| Summary: | 碩士 === 國立東華大學 === 光電工程學系 === 99 === White LED based on phosphor conversion an important approach for energy saving and mercury-free lighting in the near future.Two approaches have been proposed for phosphors converted white LED using either the blue or UV LEDs for pumping light. The blue pumped Nd:YAG LED has the advantage of high efficiency but the CRI is limited and the uniformity in angular distribution is poor. UV converted LED on the other hand have better CRI and angular distribution but the efficiency is limited. To over this low efficiency, an omni-directional coating approach is proposed previously to increase the efficiency of the UV-pumped LED. An one dimensional model is constructed to determine the material parameters and to analyze the performance. However one dimensional model is unrealistic since the advancing wave front of light propagation is the sum of the all the point sources in the medium already traversed, according to Huygen’s principle. In the present result an investigation of the angular distribution of the UV pumped LED is performed. A quasi three dimensional (Q3D) model based on the observed data is constructed to analyze the angular distribution of measured data with different material parameters.
Based on the angular distribution of phosphor-converted light, it is found that the angular distribution can be influenced by the phosphors layer thickness, concentration, ODR brightness enhancement layer, and back reflection. Angular distribution of the phosphors converted light is isotropic in the media it is generated which forms a near lambertion pattern when measured outside the media. The details shape of the emitted light depends on the refractive index differences of the effective index in the media in which they are generated and where they are observed. Thus a quasi-3D model is constructed with the primary source follow the Beer-Lambert law with a exponential decay inside the media and the loss in light is absorbed by the phosphors to generated the isotropic ally distributed light at a lower wavelength. This generated light is then absorbed by the media before they escaped the media. The validity of the model is established by fitting the measured data and then is used to explain the observed behavior of the angular distribution of the emitted light which are summarized below:
1.As the thickness increased, the distribution deviate more from the lambertian distribution since the light emitted at an angle after absorption of UV photon will travel a longer path in the phosphor layer and will experience a larger attenuation, thus less light is emitted. This attenuation increases with angle and resulted in a narrower distribution than the lambertian distribution. For the same reason, the beam also becomes narrower as the concentration increases.
2.For the ODR covered sample, the deviation becomes less since more UV light is converted near the top of the phosphor layer with the ODR present to reflect the UV light. Being generated near the top, this part of light has shorter path and has wider distribution than that generated near the bottom of the layer, thus the combined light appears wider for the high angle light and the deviation from lambertion with thickness appear to be less than that without the ODR. The same trend is observed with the concentration.
3.For the case with the back-reflector in the package to reflect the backward propagating, light from the top of the phosphor layer travels a longer distance and its contribution becomes less. But that generated at bottom are making bigger contribution as they are experiencing less attenuation. The angular distribution of the combined light narrows.
Good quantitative fit with the measured data are obtained that validate the model. The material parameters used for the fit including the absorption coefficient of the pumped light and the converted light is compared with that obtained with the 1D model. It is found that the 1D model over-estimated the primary light since the distance traveled for the 1D model is shorter than the Q3D model.
In summary, the factors that can influence the angular distribution of the light from the UV pumped phosphor converted LED are analyzed qualitatively in the present work. The influence of these factors are quantitatively analyzed by taking into account the path length variation as light travel at an angle through the phosphor layer.
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