Fig. 1 Emitter reflectance spectrum. The reflectance spectrum is a step-function-like structure designed to shift the resulting radiation toward the visible. The lower inset shows the optical thin-film structure observed by a transmission electron microscope (TEM). The upper inset presents the quantitative model for the modification of the blackbody radiation. The black line represents the reflectance spectrum, the blue line is the emissivity spectrum described by ε(λ) = 1 - R(λ), the green line is the blackbody radiation spectrum, and the red line is the modulated thermal radiation created by the product of the blackbody radiation spectrum and the emissivity ε(λ).

Fig. 2 Thermal radiation spectrum from the emitter obtained by a FTIR spectrometer at 580 K (red circles), 670 K (yellow circles), 785 K (green circles), and 870 K (blue circles). For comparison, the thermal radiation spectrum from a copper plate (emissivity ε₀ = 0.13) is shown (open squares). The thermal radiation spectrum of the plate obeys Planck’s law, and the results are fit by solid curves. The arrows mark the peak positions of the thermal radiation spectrum for the emitter and the plate for each temperature. The inset shows the 580 K thermal radiation spectrum from the emitter (solid circles) and the plate (open squares). The radiation spectrum of the emitter was fit using the product of the wavelength-dependent emissivity and Planck’s law, and the results were fit with theoretical curves obtained using Eq. (5).

Fig. 3 The total radiation intensity as a function of temperature (T⁴) for the copper plates (blue circles) and the emitters (red circles). The Stefan–Boltzmann law, I∝T⁴, is clearly shown for the plate. The red line is the theoretical fit determined by Eq. (6). The inset shows the change in temperature as a function of input power for the plates (blue circles) and for the emitters (red circles) to estimate the ratio of the energy dissipation between the thermal radiation and the conduction. The solid blue line is the curve obtained using Eq. (7), given the linear dependence of conduction loss on temperature, denoted by the solid black line. The solid red line is the theoretical fit to the power dissipation curve of the emitters obtained by Eq. (8). This leads to the conclusion that almost 80% of the input power can be converted into thermal radiation at shorter wavelengths using the emitter.

Fig. 4 The blue line represents the theoretical temperature change as a function of input power generated from Eq. (8), given an emitter with a cutoff wavelength of λ₀ = 0.7 μm. The black line is the heat dissipation term due to conduction from electric wires. The broken red line is the extracted radiative power from the emitter given the step-function reflectance spectrum described in the upper inset. The red curve in the upper inset is a theoretically derived radiation spectrum, and the green line is the spectral luminous efficiency curve. The chromaticity of the incandescent lamp investigated here centers on the red circle of the inset CIE chromaticity diagram, and its correlated color temperature is beyond 9000 K.