Remote phosphor-converted LEDs (rpc-LEDs), which rely on a phosphor layer located away from the LED chip, are a particularly attractive technology benefitting from a higher luminous efficiency and from an improved stability compared with on-chip LEDs. However, systems based on thin-film remote phosphor layers still face a low color conversion efficiency (CCE). This mostly originates from an insufficient interaction of the exciting blue light with the phosphors. To overcome this limitation, we propose to couple the thin-film converting layers to a micro-concavity array (MCA) designed to enhance the optical pathlength of the exciting light, resulting in an improved CCE. This is achieved by exploiting the excellent light scattering and retro-reflection properties of MCA. We experimentally verify that the MCA transmit 95% of the incoming blue light into the converting layer, whereby 84% of this share corresponds to scattered light. Moreover, the measured retro-reflection amounts to 21% for normally incident light. The potential of the fabricated MCA films is tested by integrating them on the illuminated side of remote light converting thin-film layers with sub-millimeter thickness. Two examples, including quantum dots (QDs)- and rare-earth phosphor- based LEDs, are investigated. Our results show that the CCE of both rpc-LEDs are improved due to the enhanced excitation of the downconverted materials and to the effective extraction of the backscattered light. Thus, the CCE values of QDs-based and phosphor-based and rpc-LEDs are increased by 8.1% and by 12.7%, respectively, compared to devices without MCA films. In the latter case, the angular color uniformity is additionally improved under the effect of light scattering.
A composite method combining energy and intensity mapping is proposed to address the issue of surface error caused by the irregular sampling phenomenon in freeform illumination lens design. In the combined method, the central region of the freeform lens is designed by the intensity mapping method, whereas the peripheral region is designed by the energy-mapping method. Furthermore, an iterative feedback optimization is added to scale out the application in extended light sources. As an evaluating example, a freeform lens with a 120-deg viewing angle, fitted with an appropriate number of points is designed by the proposed combined method. Compared with that designed by the energy method, the lens forms a more uniform illumination on the target surface without the appearance of a hot spot in the central region. The proposed method also exhibits superiority in extended-source design, where only a one-time optimization is needed to achieve the preset uniformity with the proper choice of coefficients.
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