Normal spectral emissivity of Si melt in visible and infrared regions was determined by the direct measurement of thermal radiations from the melt and a blackbody cavity which was located close to the melt. The spectral emissivity slightly decreases with wavelength. The emissivity slightly changes with temperature. The spectral emissivity values in visible and in infrared region are 0.27 and 0.21, respectively. The wavelength dependence of the emissivity can be interpreted by a dielectric response of free electrons in the melt.

Microstructure of porous silicon (PS) was observed through high-resolution transmission electron microscope to study a relationship between the microstructure and a photoluminescence (PL) from PS. Three PS samples were made through anodization with different current densities. The samples had different PL spectra and were supposed to have different microstructures. Threadlike structures of Si and Si crystallites in the PS layer were observed in all the samples. The threadlike structure has the same density in all samples and has very little structural change, in spite of a significant change of PL spectra. The density of the Si crystallites varies for the samples and has a strong correlation with the PL intensity. No particular change of crystallites shape was observed. The size of the crystallites ranges from a few nanometers to a few tens of nanometers. Although no positive evidence to a quantum confinement is indicated, the Si crystallites are responsible for a light emission from PS.

The normal emissivity of liquid germanium was determined from the direct measurement of the thermal radiation from the liquid surface and a blackbody. The monochromatic emissivity in the visible region and the average emissivity in the near infrared region were measured to investigate the wavelength dependence. The emissivity of the liquid has little dependence on the wavelength and is 0.217±0.002 for the visible region (600–900 nm) and is 0.200±0.005 for the near infrared region at the melting point. The emissivity has a weak dependence on the temperature from 910 to 1160 °C. The weak dependence agrees well with a dielectric response of the free electrons.

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Normal spectral emissivity of liquid Si was determined by direct measurement of thermal radiations from a surface of the Si melt and a blackbody cavity. The spectral emissivity has little dependence on the wavelength. The emissivity is 0.27 for the wavelength from 500 to 800 nm and is about a half of that of solid silicon at the melting point. Temperature dependence of the emissivity is very small in the temperature range from the melting point to 1550 °C. Free-electron model with a plasma frequency and relaxation time of the order of 1016 Hz gives a good agreement with the experimental result. That indicates the dominant effect of the free electrons on the optical properties of the liquid Si.