A SIMPLIFIED MODEL OF LIGHT LOSS DUE TO STRUCTURAL MEMBERS IN MULTISPAN GREENHOUSE ROOFS

A method of expressing light loss associated with the regularly spaced structural members in a multispan greenhouse as a function of solar or sky elevation is presented. Transmission is related exponentially to a function (1 + sin a)α/2), where a is the solar or sky elevation, and α is an obscurity coefficient, characterizing the total fraction of sky blocked out. For typical houses this function is shown to predict losses accurately for angles of elevation greater than 10 °, and moreover is independent of azimuth over the same elevation range, to an accuracy usually better than 10%. The coefficient α is only slightly dependent on skylight distribution, and can therefore be used to characterize a structural configuration. For this purpose, an expression correlating α to measured values of light loss is also developed. © 1997 Elsevier Science Ltd. All rights reserved.