Color Shift Failure Prediction for Phosphor-Converted White LEDs by Modeling Features of Spectral Power Distribution with a Nonlinear Filter Approach

Dynamic prediction of optical and chromatic performances for a light-emitting diode array based on a thermal-electrical-spectral model

Light-emitting diode (LED) arrays have attracted increased attention in the area of high power intelligent automotive headlamps because of their superiority in disposing of the power limit of an individual LED package and controllably luminous intensity and illumination pattern. The optical and chromatic performances of an LED array do not equal to the sum of individual LED packages' performances, as the thermal interactions between individual LED packages can't be ignored in the actual application. This paper presents a thermal-electrical-spectral (TES) model to dynamically predict the optical and chromatic performances of the LED array. The thermal-electrical (TE) model considering the thermal coupling effect in the LED array is firstly proposed to predict the case temperature of each individual LED package, and the Spectral power distributions (SPDs) of individual LED package is then decomposed by the extended Asym2sig model to extract the spectral characteristic parameters. Finally, the experimental measurements of the designed LED arrays operated under usage conditions are used to verify the TES model. Some validation case studies show that the prediction accuracy of the proposed TES model, which is expressed as a quadratic polynomial function of current and case temperature, can be achieved higher than 95%. Therefore, it can be concluded that this TES model offers a convenient method with high accuracy to dynamically predict the optical and chromatic performances of LED arrays at real usages.

The Protective Effects of Blue Light-Blocking Films With Different Shielding Rates: A Rat Model Study

Purpose

To examine light emitting diode (LED)-induced retinal photochemical damage and assess the protective performance of blue light-shielding films with different shielding rates in Sprague-Dawley rats (SD rats).

Methods

SD rats were randomly divided into five groups: blank control (group I), white LED illumination (group II), and white LED illumination combined with shielding of blue light of wavelength 440 nm at 40%, 60%, and 80% (groups III, IV, and V). The illumination was 200 lux. All animals underwent electroretinography (ERG), hematoxylin-eosin (H&E) staining, immunohistochemical (IHC) staining, and transmission electron microscopy (TEM) observation after 14 days of dark-adaptation before illumination, after 14 days of cyclic illumination, and after 14 days of darkness for recovery following illumination.

Results

ERG showed retinal functional loss after LED light exposure. However, retinal cell function was partly recovered after a further 2 weeks of dark adaptation. H&E staining and TEM revealed increases in photoreceptor cell death after illumination. IHC staining demonstrated that oxidative stress was associated with retinal injury. Although retinal light injury was discovered in the LED light-exposure groups, shielding 60% of blue light of wavelength 440 nm (bandwidth 20 nm) protected retinas.

Conclusions

Cyclic illumination of low light intensity (200 lux) for 14 days produced retinal degeneration; shielding 60% of blue light may protect retinas from light damage.

Translational Relevance

This study found the effective shielding rate that could protect retinas from light damage when shielding specific narrow-band harmful blue light; thus providing a more normative method for protecting eyes from blue light hazard.

A Heat-Stimulated Luminous Fiber Using Heat-Sensitive Green TF-G Pigment

In this study, we fabricated a heat-stimulated luminous fiber (HSLF) by wet spinning. The HSLF consists of Sr₂ZnSi₂O₇: Eu²⁺, Dy³⁺ (SZSO), Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ (YOS), and heat-sensitive green TF-G pigment (HSGP). SZSO and YOS serve as a source of luminescence to yield a long afterglow system. HSGP is a heat-stimulating agent which develops the link between luminescence and temperature for HSLF. The luminescence of the HSLF is dull below 30 °C, but vivid above 30 °C. The luminescence of HSLFs can be stimulated by low heat (human body temperature). Emission spectra were recorded at 20 °C and 30 °C to investigate the heat-stimulated luminescent performance of HSLFs. HSLF is a smart material which can discern the exciting light to change color because of the photo-thermal effect. This characteristic provides optimum conditions for SZSO and YOS to store energy. The results demonstrated that most luminescence from SZSO and YOS could be absorbed by HSGP at 20 °C, but the luminescence could be liberated at 30 °C. The heat-stimulated phenomenon could also be verified by afterglow and the naked eye.

Photometric and Colorimetric Assessment of LED Chip Scale Packages by Using a Step-Stress Accelerated Degradation Test (SSADT) Method

By solving the problem of very long test time on reliability qualification for Light-emitting Diode (LED) products, the accelerated degradation test with a thermal overstress at a proper range is regarded as a promising and effective approach. For a comprehensive survey of the application of step-stress accelerated degradation test (SSADT) in LEDs, the thermal, photometric, and colorimetric properties of two types of LED chip scale packages (CSPs), i.e., 4000 °K and 5000 °K samples each of which was driven by two different levels of currents (i.e., 120 mA and 350 mA, respectively), were investigated under an increasing temperature from 55 °C to 150 °C and a systemic study of driving current effect on the SSADT results were also reported in this paper. During SSADT, junction temperatures of the test samples have a positive relationship with their driving currents. However, the temperature-voltage curve, which represents the thermal resistance property of the test samples, does not show significant variance as long as the driving current is no more than the sample’s rated current. But when the test sample is tested under an overdrive current, its temperature-voltage curve is observed as obviously shifted to the left when compared to that before SSADT. Similar overdrive current affected the degradation scenario is also found in the attenuation of Spectral Power Distributions (SPDs) of the test samples. As used in the reliability qualification, SSADT provides explicit scenes on color shift and correlated color temperature (CCT) depreciation of the test samples, but not on lumen maintenance depreciation. It is also proved that the varying rates of the color shift and CCT depreciation failures can be effectively accelerated with an increase of the driving current, for instance, from 120 mA to 350 mA. For these reasons, SSADT is considered as a suitable accelerated test method for qualifying these two failure modes of LED CSPs.