity of Luminance (U_L) is 0.92, threshold increment (TI) is 1.64%, uniformity of horizontal illuminance (U_HE) is 0.66, and surround ratio (SR) is 0.52. Finally, a compact freeform lens capping an LED was constructed to realize the optical model. The performance of the luminaire designed with arrayed units of an LED capped by the freeform lens is as follows: U_O is 0.80, U_L is 0.85, TI is 3.57%, U_HE is 0.56, and SR is 0.51. In addition, the luminaire provides 19,600 lumens of luminous flux, of which 68% and 54% are projected on the target area and roadway alone, respectively, while 27% is absorbed by the surrounding materials.