Solution.--Suppose that we light two-thirds of
this room from the front and the remainder from the rear. For the front
we find, therefore, a room 60 feet long and 25 feet wide, 17 feet high,
street 65 feet wide, opposite building 50 feet high, highest angle 5°
below horizontal, lowest angle 45° below horizontal, light for fine
merchandise. From the tables we find that for such a room we should need
90 square feet of L and J
prisms. For the rear we have a room 25 feet wide, 30 feet long, 17 feet
high, 25-foot alley, opposite building 80 feet high, same angles. For
this we need 100 square feet of A
canopy.
Corner Store.
In many cases a store can be lighted from two
sides, such as a corner store, and it may be desirable to place prisms
in both fronts. In these cases an arrangement which has been found very
satisfactory, and which gives increased results, has been to consider
the total amount of prisms which would light the store, first from one
side and then from the other, taking two-thirds of this total amount,
and distributing it uniformly in both fronts.
Example 8.--A room 40 feet wide, 50 feet long, 16
feet high, to be used "For Fine Merchandise," with no particular obstructions,
is a corner store and is to be lighted from two sides. The 40-foot front
faces a street 45 feet wide, opposite building 55 feet high. The 50-foot
front faces a street 60 feet wide, opposite building 50 feet high.
Required, the kind and quantity of prisms. Highest light 5° and
lowest light 40° below horizontal. Prism plate, 10 feet from
sidewalk.
Solution.--Our zenith-tangent for the 40-foot
front is 1.00, and for the 50-foot front is 1.50. The prisms necessary
to light a store "For Fine Merchandise" from the 40-foot front is 155
square feet, using 90 per cent O
and 10 per cent K. The area of prisms needed to light a store from the
50-foot front is 138 square feet, using 90 per cent
L and 10 per cent K. The
sum of these two areas is 293 square feet; two-thirds of these two areas
is 196 square feet, which is distributed uniformly on two sides.
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