Increasing the Brightness of Light Sources
In modern illumination systems, compact size and high brightness are important features. Light recycling allows an increase of the spectral radiance (brightness) emitted by a light source for the price of reducing the total radiant power. Light recycling means returning part of the emitted light to...
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|In modern illumination systems, compact size and high brightness are important features. Light recycling allows an increase of the spectral radiance (brightness) emitted by a light source for the price of reducing the total radiant power. Light recycling means returning part of the emitted light to the source where part of it will escape absorption. As a result, the output brightness can be increased in a restricted phase space, compared with the intrinsic brightness of the source. In this work the principle of light recycling is applied to artificial light sources in order to achieve brightness enhancement.
Firstly, the feasibilities of increasing the brightness of light sources via light recycling are examined theoretically, based on the fundamental laws of thermodynamics including Kirchhoff's law on radiation, Planck's law, Lambert-Beer's law, the étendue conservation and the brightness theorem. The theory of light recycling can be derived from first principles.
From an experimental viewpoint, the radiation properties of three different kinds of light sources including short-arc lamps, incandescent lamps and LEDs characterized by their light-generating mechanisms are investigated. These three types of sources are used in light recycling experiments, for the purpose of
1. validating the intrinsic light recycling effect in light sources, e. g. the intrinsic light recycling effect in incandescent lamps stemming from the coiled filament structure.
2. acquiring the required parameters for establishing physical models, e.g. the emissivity/absorptivity of the short-arc lamps, the intrinsic reflectivity and the external quantum efficiency of LEDs.
3. laying the foundations for designing optics aimed at brightness enhancement according to the characteristics of the sources and applications.
Based on the fundamental laws and experiments, two physical models for simulating the radiance distribution of light sources are established, one for thermal filament lamps, the other for luminescent sources, LEDs. Both are validated with high resolution measurements. The physical models are capable of analytically modelling the radiance distribution with few required parameters (geometry, material properties and operating conditions). They are widely applicable to any kind of sources with similar light-emitting mechanisms. Combining the advantages of conciseness, high accuracy and wide applicability, the physical models can be integrated into ray-tracing software.
As validation of the theoretical and experimental investigation of the light recycling effect, an optical device, the Carambola, is designed for achieving deterministic (as opposed to stochastic) and multiple light recycling. The Carambola has the function of a concentrator. In order to achieve the maximum possible brightness enhancement with the Carambola, several combinations of sources and Carambolas are modelled in ray-tracing simulations. Sources with different light-emitting mechanisms and different radiation properties (optical thickness), and Carambolas with different geometries and optical properties are used. It is concluded that a high-pressure xenon lamp is suitable for light recycling with the Carambola due to its moderate optical thickness, its continuous spectrum and its geometrical features. In the ray-tracing simulation, the suitability of the XBO lamp and the Carambola optics for light recycling is confirmed.
A prototype five-point reflective Carambola was manufactured from aluminium, for the purpose of experimentally demonstrating a brightness increase.
The Carambola is tested with different sources. The experimental results are below the theoretical expectation, i.e. the measured brightness enhancement factor is lower than the designed factor and the modelled factor. The discrepancies are explained.
A real Carambola must have a high reflectivity and an accurate shape, if a significant effect of light recycling is to be shown. The brightness enhancement by light recycling with the Carambola is expected to be improved greatly by enhancing the surface quality of the reflectors, using suitable sources with small optical thickness, compact size, (and in the case of LEDs) high intrinsic reflectivity and external quantum efficiency. The Carambola is expected to be utilized in applications such as high brightness displays.
In conclusion, both theoretical investigation and experimental measurements on light recycling demonstrated a significant increase of the brightness of optically thin sources.