Handbook: 80 of 287
TABLE OF ZENITH-TANGENTS EXPLAINED.
Luxfer Prisms distribute in a room the light
which falls from the sky upon the window. The vertical angle (z) of the
lowest light falling upon a prism plate has been called the "zenith
distance." The tangent of this angle has been called the "zenith-tangent"
(zt). This tangent is obtained by dividing the width (a) of the street by
the height (b) of the opposite building, i.e. zt=a/b. Upon this
zenith-tangent depends almost
entirely the kind, quantity and method of installation of prisms which
must be used to light a room satisfactorily. The tables on pages
give this zenith-tangent for various conditions. It is to be noted that
the "street width" (a) is the distance from the prism plate to the face
of the opposite building, and that the height of the opposite building