Merchandise" light if we throw the lowest light 45° below the
horizontal. Therefore, turning to the right-hand column of this page,
and to the row corresponding to the nearest zenith-tangent 1.70, we find
that we should use M and J
prisms. We find the M is in
heavy type, which indicates that it is the major prism and constitutes
90 per cent of the prism plate. J is the minor prism and constitutes
10 per cent of the prism plate.
In order to obtain the area of the prism plates,
we turn to page 203, of the Table of Luxfer Prism
Areas corresponding
to the room width 20 feet, and to the class of illumination headed "For
General Merchandise," find the fourth column corresponding to our major
M prisms and the fifth row
corresponding to the length of the room, 60 feet. The area of prism plates
required is 58 square feet. We should order, therefore, a prism plate 20
feet wide (width of room) and 3 feet deep, 90 per cent
M's and 10 per cent J's.
Example 2.--A room 25 feet wide, 90 feet long,
17 feet high, light walls, is to be used for a jewelry store, with no
particular obstructions, facing a street 65 feet wide, the opposite building
50 feet high. Required, the kind and quantity of prisms.
Solution.--Our prism plate is probably 10 feet
from the sidewalk. The zenith-tangent is, therefore, 1.62. We find that
the major prisms are M, and
the minor prisms J. Assuming that the highest light is horizontal and the
lowest light is 45° below horizontal, we should distribute them as
90% and 10%. To find the area of the prism plates required, we turn to
page 204 corresponding to room width 25 feet, and to
the table headed
"For Fine Merchandise," we select the fourth column, which is headed
M, because this is our major
prism, and to the row corresponding to 90 feet, and find that the area
of the prism plates should be 147 square feet. We should have, therefore,
for the front, a plate of prisms 25 feet wide (width of room) and 6 feet
deep, 90 per cent M's, 10
per cent J's.
Example 3--A room 40 feet wide, 50 feet long, 14
feet high, light walls, to be used "For Storage," having no particular
obstructions to light, faces a 25 foot alley, opposite building 80 feet
high. Required, the kind and quantity of prisms.
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