To make incandescent light bulbs a viable alternative to LEDs, it is required to convert with a high efficiency the broadband thermal radiation of the incandescent material into visible light. The ideal way to achieve such an efficient conversion consists in placing a cold bandpass filter above the incandescent material, with a 100% transmittance in the visible and a 100% reflectance in the other spectral regions. By reflecting back the non-visible components of thermal radiation onto the incandescent material, the filter triggers a recycling mechanism in which such components are re-absorbed and the corresponding power can be reused in subsequent thermal radiation events. However, in a real device, the cold filter has non-ideal optical properties and, more importantly, it has to be separated from the incandescent material by a gap to avoid thermal damage. Radiation leaks from the sides of this gap and gets lost into other parts of the device, thus lowering the conversion efficiency. Arny Leroy and coworkers demonstrate, both theoretically and experimentally, that placing specular reflectors on the sides of the gap between the cold filter and the incandescent material mitigates radiation leakage and improves device efficiency. A 10% gain is reported when using a tungsten emitter and silver-coated polished copper mirrors as reflectors.

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