The photoelectric and thermionic effects are combined in an illustrative experiment to demonstrate that solar light and heat can be converted into electrical energy simultaneously. When an electron is ejected from the cathode and is collected by the anode, a difference in chemical potential develops between the anode and cathode Fermi levels. Work can be extracted as the electron returns to the emitter Fermi level via the load. When the electron is not thermalized, it is said to be a "hot" electron. Ross and co-workers have predicted that the AM1.5 efficiency limit for a hot carrier conversion system, 66%, is greater than that for a purely thermal system, 52%, or for a quantum system, 33% (e.g., a photovoltaic cell). The present work was undertaken to provide an easy to reproduce experimental format to explore these concepts. As an example suitable for a student laboratory, a commercial vacuum phototube is used as a quantum and thermal energy converter. An S1 photocathode comprised of Ag2O:Cs is employed at low temperatures, T < 100degreesC, to demonstrate that the power converted by a heated and illuminated phototube is greater than that obtained either heated in the dark, or under illumination at room temperature. Although the conversion efficiency and power production is small in this example (approx. 10(-3)%), the experiment demonstrates how two forms of solar energy can be simultaneously utilized. It also promotes a thermodynamic approach to the evaluation of solar converters. The use of cesiated III/V materials (e.g. InGaAsP:Cs) as photocathodes is discussed as a possible research pathway for realizing efficient hot electron devices. (C) 2004 Elsevier B.V. All rights reserved.