Spectral and Directional Variations of Emissivity — Lesson 4

This lesson covers the concept of emissivity of metals and its variation with temperature and wavelength. It explains how the emissivity of a metal depends on its surface temperature, while the absorptivity depends on both the surface temperature and the source temperature. The lesson also discusses the impact of surface finish and oxide layer on the emissivity of metals. For instance, it explains how a thin oxide layer can significantly increase the emissivity of metals. The lesson further explores the use of selective coatings in solar collectors to achieve higher temperatures and the role of emissivity in the efficiency of tungsten filament lamps.

Video Highlights

03:15 - Explanation of the concept of selective surfaces and their properties.
08:07 - Discussion on the dependence of the directional spectral emissivity of materials and the impact of an oxide layer.
18:00 - Explanation of the Stefan Boltzmann law
19:38 - Discussion on the effects of surface finish and oxide layer on the emissivity of materials.
29:51 - Explanation of the temperature variation from a tungsten filament lamp and its efficiency.
44:38 - Discussion on the computation of total properties from spectral properties and the impact of surface finish on emissivity.

Key Takeaways

- Emissivity of a metal depends on its surface temperature, while absorptivity depends on both the surface temperature and the source temperature.
- The emissivity of metals can significantly increase due to the presence of a thin oxide layer.
- Selective coatings are used in solar collectors to achieve higher temperatures.
- The efficiency of tungsten filament lamps is limited due to the maximum achievable temperature of the filament.
- The emissivity of a material can vary with wavelength and angle, and this variation can be used in the design of many engineering systems.