In this thesis the degradation and failure mechanisms of organic materials in the optical part of LED-based products are studied. The main causes of discoloration of substrate/lens in remote phosphor of LED-based products are also comprehensively investigated. Solid State Lighting (SSL) technology is a new technology based on light emitting diodes as light sources. This technology, due to its several exceptional characteristics such as lower energy consumption, longer lifetime, and higher design flexibility with respect to the conventional lighting technology, has become very attractive to both manufacturers and consumers. It is applied in a variety of applications such as general lighting for in-door and out-door applications, and for automotive. Reliability in the highly demanding and fast growing SSL market is a key challenge, which requires special attention. A SSL system is typically composed of an LED engine with an electronic driver(s), integrated in a housing that also provides optical functions, thermal management, sensing and/or other functions. Knowledge of (system) reliability is crucial for not only the business success of today!s SSL products and applications, but also to gain deeper scientific understanding which will enable improved product and application design in the future. A malfunction of the system may be induced by the failure and/or degradation of any subsystem or interface. A comprehensive and in-depth understanding of failure and degradation behaviors of different SSL system components would obviously result in a more effective and reliable design as well as a proper selection of materials and anufacturing techniques. Package-related failure mechanisms. that result in an optical degradation, colour change, and severe discoloration of the encapsulant are listed as carbonization of the encapsulant, encapsulant yellowing, and phosphor thermal quenching. Among different materials used as an encapsulant or substrate for the phosphor in remote phosphor design, PolyCarbonate (BPA-PC) is chosen in this research. In order to study the main reason(s) of discoloration and consequently to define lifetime, a series of experiments are performed under different external stresses (temperature range of 100 to 140 ºC and radiation of blue light with 450 nm wavelength). A highly accelerated test set-up was designed to control these stresses and monitor light output of the system at the same time. Evaluating and analyzing of chemical and optical characteristics of samples during ageing in this specially designed highly accelerated test set-up are performed using a wide range of techniques including UV-Vis, FTIR-ATR, and X-ray photoelectron spectroscopy (XPS), Lambda spectroscopy and Integrated Sphere. The results show that increasing the thermal ageing time leads to yellowing, loss of optical properties, and decrease of the light transmission of the relative radiant power value of both pure and commercial BPA-PC plates. Thermally induced oxidation reactions of BPA-PC are found to be the major reason of the yellowing and discoloration. The major effect of light intensity in remote phosphor is believed to be increasing the temperature of the phosphor, and therefore enhancing the kinetics of thermal ageing. Photo-fries products are found in photo-thermally aged BPA-PC plates, aged under blue light radiation at elevated temperature of 140 ºC, and believed to have a contribution to the discoloration. The XPS analyses of aged samples confirm that discoloration is associated with surface oxidation. A significant increase in the signal ratio O1s /C1s in the XPS spectra of degraded specimens is observed. During thermal ageing, the C-H concentration decreases and new oxide features C=O and O-C=O form, with the latter being a support for oxidation at the surface being a major reaction during discoloration. Results also show that irradiation with blue light during thermal ageing accelerates the kinetics of discoloration and the increases O1s /C1s ratio in XPS spectra. The accelerated optical degradation and reliability of two different commercial BPA-PC plates under elevated temperature stress are studied as well. The reliability model, explained in this thesis, is indeed a useful framework to incorporate kinetics of (photo)-thermal ageing of BPA-PC and YAG:Ce phosphor into the life-time prediction models. It is shown that increasing the exposure time leads to degradation of BPA-PC optical properties, i.e. decrease of light transmission and increase in the yellowing index (YI). By increasing the temperature, the rate increases, meaning that lumen depreciation takes place at shorter time. The reaction rate follows the Arrhenius acceleration law. The thermal stability and life time of remote phosphor lens plates are also studied. The photometric properties of thermally-aged plates, monitored during the stress thermal ageing tests, show a significant change both in the correlated color temperature (CCT) and in the chromaticity coordinates (CIE x,y). It is also observed that there is a significant decay both in the phosphor yellow emission and in the blue peak intensity, with yellow emission being more affected, inferring that the main reason for the optical degradation of thermally-aged BPA-PC plates could be ascribed to yellow conversion of blue light. As final conclusions, among different existing stresses including light intensity, humidity and heat, thermal stress has a more pronounced influence on the ageing of encapsulants in optical parts in LED-based products. Also it is shown that the rate of lumen depreciation is highly dependent on temperature; the higher the temperature the faster the kinetics of color shifting and lumen depreciation is. The effect of light intensity is increasing the temperature in phosphor plates. Reliability of optical components in LED-based products can be well described by the Arrhenius equation and generalized Eyeing equation. Coating the BPA-PC by a graphene monolayer can significantly enhance the optical properties and stability of BPA-PC, used as substrate in remote phosphor plates. Graphene decreases the oxidation kinetics of BPA-PC and acts as a barrier for moisture and oxygen diffusion.