TABLE OF LUXFER PRISM PAVEMENT AREAS EXPLAINED.
The tables on pages
236-245 show the areas of
Luxfer multi-prism pavements necessary for different basements. Each
half page contains all the variations of basements of one width. Two
classes of illumination have been considered, one is "For General
Merchandise," the other "For Storage." The "beam" referred to is the
head-beam (b) at the inner edge of the pavement and supporting the same.
It is ordinarily from 4 to 20 inches in depth. The table provides for
four different depths--0, 6, 10 and 14 inches. In order to find the
square feet of multi-prism plates needed for a basement, select the table
containing the given width of basement, select the class of illumination
desired, under this select the column corresponding to the depth of beam,
and the row across this table corresponding to the length of the
basement; the square feet of product is found at the intersection of this
row and column. The illumination obtained is independent of the
zenith-tangent, provided the zenith-tangent is larger than 0.8; if the
zenith-tangent is less than 0.8, the area of the prism plates should be
multiplied by the quotient of 0.8 divided by the zenith-tangent, i.e.
0.8/zt. It is assumed that the basement is of reasonable height, say at
least 7 feet in the clear, and that its walls and ceiling are light,
and that it is plastered, or, at least, that the joists do not show.
Example 10.--A basement 80 feet long, 30 feet
wide, is to be given a fair basement light from one end, the street
being 60 feet wide, opposite building 50 feet high, depth of head beam
below the surface of the prism pavement is 10 inches,