Radiation from very hot objects (such as the Sun and incandescent light-bulbs) is of tremendousutility, yet a large fraction of the radiated energy is wasted: for example, in solar cells, the mismatch between the Sun's spectrum and the cell's absorption profile limits efficiency 1 ; in incandescent light-bulbs, most of the energy is lost as heat 2 . For moderate temperatures, shaping thermal emission is possible through wavelength-selective radiators such as photonic crystals 3-13 .However, thermal emission tailoring at elevated temperatures (>1000K) remains exceedingly difficult [14][15][16][17][18] . Here, we address this challenge by coupling the hot emitter with a cold-side nanophotonic interference system. In particular, we show that a plain incandescent (3000K) filament, surrounded by an interference system uniquely optimized to reflect infrared and transmit visible light for all angles, becomes a light source that reaches luminous efficiencies close to the limit for lighting applications (~40%), surpassing all existing lighting technologies. In an experimental proof-of-concept, we demonstrate efficiency approaching that of commercial fluorescent or LED bulbs, but with exceptional reproduction of colors and high and scalable power. These results showcase the potential of spectral tailoring and enable a new high-temperature frontier in optics, with applications in thermophotovoltaic energy conversion 3-5 and lighting.2 Consider a thermal emitter of emissivity sandwiched between two identical structures of reflectance R(λ) and transmittance T(λ), separated by a small gap, as shown in Fig. 1c. In general, the emissivity of a high temperature emitter depends on temperature and wavelength.Such an emitter can be made of uniform, un-patterned, bulk material (e.g. refractory metals such as tungsten or tantalum), but can also be a photonic crystal with wavelength-selective emission properties. Similarly, in the simplest form, the filtering structure is a layered stack of materials of different refractive index, but can also be a 2-dimensional or a 3-dimensional photonic crystal.By tracing the reflected radiation in the cavity surrounding the emitter, we show (see Supplementary Discussion) that the effective emissivity of this emitter-tailoring-structure system can be expressed as (1) where is the original emissivity of the thermal emitter and F is the view factor characteristic to the geometry. The view factor equals the proportion of the radiation leaving the emitter that is intercepted by the enclosing surface. Expression (1) highlights the potential to tailor thermal emission by designing the surrounding cold-side structure properties. In such a manner, the radiation spectrum of extremely high temperature emitters can be modified without the need for any structural patterning of the emitter surface. The cavity effect due to the surrounding structure results in a portion of the power emitted at unwanted wavelengths to be reabsorbed by the thermal emitter. Applying a similar analysis as before and using Kirchoff's...