The field of solar energy conversion has become an important discipline with a recognized potential to significantly contribute to the world supply of energy. It is diversified and encompasses a wide variety of disciplines - from mechanical engineering to physics, from biology to architecture, from ocean science to agriculture, from chemistry to atmospheric science, to name some of the major fields. It involves fields which have matured to the engineering aspects, such as the conversion of solar energy into heat or of wind into shaft work. It includes other fields in which more basic science research is necessary to unravel the micro-structures of nature, as, for example, for photovoltaic conversion or for certain bioengineering tasks. Several of these fields have elements which have been common knowledge for centuries but sometimes forgotten at times of cheap energy supplies, while others have barely started with first studies. Most of the fields have seen during the last decade a substantial advance in sophistication, in theoretical understanding, in demonstrated feasibility, in developing hardware, in field testing, with some moving into a phase of initial commercialization.

A number of significant changes have occurred in Advances in Solar Energy since Volume 1 appeared in 1982. The delays in publication of the second volume are the result of reorganization of the American Solar Energy Society, and the negotiation of a new publishing arrangement. Beginning with this volume, Advances is now published jointly by the Society and Plenum Press. The Editorial Board has been enlarged to be more representative of the different fields of solar energy conversion. Production of Advances is being expedited through the use of modern word processing equipment and the 'lEX typesetting-editing program. We have gone to a single-column format to ease the problems of presenting long equations, and we expect that the user of the volume will find it easy to read. The use of 'lEX will make last minute updates possible. The external appearance of the volume matches that of Volume 1. We expect that future volumes of this annual will be proceeding on schedule. We invite comments from users and correspondence from prospective authors of critical reviews. Karl W. Boer John A. Duffie CONTENTS CHAPTER 1 The Measurement of Solar Radiation Ronald Stewart, Daniel W. Spencer and Richard Perez 1.1 Abstract 1 1.2 Characteristics of Pyranometers ....................................... . 2 1.3 General Features of a Pyranometer ................................... . 3 1.3.1 Instrument Sensitivity 4 1.3.2 Response with Time 4 1.3.3 Sensitivity 4 1.3.4 Responsivity ................................................. .

Advances in Solar Energy is back on schedule. Volume III contains a number of interesting reviews of the different fields in solar energy conversion. We appreciate the many encouraging comments received after the second volume appeared and have incorporated some of the suggested changes. Even though most of the reviews are invited through our editors, we are always open to suggestion about subjects of importance that are ready for a com prehensive and critical review and have not been recently covered, or about potential authors. I would like to take this opportunity to thank Professor John A. Duffie for his invaluable help in starting the Advances in Solar Energy series. Although he has recently taken full responsibility as editor-in-chief for the Solar Energy Journal, his continued assistance as a member of the Board of Editors is greatly appreciated. The diligent work of the many active editors is gratefully acknowledged and constitutes the basis for a valuable review periodical with outstanding contributions. The typesetting was done by Sandra Pruitt in the Delaware office, using the TEX-program with laser print-out. Her organization and patience in coordinating with the authors, and her technical skill and diligence in preparing the submitted copy permitted the timely and high-quality assembly of this production. We wish to commend her for efforts well beyond the call of duty. The accommodating help from Plenum Press and its production staff deserves our grateful acknowledgement."