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The local dimming mini-LED backlight has become one of the most important technologies for liquid crystal display (LCD), facilitating the improvement on high dynamic range (HDR) and power saving. The halo effect still limits the performance of LCDs employing mini-LED backlight. The impact of the number of local dimming zones on the contrast ratio has been investigated. The halo effect has also been investigated under four typical local dimming algorithms and different local dimming zones. It has been experimentally demonstrated that the halo area gradually shrinks with increasing the number of local dimming zones, contributing to the improvement on the HDR of LCD.

We present a high-resolution non-contact method for the measurement of two-dimensional (2D) phosphor surface temperature distribution (PSTD) of phosphor-coated light-emitting diodes (pc-LEDs) based on advanced hyper-spectral imaging technology. The studied pc-LEDs with a surface-mounted device structure are mainly consisted of blue InGaN/GaN-based LED chips (with a spectral emission at about 456 nm), red (Sr,Ca)AlSiN3: Eu2+ phosphors (with a spectral emission at around 623 nm), and transparent silicone gels. Experimental temperature results are compared with those of micro-thermocouple ( $\mu $ -TC) and infrared thermal imaging (TI), and fairly good agreements can be noticed. However, the spatial resolution of this proposed method is more than one order of magnitude higher than that of TI method, and the proposed method provides more detailed surface temperature information of phosphors than the latter. Therefore, we believe that this proposed method can serve as useful tools for the non-contact measurement of PSTD in pc-LEDs with or without optics lens.

In this work, the genetic algorithm is employed to optimize both circadian action factor (CAF) and color quality of laser-based illuminants (LBIs) with three, four, and five spectral bands to disclose its possible use in two common white lighting applications, i.e. bedroom lighting and office lighting. Comparing all LBIs at a correlated color temperature (CCT) of 3000 K and a color rendering index of 80, the CAF of four-band LBIs reaches a minimum of 0.238 and maintains at a possibly highest luminous efficacy of radiation (LER) of 422 lm/W among all cases. The performances of white LBIs are also compared with those of white light-emitting diodes (LEDs). The results demonstrate that, under the same conditions of color rendering and color temperature, both four-band LBIs and four-band LEDs exhibit the largest circadian tunability of about 4.7, while four-band LBIs possess much higher LER at the same time compared with four-band LEDs. In addition, for the display application, the investigation on the optimal circadian tunability as a function of color gamut at two CCTs (3000 K and 6500 K) is also performed. We believe that this study can serve as a useful guidance for the application of LBIs in both the healthy general lighting and display.

For humans, nonvisual biological effects, which can be parameterized by the circadian action factor (CAF), are important for their health and work performance. In this paper, an approach by adjusting electrical currents of three-primary light-emitting diodes (RGB-LEDs) to tune the CAF and color coordinates to be close to those of the standard light with various correlated color temperature (CCT) (2700 K-6500 K) has been reported. This approach is based on deriving an empirical formula between the optical power and the electrical current. In order to meet the requirement of both high color rendering index (CRI) and better CAF, we theoretically perform further spectral optimization on RGB-LEDs and have found that peak wavelengths of 466, 547, and 615 nm for the blue, green, and red color component, respectively, turn out to be the best peak wavelength combination for obtaining both high CRI and high luminous efficacy of radiation of RGB-LEDs, while the CAF, CCT, and color coordinates of RGB-LEDs approach those of the standard light.

In the light-emitting-diode (LED) industry, it is a great challenge to simultaneously achieve high efficiency values and excellent light quality in practical applications. In this paper, we have designed microscale-roughness-controlled-surface modules to optimize rectified mellowness index, quantum efficiency, correlated color temperature, and color rendering index; we have also discovered a mechanism that reduces the total internal reflection trapped within the diffuser, maximizes phosphor fluorescence probabilities, and randomizes photon emissions. The first effect subsequently leads to the photon-propagation-direction rectification, such that incident photons are allowed to escape from the top but are discouraged to reversely exit from the bottom. In addition to the discovery of this mechanism, we have proposed a previously undefined criterion that evaluates the uniformity of various optical parameters exemplified by wavelengths, solid angles, and spatial distributions. Current findings offer a guide on future research for human-eye-comfort-related uniformities.

By modifying traditional remote phosphor-diffuser-packaged light-emitting diode systems, we have managed to increase the luminous efficacy from 145.7 to 162.3 lm/W. One mechanism responsible for this achievement is associated with randomizing the directions of light beams transmitting through an interior diffuser, whose position is optimized based on an overall merit. The other mechanism is identified as the gradual attenuation of the undesirable blue-ring image along the distance from the diffuser toward the phosphor base. The identification of these two mechanisms is verified by luminous efficacy measurements, 3-D image plots, and combined Monte Carlo algorithm ray tracing simulation results. In addition, merits related to correlated color temperatures and issues pertaining to costs are briefly discussed.

InGaN/GaN light-emitting diodes (LEDs) with a graded-composition multiple quantum well (GQW) were designed not only to investigate the effect of polarization field on the efficiency droop in InGaN/GaN multiple quantum wells (MQWs) but to find out possible solutions to prevent or reduce the efficiency droop in GaN-based LEDs as well. Pulsed electroluminescence measurement, to avoid heating effects partly, revealed that the output intensity peak occurs at ~280A/cm2 current density for the GQW structure, whereas for a conventional structure, it occurs at a much lower current density of ~150A/cm2. The relative quantum efficiency indicates that the efficiency droop in GQW LEDs was reduced effectively due to the reduction in polarization field in the active layer. In the test GQW structure with inverse indium composition variation direction, electrons escape from the active region, which is in company with rapid saturation of output power and decrease in efficiency observed at high current density. It indicates that at high current density, the electron leakage is obvious. These results suggest that the polarization field in the active layer and the consequent electron leakage are probably the main mechanism responsible for the efficiency droop at high current injection levels.

Three- and four-hump InGaN-based white light-emitting diodes (LEDs) have been computationally investigated. The investigation includes three-hump LEDs precoated with green- and red-emitting quantum dots (QDs) and four-hump LEDs precoated with green-, yellow-, and red-emitting QDs. Results show scotopic/photopic (S/P) ratios $>$ 3.80 and color rendering indices (CRIs) $\geq$ 70 for three-hump LEDs and S/P ratios $>$ 3.90 and CRIs $\geq$ 70 for four-hump LEDs under a correlated color temperature (CCT) of 45 000 K. Furthermore, for both three- and four-hump LEDs under the condition of CCT $>$ 5000 K, optimal results in this paper exhibit capabilities of exceeding $\hbox{S/P} = 2.50$ and simultaneously satisfying the criterion of CRI $\geq$ 70. Our simulation procedure is conducted under variations of nine CCTs, several wavelengths, and peak heights. Optimized results of S/P ratios and CRIs are identified and retained through filtering off a great number of unqualified candidates. Finally, present findings may help improve the quality of eye visions pertaining to LED illumination and help reduce the electrical energy consumption related to LED usage.