There is no particular reason for having a brilliant
light at the front of the store, so we should order a plate 90 per cent
N, 10 per cent L. The
dimensions of the prism plate are to be found from the TABLE OF
LUXFER PRISM AREAS.
Whenever the prescription is
"A," the meaning is that the
prism plate must be installed in an independent frame (canopy) set at an
angle of about 50° to the vertical. Any one of three canopies "A,"
"C," and "1/C" may be used, as explained on
page 85.
Diagonal Canopy.
When a canopy is installed over a single opening
most of the light is thrown directly through the window, illuminating
brightest a strip in front of the opening inside which is not much wider
than the width of the window. When the window openings are quite narrow,
it is desirable, oftentimes, to install a canopy which is wider than the
window opening. In this case, the brightest light from the prisms at the
sides of the canopy is throw directly against the wall of the building
adjoining the window, and does not enter the room. In order to obviate
both of these difficulties whenever they occur, it is customary to place
at the side of each canopy to be installed over a single opening, some
diagonal prism lenses. (See detail drawings.) These diagonal prism
lenses are named "rights" and "lefts," "rights" going in the right-hand
part of the canopy, looking at the prism side, and the "lefts" going in
the left-hand side. If a room is to be lighted by one long canopy, of
course these diagonals are not needed, unless it is to throw light around
columns or other obstructions. In cases of canopies over single windows,
the quantity of these diagonal prisms depends largely upon the conditions
of the room to be lighted. It is customary, however, to use about 70
per cent of the body prisms, either "A" or "C," and about 15 per cent
"rights" and 15 per cent "lefts" of the corresponding letter.
Tilted Prisms.
We have specified that the highest building across
the street shall be taken into consideration in obtaining the zenith-tangent,
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