No abstract

Most of the commercial white LED lamps use blue chip coated with yellow emitting phosphor. YAG:Ce 3+ phosphor is coated on blue chip to obtain white light. Though this is commercially successful, there are several drawbacks such as "halo effect", poor colour rendition, etc. In recent years several efforts have been made to improve LED lamp performance. In this paper modification of YAG:Ce phosphor for improving CRI, by introducing Gd 3+ , Pr 3+ or Tb 3+ at Ce 3+ site is reported.

Solar simulators based on light emitting diodes (LEDs) have shown great promise as alternative light sources for indoor testing of PV cells with certain characteristics that make them superior to the traditional solar simulators. However, large-area uniform illumination, more suitable for larger cells and module measurements still remain a challenge today. In this paper, we discuss the development and fabrication of a scalable large-area LED-based solar simulator that consists of multiple tapered light guides. We demonstrate fine intermixing of many LED light rays and power delivery in the form of a synthesized air mass (AM) 1.5 spectrum over an area of 25 cm by 50 cm with better than 10 % spatial no uniformity. We present the spectral output, the spatial uniformity and the temporal stability of the simulator in both the constant current mode and the pulsed-mode LED operation, and compare our data with the International Electro technical Commission (IEC) standards on solar simulators for class rating. Although the light intensity with our current design and settings falls short of the standard solar AM 1.5intensity, this design and further improvements open up the possibility of achieving large-area, high power indoor solar simulation with various desired spectra.

In this paper we have reported green emission for europium activated strontium thiogallate. The phosphor is prepared by the precipitation method followed by heating at reducing atmosphere produced by the burning charcoal and characterized using XRD, PL and EL techniques. The phosphor SrGa 2 S 4 :Eu 2+ gives intense green emission at 525 nm when excited by 400 nm wavelength. LEDs were also fabricated by coating europium activated strontium thiogallate on 390 nm LED chip. Intense green LEDs were obtained which can be useful for new LED applications. This phosphor can also be a prominent green component for the fabrication of white LEDs from UV diodes.

YAG:Ce phosphor coated on blue-chip gives many advantages, except the color rendering of the LEDs fabricated by this method is poor due to the lack of red and green colors. This problem can be sorted out by choosing different methods out of the available two different methods. For the first method, the n-UV (near-ultraviolet) LED is coated with a mixture of intense blue, green, and red phosphors to make a white light-emitting diode. Also, if the yellow color phosphor is added, lamps can give better CRI values. So many papers are reported on this type of white LED fabrication technique. In this paper, we have reported the fabrication of white LED lamps by coating three different phosphors i.e. blue, green, and red in the appropriate amount to be mixed and coated on the near UV LED chips. This approach is different from the existing reports because we are using near UV (405-407 nm LEDs) not the UV LED chips in the fabrication of white LEDs in this paper.