The solar-powered LED lighting system has been commercialized for a long time. The system usually consists
of a DC to DC converter in order to convert the battery voltage into a fixed voltage or current for the LED lighting
luminaire. This will cause energy loss and system reliability due to the failure of DC/DC converter. In the present study,
we develop a special technique to drive the LED luminaire directly from battery utilizing PWM technique in order to
remove the DC/DC converter. However, instantaneous current overdriven can occur easily due to the variation of
battery voltage with the state-of-charge of battery.
In the present study, we setup a thermal chamber with temperature variation to within 40±3°C. A LED luminaire
was specially designed for the LED reliability test with four different circuits with each circuit connecting three LED
lamps serially. A driver is designed to provide 4 kinds of power inputs to LED: (a) 350mA constant current, (b)
700mA,100Hz, duty cycle=50%, (c)700mA, 10K Hz duty cycle=50% and (d) 1050mA, 100Hz, duty cycle=33%. The
tests were performed simultaneously to compare light decay between normal drive condition (a) and other PWM
driving conditions (b, c, d). The accumulated total test time so far is more then 7,032 hours and has shown no
significant light decay in 4 different loops. This reveals that the PWM technique directly driven by battery is feasible
and is able to reduce energy loss of DC to DC converter in the solar lighting system.
We investigated the luminance enhancement, spectral shift and image blur of the OLED with the microlens array
film (MAF) attachment experimentally and theoretically. Higher density, larger curvature, and smaller diameter of the
microlenses extracted more light from the substrate mode. The maximum improvements of the luminance at the normal
direction and the total power were 42.5% (80%) and 45% (85%) from our experimental (simulation) results, respectively.
The differences between the theoretical and experimental results may come from the non-Lambertian radiation of OLED
and the imperfection of the microlens array film. From observing the planar OLED, the peak wavelength is blue-shifted
and the full width at the half maximum (FWHM) decreased with respect to increasing viewing angles due to the
microcavity effect. When the MAF was attached, the spectral peak had a further blue shift (5 to 10 nm at different
viewing angles) compared to that of the planar OLED and it came from the light extraction of the MAF from the
substrate mode.
We also quantitatively investigated the "blur width" of the OLED with MAF attachment. Higher image blur was
observed as accompanied with higher extraction efficiency which showed a tradeoff between the image quality and
extraction efficiency. It means that the MAF attachment is more suitable for OLED lighting application, rather than
display application. To reduce the image blur and keep the high extraction efficiency at the same time, we re-designed
the arrangement of the microlens arrays on the film. In our optimized case, we found that the blur width can be reduced
from 79 μm to 9 μm, while the extraction efficiency is kept nearly the same. It shows a possibility to use the microlens
array film on real OLED display for improving the extraction efficiency without image quality degradation.
This study proposed a novel approach to replace the traditional surface plasmon resonance (SPR) bulk prism by
microlens arrays (MLAs). It demonstrated the effect that coupling SPR on the optical response of microlens arrays
structure. Surface plasmons are features specific to the interface of metal-dielectric. They are due to charge density
oscillations in the metal, accompanied by electromagnetic field dissipation in the metal and in the dielectric. SPR
biosensor bulk prism technology has been commercialized and SPR biosensors have become a central tool for
characterizing and quantifying biomolecular interactions. We will used this microlens arrays coupling SPP phenomenon,
which gives rise to selective spectral response due to a local field enhancement interrelating the optical and biochemical
domains.
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