Lighting systems using light emitting diodes (LEDs) for providing white light is at the point of surpassing conventional lighting technologies such as fluorescent lamps with regard to light output, light quality and efficiency.
However, it is difficult to produce an LED which emits white light with a uniform color at a desirable color temperature. Therefore, various types of wavelength converting elements are often used to convert light emitted by LEDs. As an example, LEDs emitting light in the blue region of the spectra are often combined with a yellow phosphor material for achieving white light. Light from the LED entering the phosphor undergoes wavelength conversion, and the degree of conversion is related to the distance which the light travels through the phosphor. Accordingly, the wavelength of the light emitted from the phosphor element depends on the distance between the light source and the light emission surface of the phosphor element. Thus, assuming a phosphor element having a flat light emission surface, the color output is angle dependent.
An angle dependent color output will lead to that light emitted from the surface of a phosphor element at an angle close to the normal of the surface is closer to the blue end of the spectra compared to the more yellowish light emitted essentially parallel to the surface. This color variation phenomenon is known as Color-over-Angle (CoA) variation. CoA variation is undesirable as it produces a ring-like effect in the light-output from the phosphor element where a yellowish ring may be seen around the edges of the preferably white light beam.
Several different approaches for CoA correction have been tried in order to overcome the above effect, such as by grading the phosphor layer or using dichroic filters. However, these methods may reduce the efficiency and increase the complexity, and thereby the manufacturing costs of a lighting system. Another proposed solution is to introduce microstructures on top of a collimating structure.